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post #1 of 18 (permalink) Old 11-27-2012, 11:49 PM Thread Starter
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carb tuning; can't find Norm's procedure

Hi everybody, new posting here, but been scouring these forums for a little while now.

For the first time I'm going to tune the round top SUs on my recently acquired 71 240z. I've been looking for all the info I can get, and I've found dozens of posts mentioning Norm's write-up on tuning. But whenever I find a link to it, I just get sent to the front page at Zcar.com.

So here's how I understand what I've read so far:

-Engine up to operating temp
-basically shut down one carb by pushing up that little rod on the bottom of the carb
-adjust the mixture on the other carb to the best, smoothest idle
-fix the mixture on the other carb by the same method
-use (unisync?)that air flow meter and adjust the idle screws on each carb until they're each pulling the same air flow

Is there anything I'm misunderstanding? Alternatively, does anyone know where I can find Norm's legendary write up?

thanks in advance.
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Tuning your SU carburettors by Scott Fisher
I’ve been meaning to write this up for some time, ever since I did the SU Performance Tuning 101 a few months ago. This one is more like Basic SU Adjustment for Happy Driving.
The trick to tuning SU carbs is to understand that there are two things you need to get right: the air flow, and the fuel mixture. While they are interconnected, they are also independent, and need to be measured and adjusted independently.
Special Tools
You will probably need to arrange to buy or borrow a Unisyn flow meter. The Unisyn is the usual gauge for getting the air flow balanced between the two carbs. This costs about $20 and is simple to use. It consists of an adjustable opening (same size circumference, but with a disc on a threaded rod that you can screw tighter or looser) that you use to set the level of a little float that rises or falls in a glass tube at the side of the gauge.
For the fuel mixture, I have become sold on a device called the Gunson ColorTune (maybe ColourTune, as it’s a British co.). This is a spark plug with a crystal pressure- and heat-resistant window in it that lets you see into the combustion chamber while the motor is running. The color of the flame indicates the mixture richness. It costs about $40, and while it’s not absolutely essential, it makes life so much easier that it’s worth the cost.
If you don’t have a Gunson, I’ve included the standard directions here for determining correct mixture (step 4 of the Adjusting Mixture procedure).
To tune SU carbs, first locate the following components:
Throttle linkage nuts. These are the things that connect the throttle linkage (the bar connected to your foot through whatever means your car uses, cables or rods) to the carburetors’ throttle levers.
Throttle stop screws. These set the idle speed for each carb, and are located typically behind the dashpot, on the same side of the carb to which the throttle linkage connects.
Mixture adjusting nut. This is the lower of the two nuts at the very bottom of the carburetor. Later SU carburetors of the HIF type have integral float chambers, on which the mixture is adjusted by turning a screw. You’ll need to experiment (and I explain how) to see which way makes this richer and which way makes it leaner.
Lifting pins. These are little wobbly metal pins under the dashpot. When you push up on the pin, it raises the piston in the dashpot. Find these; they’re crucial if you don’t have a Colortune. If you don’t have or can’t find them, you can raise the piston with a flat-bladed screwdriver pushed down the throat of the carb and twisted to lift it.
The bridge. This is the part inside the carburetor, where the gas jet opens into the airstream. You’ll see a needle inside the jet, and the jet itself should be a few fractions of an inch down from the bridge itself. The jet is the brass tube that sits in the center of the bridge, with a tapered needle poking down into it.
The choke linkage nuts. Comparable to the throttle linkage nuts (and usually the same size), but on the linkage that goes between the choke cable and the mixture adjustment mechanism. They make sure that both carbs are enriched when you pull on the choke.
Balancing The Air Flow
1. Start with the engine warmed up to operating temperature and perform your standard ignition tune-up (points gap, timing, spark plug gap, new condenser, etc.) first. If you’ve got a timing light and a dwell meter, you can verify all that stuff independent of the way the car is running. When it’s warm, shut the motor off and remove the air filters.
2. Begin by balancing the air flow. To do this, first loosen the throttle linkage nuts. Leave them connected, just loosen them half a turn or so.
3. Back out the throttle stop screws till you can see that they are just touching the throttle stop. Then open each carburetor (that is, lower the throttle stop screw) 1-1/2 turns of the throttle stop screw and start the engine. It will probably idle at about 2000 RPM; don’t worry.
4. Put the Unisyn over either carb and adjust the orifice in the Unisyn till the little float at the side rests at the middle of its graduated tube. (Pre-diagnostics: if the idle drops and the car wants to die when you slap on the Unisyn, the carb is too rich; if the idle soars upwards, it’s too lean.) Hold the Unisyn over the carb for only long enough to see the level of the float, then remove it.
5. Place the Unisyn on each carburetor in turn to check its flow, adjusting the throttle stop screws until both carburetors register the same position on the graduated tube of the Unisyn. (The float will probably move either up or down in the tube, which is why you want to center it in Step 4.) When both carburetors flow the same amount of air, tighten the throttle linkage nuts, adjusting for the amount of free-play between the linkage and the throttle stops that your manual calls for (probably about 0.006″). Your goal should be to achieve the lowest possible idle with both carbs balanced and the engine running smoothly. (Note that the idle speed will very probably rise as you get the mixture correct.)
If you’ve taken more than five minutes to do this, rev the engine to over 2500 RPM (assuming the idle isn’t already that high) for thirty seconds or so to clear the spark plugs. Then adjust the mixture.
Adjusting The Mixture:
Note: in the following procedure, one “flat” is the basic increment of adjustment, and refers to 1/6 of a turn of the mixture adjusting nut. This corresponds to the flat faces on the nut.
I’m going to give instructions for SUs with the separate float chambers. If you have the HIF integral-float carbs, you’ll have to look in a manual to see whether you turn the mixture screw to the right or the left to make it richer or leaner; I’ve done that once but I can’t remember. Alternatively, you can — with the motor shut off — peer down the throat of the carb and turn the mixture screw while watching the top of the jet. Remember that moving the top of the jet up will lean out that carb, while moving the top of the jet down will richen it.
1. Shut the car off and loosen the choke linkage nuts.
2. Adjust the mixture nuts (screws) fully lean.
For separate float-chamber cars, this means raising the mixture nut all the way up against the bottom of the carb (or rather, against the spring). For HIF carbs, you can try turning the screw while looking down the throat to see which way the jet is moving. In either case, the idea is to zero out the jet: raise it all the way up in the bridge.
3. Now drop the jet an equal amount — two full turns for HS-type carbs, two full turns (I believe) for HIFs. Then start the car.
Note: In the following step, you might want to consider adjusting the carburetors one-half a flat too lean, as the mixture will be enriched when you put the air filters (which restrict air flow) on at the end of the tuning process.
4. Raise the lifting pin (or use a screwdriver if you don’t have the pins) so that the piston rises no more than 1/16″. Listen to the engine’s exhaust note and compare it to the following conditions:
If the exhaust note rises and stays high till you drop the piston, this carburetor is adjusted too rich. Turn the mixture nut one flat (one-sixth of a turn) up, moving the jet toward the bridge, then repeat Step 4.
If the exhaust note falls and the car sounds as though it is going to stall, this carburetor is adjusted too lean. Turn the mixture nut one flat (one-sixth of a turn) down, moving the jet away from the bridge, then repeat Step 4.
If the exhaust note rises briefly and then settles back down to something like the original RPM level, this carburetor is set correctly. When you have achieved this setting for both carburetors, continue with Step 5.
5. Tighten the choke linkage nuts so that the choke cable will pull an equal amount on both mixture nuts when you pull the knob.
6. At this time, I find I usually have to adjust the idle again because getting the fuel mixture right usually changes the idle speed. Since you know you have the throttles synchronized, I normally just adjust the idle without loosening the throttle linkage. The easiest way is to screw one of the screws out till it doesnt’ even touch the throttle stop, then use the other to get the idle speed right. When that’s done, you can screw the other stop screw down till it just touches the stop on that carb and you’re set.
7. Replace the air filters and go for a test drive!
Notes
SU carburetors are most fuel-efficient when slightly lean, and provide the most power when they are slightly rich. You can use this knowledge to provide a certain amount of tuning for the kind of driving you do. If you learn to read spark plugs, you can get a basic idea of what your engine’s condition is and make fine adjustments to the mixture nuts accordingly.
If you have a ColorTune, you simply install it in place of one of the plugs, then adjust the carburetor that feeds that cylinder (the front carburetor for 1 & 2, the rear for 3 & 4). The ColorTune will let you see the color of the flame. White flashes mean too lean; yellow flame means too rich. Blue (like a Bunsen burner) is correct, and blue with a faint orangish tinge is the best for power.
You can also modify your car’s throttle response characteristics slightly by adjusting the viscosity of the oil in the dashpot damper. SUs are set up so that a thicker oil will resist the piston’s attempt to rise in the dashpot for just long enough that the engine’s increased load (when the throttle is opened) will pull more fuel across the bridge; this enriches the mixture and temporarily bumps power up to help the engine achieve higher speed more readily.
If you modify your engine, you will probably need to modify your needles, as it is the needle profile that determines the mixture curve for different air-fuel loads.
If you experience uneven idle, hunting, or an idle that changes (rises or falls) as the engine’s temperature climbs or drops, you probably have vacuum leaks. The most serious fault on most old SUs is wear in the throttle shaft area. To test for this, spray some carburetor cleaner on the outside of the throttle shaft; carburetor cleaner is non-combustible, and if the engine speed drops, it means some of this is getting into the air stream from outside the carburetor. You may also have leaks from the manifolds, from tubing such as the vacuum advance line to the distributor (if fitted), or from other places; the carb cleaner trick works well for locating those leaks as well.
Other problems that SU carbs experience involve dirt in the dashpot and occasionally in the float chamber. The dashpot is a precision piece of machining that involves very close tolerances so that the piston doesn’t stick or bind when it rises and falls. A little grit between the piston and the dashpot can make the car jerk and sputter. Take the dashpot off, wipe the insides down with carb cleaner and a lint-free, clean rag, then reinstall it, getting the screws down tight. Also, don’t swap the pistons between dashpots; they’re matched to one another so that the clearance between the piston and the wall of the dashpot makes a tight seal but permits easy rising and falling.
Dirt in the float bowl basically shuts off that carburetor (or can make it flood open, depending on whether the dirt is wedging the valve open or closed). You can try rapping on the float bowl with the handle of a screwdriver, but your best bet is to take the cover off, clean out the valve fittings, and reinstall everything, with a new fuel filter for good measure.
Some older SU models also have adjustable floats, in which you need to set the float height (which basically equals the fuel level in the float chamber) by bending a brass rod. These carburetors were replaced in the mid-1960s with carburetors that had fixed, plastic floats which are basically trouble-free unless abused. The stop at the back of the floats can break if they are installed badly, and the brass pin that holds them in place can wear an oval hole in the float pivot. New floats are fairly inexpensive and aren’t a bad idea if you’re doing a rebuild.
Grose-Jets are very popular with some people and a big pain for others. It appears — and this is just conjecture — that Grose-Jets work best in cars with adjustable floats, as they are longer than the stock SU float valves. The standard failure for Grose-Jets is to flood the carburetor. I have never had problems with the stock SU float valves or floats.
Like this? Make sure you pop in and say HI to the other Datsun owners in the Datsun Forum!
——————————————————————————–
Tuning Your S.U. Carbs by Roger Garnett
Well, it’s not really that hard to set up SU’s, just different. Of course it always gets more interesting when you have more than one… There is a very good Haynes SU carb manual available, reccommended reading. The basic syncing process also applies to Zenith-Stromberg’s, but the adjustment mechanisms are different. Here is a laymans guide to adjusting SU’s (long):
step 1-
Tune up the rest of the engine- REALLY! clean or replace, and set the points, set the timing, plugs, valve lash, and remove the air filters. (have new ones ready) All of these things can affect the setting of the carbs, which should be done LAST, (if at all). The carbs rarely need to be adjusted, once set. Also replace/install the gas filter. Of course, it helps if the carbs are in good mechanical condition as well. But you can consider a rebuild once you have gotten things working first!
step 2-
clean the carbs! use gum-out or similar stuff, clean all external linkages, shafts, and stuff.
step 3-
Remove the float bowl covers, clean the float bowls, remove old sediment, and check/adjust the float setting. (turn the cover upside down, and get a *1/8″ in drill bit, set the drill bit accross the cover, the float tab should just touch the bit.) Make sure the needle is moving and seating properly. This is just like *most* floats. Replace the cover.
* This is for HS4 SU’s- (1/8-3/16″) if you are dealing with 1″, H’s, HS2′s HS6′s, HIF’s, etc.- check the spec for your carb.
Note: You can check for matching float settings, after setting the mixture, by removing the pistons, and peering down at the jets. The fuel level should be about the same on both carbs, a little below the top surface of the jet. (After car has been run only)
step 3b- Go get a pint of ale, or something close, and set it nearby.
step 4-
Remove the piston covers. CAREFULLY remove the piston, DO NOT BEND THE NEEDLE. Set the piston down on a clean wadded rag to prevent rolling. Clean the inside of the carb. Check operation of the throttle. Check the throttle shaft slop- this is the most common place for wear on an SU, and is often where air/vacumn leaks occur. The bushings and shafts can be replaced, but it requires some machining. A small amount of leakage can be tolerated, the car just won’t idle as evenly. Clean the piston. Stare in awe at the odd carborator design, simple and effective, (constant velocity). Dump the old oil out of the damper if you haven’t already spilled it. clean. Reassemble, check piston movement, raise it, then let go, it should fall freely. If not, check assembly again, make sure the piston isn’t binding against the carb body, it should ride only on the damper shaft. Do not strech the spring. When all is operating properly, fill the damper with Marvel Mystery Oil for light damping, or use motor oil for heavier damping. (I use MMO) If you get “flutter” on, acceleration, you might try the heavier oil.
step 5-
Start the car and warm it up, then turn off/disconnect/otherwise disable the choke mechenism. (Loosen the nuts on the clamps so that the choke stuff isn’t doing anything) This will get set later. (Later Zenith-Strombergs have a thermostatic choke, not a cable.)
step 6-
Check coarse throttle adjustments- make sure the throttle cable pulls on both carbs equally, and returns completly when released. This is adjusted by loosing the set screws on the throttle shaft and matching the two sides. You can also adjust the cable length at this time, using the cable set screw/retainer at the end of the cable. You can check the float adjustments now by removing the piston & cover, and looking at the fuel level in the needle seats. Both carbs should be about even, a little below the top surface of the jet. If not, readjust one or both floats to match the level.
step 7-
Syncronize the throttles- if you have a uni-syn, here’s your chance to use it, (or other air flow guage), if not use a tube and listen to the airflow. The Uni-Syn is much easier to use, and can result in better balance. Alternatly adjust the idle screw on each carb, attempt to set the idle as low as possible (~800-1000 RPM). Adjust until the airflow is *close* to the same at each carb. The engine may now be running rough, just keep the idle speed high enough to keep running. Give the throttle a quick snap to make sure everything is settled, then check sync again. Periodicly snap the throttle to make sure everything is seated. Large differences in where you can adjust the two carbs may indicate air/vacumn leaks, or other problems, such as a bad valve)
Magic Time- Relax, and shake your voodoo rattle…
step 8-
Adjust the mixture- this is done with the spring-loaded hex fitting under the carb, where the fuel supply tube enters from the float. Turning the fitting raises and lowers the needle seat. Pick a carb, and turn the fitting 3 flats (1/2 turn), first in one direction, then back 3, then 3 in the other direction. Note where the engine runs better, idle speed should increase. Turn to the best setting. Repeat this proceedure until you get the best operation you can, (higest idle speed), keeping track of flats turned will help you remember where you were. If you get lost, turn all of the way in, then back out 12 flats and start again. Periodicaly snap the throttle and push up on the fitting to make sure everything is seated. Note: Type HIF carbs (With integrated float bowl) no longer have the hex nut to adjust the mixture. Instead, there is a screw to twiddle, on front of the front carb, and behind the rear. The screw is connected to the needle seat through a temperature compensated gizmo, which is said to make the carbs more stable. Adjustment can be done in much the same way, by counting turns/flats of your screwdriver. There is less adjustment range than with the the basic models. When you think you’re close, stop, uncramp your fingers, breath deep, and do the same to the other carb. Then retune the first carb, and then the second again. This serves to match the mixture of the 2 carbs, and prepare you for the beer sitting over there in the sun. (why do you think the British drink warm beer?)
step 9-
repeat step 7, setting the idle speed as low as possible, and re-syncing the idles. Now go back and readjust the mixtures. After a couple of iterations, the engine should be running smoothly (controlled by mixture) and at a low idle. Repeat as necessary. Set the final idle to 800-1000 RPM, depending on the condition of the rest of the engine.
This is a standard mixture test, performed AT IDLE: Under operation, (air filter off) lift the carb piston by 1/16″ with the lifting pin or a screwdriver, which leans the mix a tad. If: -RPM’s rise and stay up, that carb is rich. -RPM’s rise briefly, then drop, mix is about right. -RPM’s fall, engine gets rougher- mix is lean.
[where is step 10--Ed]
step 11-
Adjusting the choke- I won’t get into the temperature compensation in the type HIF, or the Thermostatic choke in the later strombergs. Check the manual for more info. The choke is supposed to do two things; the first half of travel moves a cam on each carb which opens the throttle, for warm up. The second half pulls down on the needle seat to enrichen the mixture, for starting. Start with the choke in the off position (knob in). Adjust the so that the cam only starts moving the throttle after you start pulling out on the cable (adjust with shafts and adjusting screws). Try to get both carbs adjusted the same, so that both screws begin to hit the cam at the same time. This is not real critical, but you can use your Uni-Syn to match air-flow on both sides, with the choke partly engaged. After the cable is about halfway out, it should start engaging the lever which pulls down on the needle seats. Adjust the linkages so both carbs are acted on equally. You can do this by adjusting for even running of the engine. Of course, for a warm engine, the richness of this mixture will cause some roughness. Make sure the needle seats return freely when you release the choke.
step 12-
Drink that warm beer (only one, no DWI now…) it will taste great at this point!, go wash up, and go for a ride.
Notes: These procedures assume that your engine/carbs are in reasonable operating condition. If something is malfunctioning/leaking, etc, this should still help, but the results may vary. For instance, if you have leaky carbs, worn needles, engine modifications, etc, you may find things work better if you tune for optimum performance at open throttle rather than idle.
The first time through carb adjustments can be confusing, once you’ve done it, all of the stuff in the manuals makes sense. Go back and read them again- As always, I reccommend Bentleys, which is a repro of the original factory manuals, and then Haynes, and throw out the Chiltons. (orginal factory manuals are to be read in a clean enviroment, repros are for smearing grease all over, except, if that’s all you got, use it!) Haynes has an excellent manual just for SU carbs, it covers operation, theory, rebuild of all models, and has needle charts for hundreds of car/engine/carb setups. They also have a manual for Zenith-Strombergs, which, while similar, are a whole ‘nother beast. -Saftey Fast!
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post #5 of 18 (permalink) Old 11-28-2012, 01:16 AM
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SU carburetors got you down? Does it seem like everyone you ask for help says, "Those carbs are junk." If that's you, read on:

When I had my first Z, the ill-fated '73, I had no luck getting help in setting them up. Although it had the earlier '72 (Round-Top) SUs, I couldn't figure out how to set them. I ended up installing the Holley Four-Barrel, which worked great.
However, I have since learned how to set up the SUs, and I'm very happy with them. There are good write-ups on balancing the carbs found at the Internet Z-Car Club web site, so I won't cover that here. Setting balance is fine and dandy, but the mixture is equally important. Some folks may tell you that you need to spend $50 or so for a Color-Tune. Bah Humbug!!
On IRC one night, Doug Antelman told me how to set the mixture without a Color-Tune. Having given this information out on several instances, I decided to do post it here for the world to see.

Warm the engine up, then shut it off:
  • 1) Turn both carbs' adj. nuts all the way in (lean)
    2) Turn both carbs' adj. nuts out three turns (richen)
Re-start, and:
  • 3) Set balance with Uni-syn
    4) Turn in one carb 1/2 turn, and listen for decrease in RPM
    5) Turn the other carb in 1/2 turn, and listen for the same decrease in RPM
    • a) if the RPM difference isn't the same, you may need to turn one in more than the other to get them the same. Don't do more than about one full turn difference. The differences are pretty slight, and if your carbs are worn out a bit, you may be unable to detect a difference. If you can't, don't worry too much....
    6) Repeat 4 & 5 until the car stalls
    • a) it may not stall, if the carbs are worn out. If it doesn't stall, turn the adjustment nuts all the way in, and continue
    7) Turn both carbs out two full turns
    8) Rebalance, and see how it runs
    • a) depending on the condition of your carbs, you may need to go richer than that. I.E., on my L28/E88/SU, with good carbs, I'm at about 2.5 turns out from stall. My wife's L24/E88/SU, with worn-out throttle shaft bushings, has to be set at about 3.5 turns out from all the way in out to get off-idle performance acceptable. Although, if my car stalls at 1 turn out, our cars are set the same (1.0 + 2.5 = 3.5)
Additionally, elevation and temperature can affect tune. The best way to get them spot-on is to get the car running properly, then adjust the carbs as necessary to get the best performance. Also, the front or rear carb may need to be richened or leaned to match the other from step eight. Once you get it running properly, drive it for a while, then pull the plugs. You want all six plugs to look the same. If three of the plugs are darker (dull black - glossy black is probably oil), lean that carb out 1/2 a turn and recheck. Of course, any modification requires rebalanacing the carbs.... Here's some symptoms of incorrect adjustment. Of course, all these problems can be the result of other problems, so consider this chart when attempting to diagnose problems that appear after tuning the carbs.
Too Lean?

Too Rich?

Pinging Smoking Poor low RPM performance Fouling plugs Running hot Poor mileage
And now a few words on carb fluid. I've found the level and type of fluid in the carbs to be of the utmost importance. What concerns me is not the fact that it's critical, but the fact that it's seldom mentioned. I've corrected some pretty major-apperaring drivability problems, just by adding oil to the carbs. I've heard of people running the carbs dry, and others who used water. I can't vouch for their success, only my own. So here's what I've found:
Type of fluid

Application

Comments

20 wt Factory Original Probably works great on brand new carbs, but there aren't a lot of those around these days! ATF Performance Works very well. Thin enough to be responsive, but thick enough to smooth out off-idle performance. Requires that carbs be in good tune, and that throttle shaft bushings not be too badly worn out. 20/50 Moderate Driving / Worn Carbs Thicker than either of ATF or 20 wt. Overall performance is similar, but responsiveness suffers. Makes the vehicle run better at low RPM. Thick enough to counteract leaking throttle shaft bushings and poor tune.
After passing this on countless times via email, I figured I'd post some info on setting the needle position. Many of these cars have been around the block a few times, so often times, the carbs are way out of whack - too far to straighten out with the above procedure. F'rinstance, on my '66 1600, the rear carb was about five turns out, whereas the front carb was at around two. Yikes! So, when you encounter something like that, or when you just can't get them lean enough, try this:
  1. Warm up the car
  2. Shut it off and remove the carb domes (take out the dipsticks first)
  3. Remove spring from dashpot (the piston-thing that lives inside the dome)
  4. CAREFULLY remove the dashpot - see the needle on the bottom?
  5. Loosen the set-screw that holds the needle so that the needle can move a bit
  6. Turn the adjusting nuts all the way in (located underneath the carb throat, where the fuel line enters from the float bowl)
  7. Pull the needles out about 1/8" or so, you should see the shoulder of the needle sticking out past the dashpot body
  8. CAREFULLY reinstall the dashpot, and push down ever-so-gently to seat the needle fully in the nozzle (that's the hole the needle sits in)
  9. Pull the dashpot back out, snug down the needle set screws, and put them back together.
  10. Set the balance and mixture as above.


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http://www.dbraun99.com/Setting_SU_Carburetors.pdf

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HOW SU CARBS WORK?

This section does not go into how SU carbs actually work but I would very much recommend reading the following which beautifully illustrates how these carbs operate.

http://www.zparts.com/zptech/articles/mal_land/ml_sucarb2/images4/SUcarb_111601d.htm

MYTHS AND LEGENDS

Before we get into the detail, it is worth saying something about a few myths and legends.
· SU Carbs go off over time ...” actually not true unless the carb has worn spindles for example. The main reason that an SU carb appears to go off tune is because the engine condition changes or there has been a change in the fuel used, maintenance is poor or the temptation to fiddle has not been resisted.
· Don’t use dashpot oil / no dampers / no piston springs” The SU carb is a precision instrument and every part of it is present for a specific and important reason. For example, the spring ensures that the piston is pushed downwards against the upward force on the piston of atmospheric pressure. This keeps the piston at a specific height exposing a specific section of the needle. Dashpot oil has a number of functions including slowing the upward movement of the piston (as the vacuum depression by the engine increases with engine speed), temporarily increasing the richness of the mixture. It also dampens out what would otherwise be a constantly oscillating piston. Like nature, each item on an SU carb is there for a reason
· See what others recommend as the right needle for your car” True if the car is unmodified but so very few cars of 40 plus years old will be in the standard factory specification. By all means ask what needles people with a similar set up are using but then work logically from that needle to the needle you need for your setup (more details below).
· if you want more power and torque, go for Webers” Whilst Webers may ‘look the part’ they cost a lot more than twin SUs and are a lot more complicated to set up. I have read that a well set up SU is more than a match for most Weber set ups.

HOW TO TUNE AN SU CARB?

First check what parts your carbs are using. A surprising number of carbs are modified over the years or use non-standard parts, to the blissful ignorance of their current owner.

MGCs had twin HS6 SU carbs which are:

· 1 ¾ inch diameter butterfly
· 0.100inch diameter main jet
· ST needle as standard

Original carb specification for the MGC was:
· UK - 1967/1968 Specification
AUD150F (front)
AUD150R (rear)
Needles – ST (Rich SQ, Weak CIW)
Spring - Yellow
· UK – 1969 Specification
AUD341F (front)
AUD341R (rear)
Needles – ST
Spring - Yellow
· US – 1967/1968 Specification
AUD287F (front)
AUD287R (rear)
Needles – KM
Spring - Yellow
· US – 1969 Specification
AUD342F (front)
AUD342R (rear)
Needles – BAD
Spring - Yellow

A number of methods exist for tuning SU Carbs with various levels of complexity. This section concentrates on the HS Type carb found on MGC but the principles are equally applicable on all SU carbs.

CHECK EVERYTHING ELSE FIRST

I can’t stress this enough. Before even going near your carbs make sure that you have checked at least the following:

· Agood spark reaching the spark plugs (remove one and hold it tight against the engine block with something to insulate you from the spark)
· Spark plug gaps are set correctly
· Ignition timing is set appropriate to the engine and level of modification. A hotter cam may require greater advance than a standard cam. By way of example a Kent AH2 mild road cam at 278 degrees requires a static timing advance of around 14 degrees in my experience. Make sure you are using static timing figures for static timing and not static figures for strobe timing.
· Points are not pitted. If using electronic ignition, this will not be an issue of course.
· Distributor cap is in good condition – that is that the contact surfaces inside the cap are not pitted or black. Also check that the cap itself is clean inside to avoid conduction across the surface.
· Ignition leads – check these are fully home and not cracked or damaged.
· Vacuum line – make sure this is attached to the distributor (if applicable) and that the servo take off is tight and the rubber is not perished (leading to leaks).
· Gaskets – it is not unknown for these to be put on upside down which means that the vacuum transfer holes (which allows atmospheric pressure to push the piston up to its correct height) at the front of the carbs are blocked by the gasket. This will ruin all attempts to properly tune the carbs.
· Valve clearances are correct

CLEAN FILTERS

I always recommend changing the fuel filter (if you have one) and air filter elements before tuning. If you have K&N’s give them a wash in the recommended fluid. If you have time, I would also clean out the fuel reservoirs.

CHECK FOR LEAKS AND WORN SPINDLES

Nothing will mess up all attempts to tune a set of HS carbs like an air leak. Make sure that all of the gaskets are in good condition and all bolts and nuts are tight but not over-tight.

And if there is even a hint of sloppiness in the spindles then replace these and rebush if necessary. Worn spindles will make setting idle impossible. You will set it to 800rpm, blip the throttle and it will then be at 55rpm or 1300rpm. I would also check that the throttle linkages all work and work smoothly.

CHECK FUEL RESERVOIR FLOAT LEVEL

One which is easy to miss. The level of fuel in the fuel reservoir dictates the level of the fuel in the jet (just like the water at two ends of a hose pipe full of water will be at the same level). If one is higher than the other, tuning will become confusing and difficult, as effectively different parts of each needle are exposed.

With the float assembly resting on the needle valve (upturned float chamber lid), the gap between the float lever or the float itself and the rim of the float chamber lid should be between 0.126 and 0.189 inch (3.2mm to 4.8mm) and ideally the same on each carb. With a steel float it is possible to carefully bend the steel float arm to set this. With a plastic float, it is necessary to add or remove washers from under the needle seat.

CHECK PISTON FALL

Remove the three small screws holding the dashpot to the body of the carb. Be careful not to lose the screws as they are almost impossible to find in your engine bay. I also lay them out in the same relative position on the bench.

The dashpot can be removed by hand but take care to hold the piston inside it and lift the whole assembly out vertically, so as not to bend the needle. Note how there is only one way for each dashpot to go (one of the screw holes dictates this) and make sure you know which is the front and which is the rear carb dashpot, as well as which piston goes with what dashpot (by convention the front carb is the carb nearest the radiator). I tend to rest the piston, with the dashpots over them, on the edge of the opening of a long glass coffee jar (long enough so that the needle is not resting on the bottom of the jar).

Clean the piston carefully and clean it right to the top inside. Don’t whatever you do use wire wool or a wire brush and any sandpaper OR ANY OTHER ABRASIVE MATERIAL. SU pistons and dashpots (also known as suction chambers) are matched assemblies with a defined and specific clearance between the piston and the suction chamber. Anything abrasive will ruin this and make tuning impossible. I use a degreaser followed by Brasso to do this. Ensure everything is meticulously clean and dry and then lightly coat the rod in the centre of the piston with WD40.

Don’t stretch the spring inside the piston and make sure you put it back on when reassembling (easy to forget). With both pistons cleaned, reassemble and tighten the small screws gently and evenly. The piston locates in only one orientation inside the carb due to the groove mating with a corresponding section on the piston. The piston should fall freely within the dashpot. If you tighten the dashpot screws too much or unevenly, then the piston may get stuck.

Having removed the air filters, it is possible to see the end of the piston through the front facing air inlet hole. With the engine turned off, gently insert a finger into each and pull the piston upwards to the same height, holding both pistons with separate hands. Release each piston at the same time. As the piston hits the bridge inside the carb at the end of its travel it will make a dull clunk.
If both pistons hit the bridge at the same time, this is perfect. If the pistons hit within a fraction of a second of each other, this is also probably okay. More than a second and I would investigate why they drop at different rates. Below sets out what to do if the pistons are not falling in sync.

CHECK NEEDLES

One I would recommend doing. Holding the piston carefully, look at the needle from all angles to ascertain whether it is bent. On some HS carbs including the HS6, the needle is rear biased which means that the needle is angled towards the rear of the carb, in effect pointing towards the engine.

The picture below shows a spring loaded needle on the right and a fixed needle on the left (the reason for the numbered ‘stations’ is explained below). The difference between these types is that a fixed needle is solidly fixed into the piston via a screw whereas spring loaded (often called biased) needles are held in place within the piston by a collar and a spring and can move by virtue of the spring.

· Fixed needle – needs to be precisely centred within the jet to within a few thousands of an inch. If it is not centred or is bent, the needle will rub / bind against the side wall of the jet, wearing the needle and impairing performance if it sticks. To centre a jet, remove the dash pot and piston and adjust the jet so that, when looking inside the carb, it is flush with the top of the bridge (the bridge is what the piston falls on to at its lowest point). Refit the dash top and piston and let the piston fall slowly on to the bridge. If the piston sticks just before the bridge, the needle is not centred and is fouling on the main jet. Loosen the large nut under the carb, which allows the jet to move, with the piston sitting now on the bridge re-tighten slightly, lift the piston up and allow to drop again. Repeat this until the piston drops easily then tighten the nut fully. Check again to make sure that the piston drops positively on to the bridge, then adjust the jets to a starting point (see below)
· Spring Loaded needle – around the time of the MGC, SU came up with a new mounting style for needles specifically a soft-mounted spring-loaded needle tipped a few degrees to one side which forces contact with the jet wall but only a light contact without binding. The reason for all this was emissions. A binding and worn fixed needle leads to poor emissions. A biased needle will also lead to wear but only over time. In effect, all biased needle set ups wear but this was supposed to be better than a few very bad emissions from a few fixed needle carbs. With what were seen at the time as limited lives and servicing, the biased carbs was thought to be a reasonable step forward on total emissions.

Important – if sourcing replacement parts the jet bearing for fixed and biased needles are different (the neck of the jet for a fixed needle is smaller in diameter to allow for centre-ing). When inserting a needle into the piston, whatever type you are using, the needle should sit in the cylinder with the base of needle flush with the base / face of the cylinder. Reinsert the piston carefully into the carb, making sure that you don’t bend the needle.

BLACK GOLD MAN OF DETROIT 78 280Z 80ZX 10TH ANV 83 ZX 2+2 GL

MOTOR CITY
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post #8 of 18 (permalink) Old 11-28-2012, 01:25 AM
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http://www.zparts.com/zptech/articles/mal_land/ml_sucarb2/images4/SUcarb_111601d.htm

OIL IN DASHPOT

What a lot there is written about this issue!

The oil in the dashpot is needed to restrict the upwards motion of the piston under acceleration so that the mixture becomes enriched. It is also dampens out fluctuations in the movement of the piston that would otherwise occur at low engine revs (the piston would move around constantly ‘fretting’ as the air fluctuated).

Whatever story you believe on what is best, the presence of the oil and the damper does restrict the upwards movement of the piston which in turn does mean the mixture is temporarily richer (higher velocity air movement over the jet). It also means that the mixture is not too lean as the engine vacuum increases as the throttle is opened.

I would not recommend water but the options include:
· 20W50 (in the manual)
· A mixture of 20W50 and sewing machine oil (3 engine oil to 1 sewing oil)
· Hydraulic lift oil
· Automatic Transmission Fluid (again not sure why people recommend this)

SPRINGS INSIDE THE DASHPOT

Again from what you read on the net, you would think that these springs did nothing and had no effect. They do.

· Springs hold the dashpot “down” against the vacuum. The strength of the springs will determine where the pistons sits at a given level of vacuum from the engine.
· Even more important than the strength of the spring is whether it is equal to the spring in the other carb. If the springs are different (or old such that they have different strengths now), the piston will be “held” at different positions in each carb meaning that each carb will be showing a different part of the needle profile at a given engine vaccum.
· The paint colour on the end of the spring should tell you which spring you have but may well have worn off over time.
· If you want to check a spring performance or compare two springs, then using a light-weight set of scales and a piece of cardboard folded along its length and inserted into the middle of the spring, compress the spring to that length and then read the associated spring weight on the scales. Standard springs were yellow. If damaged, replace (and I would recommend doing so as a pair for the above reason).
Yellow 8oz (load) 2.750 (length inches) (Part AUC 1167)
Light Blue 2.5oz 2.625 (Part AUC 4587)
Red 4.5oz 2.635 (Part AUC 4387)
· If you want to look at using different types of spring for modified engines, I would recommend buying “The SU Carburettor High-Performance Manual” by Des Hammill. This is a great book with a wealth of detail you can’t easily find elsewhere.

ADJUST JETS TO A STARTING POINT

There are a couple of ways of doing this.

The first is to adjust the jets so that they are just flush with the top of the jet housing (with piston removed) and then turn the adjusting nut 12 ‘flats’ clockwise. It is important to do exactly the same on the other carb, so as to keep the mixture adjustment in sync.

The second is to use a vernier measurement gauge to adjust the jet so that the main jet on both carbs are 0.050 inch / 1.25mm down from the top of the bridge within the carburettor housing. The engine will start although these settings are very unlikely to be correct. The advantage of this method is that you know the jets are in sync. It is also close enough to be able to start the engine.

Now run the engine until it reaches normal operating temperatures.

UNLOCK THROTTLE SPINDLES

Simply this means that each carb is able to act independently of the other. It is important to adjust each carb’s airflow whilst they are not linked in any way.

Now some people will tell you that you only need to undo one of the nuts / clamping bolts at one end and that there is no need to undo both ends. This is true if you are just trying to balance the air flow at idle. But once the accelerator is pressed and the throttle linkages act so that the carb comes off idle, it is important that each carb picks up equally. That means undoing both side’s nuts and setting them precisely and in sync.

BALANCE AIR FLOW

With the engine up to operating temperature, now set the air flow so that the flow is equal through both carbs at idle. There are a number of ways of doing this, all relying on turning the idle screw clockwise to adjust the airflow, achieve a balanced airflow at each carb and a sensible tickover speed. First back off the throttle stop adjusting screws so that they are both just touching their respective stops and then turn each of them exactly 1 and and a ½ full turns. Then:
· By ear using a piece of pipe placed just on the lip of the carburettor edge next to the visible end of the piston , move back and forth on each carb, compare the ‘hissing’ sound and adjust until the sound is the same.
· Use a cheap tool and get so frustrated with it that you buy a more expensive one.
· Use a tool like Unisys or Motometer to accurately balance the air flow into each carb (the middle section rotates to allow the air flow into the glass tube to be adjusted). Swapping back and forth between carbs is very quick and easy.

To adjust the flow, turn the idle screw clockwise in small increments and be sure to open / blip the throttle to ensure that it finds the new level.

Once the carbs are in balance, set the gap between the pegs on the ends of the linkage between both carbs until there is an equal amount of free play on each side – typically around 1/16th of an inch (0.15mm). I do this using a feeler gauge inserted in between the peg and the linkage and then tighten all bolts and nuts. Then check that when you pull the accelerator cable each side picks up at exactly the same time. My experience is that this takes a few times to get right.

It is also worth checking that the pegs are far enough in. This sounds odd but it is possible to have the pegs too far towards the middle of the space between the carbs meaning that when you brake or accelerate, one peg no longer operates the mechanism.

PISTON MOVEMENT IN SYNC

Balancing the above airflow only checks that the air flow is in sync at idle.

It is also worth checking to see if the pistons rise at an equal rate and do not stick or otherwise travel unequally as the engine revs rise. It is possible to use a homemade tool to do this but I would recommend the cheap but effective SU Toolkit which consists of two rods that are inserted where the (removed) dampers used to be with two perpendicular lengths of thin wire which act as pointers. As the engine revs rise, the pointers should rise equally; as the revs fall they should fall equally.

If it doesn’t rise and fall freely, this may be either that the jet is not centred correctly, the dashpot or piston needs cleaning or the suction chamber screws are not equally tight, assuming of course that the pick up by the accelerator mechanism is in sync as noted above. Just possibly you or someone might have swapped the pistons over (they are made to match the dashpot) in which case it might be worth trying them the other way around. Also check that your needles are not bent. (Note that the shaft of the SU Tool can also be used to check the gap in the fuel chambers between the hinged level and the underside face of the fuel chamber lid.)

ADJUST MIXTURE AT TICKOVER

Looking down from the top of each carb, turn the adjusting nut on the underside of the HS6 clockwise to richen the mixture (jet moves down exposing a thinner part of the needle) or anticlockwise to weaken the mixture.

Again there are a number of ways of checking the adjustment of the mixture at tickover. The principle used here and for all of these and the standard tuning methods is the implicit assumption that if the mixture is adjusted correctly at tickover, then it will automatically mean that the mixture is correct across the whole rev range. This is true if the engine is standard and unmodified and using the standard recommended needles.

So by way of example, using ST needles in an unmodified C engine with original air filters and adjusting the mixture to be correct at idle should mean that the mixture is correct right the way to the rev limit. Obviously where you have made mods such as, in my case, Kent AH2 cam, K&Ns, unleaded head, fast flow exhaust, modified inlet manifold, 123 ignition, the original standard needles will not be the best needles for my set up, even if they can be made to idle nicely.

· The ‘By ear’ method. Sounds imprecise but in the hands of an experienced tuner, often very close to perfect.
· ‘The method in the manuals’ – where the mixture is adjusted until the engine RPM just begins to drop. If you did the 12 flats clockwise originally, then adjust by turning anticlockwise (weaken) until the engine speed just starts to drop. Then turn clockwise until engine speed starts to rise, followed by one more “flat” (on the nut) clockwise. Then, using the lift pin on the underside of the dashpot mounting flange, press up until you feel contact with the piston and then lift slightly.
· If the engine RPM increases when the pin is lifted, and remains elevated, the mixture is rich – weaken the mixture by turning the adjusting nut anticlockwise.
· If the engine RPM drops when the pin is lifted your mixture is too lean - richen the mixture by turning the adjusting nut clockwise.
· If the engine RPM initially raises as the pin is lifted and then settles back to the original RPM or very slightly above, the mixture is right.
· ‘Tailpipe Emissions’ – it is possible to use emissions testing equipment to adjust the mixture. If you have one exhaust pipe at the rear then obviously by the time the exhaust gases reach this point they will have mixed completely. So you could have a front carb very rich and a rear carb very lean and it appear to be correct. The way to avoid this is to keep them in sync. Don’t turn the front carb without turning the rear carb the same amount (if you have twin tail pipes work out which one relates to which carb). The ideal air fuel ratio is between 12.8(4.3% CO):1 and 12.1 (6.3%CO):1 but at tickover it is recommended to go leaner than this i.e. the leaner the better for the engine whilst this allows for a smooth even idle. With only a visual test for emissions, the level of CO emissions is not important for the MOT but it is for your bank balance (wasting unburnt fuel) and the environment:
· 2.5% CO – Lower end of ‘ideal’at tickover
· 3.5% CO – Upper end of ‘ideal’ at tickover
· ‘O2 Sensor’ – See below for more details

ADJUST TICKOVER

With the mixture adjusted correctly, the tickover may need to be adjusted. Again, it is important to undo all of the bolts and to turn each idle screw exactly the same amount to keep the carbs in sync.

TEST DRIVE & REPEAT LAST TWO

Take the car for a test drive and if necessary adjust the mixture. Cs have a tendency to like to run slightly rich.

ADJUST CHOKE

Pull the choke until the jets are just at the point of moving but have not moved. Screw both fast idle screws in until they just touch the fast idle cam. Alternately turn each fast idle screw until the idle speed is increased to 1,000 RPM with the engine warm.

REATTACH AIR FILTERS

CHECK FOR FUEL LEAKS

TUNING USING A GAS ANALYSER – O2 SENSOR

The huge difference here is that you are no longer simply tuning at tickover. By mounting an onboard Air / Fuel monitor (such as http://www.ngk.com/afx/ - I would of course recommend only doing this in such a way as to be able to remove all wiring once tuned i.e. temporary mounted), it is possible to see the air fuel (AF) ratio across the entire engine rev range. These work by using an O2 sensor inserted into a welded in boss in your exhaust pipe connected to a monitor. ‘Wideband’ monitors are the only way to go. It is also possible to wire two sensors and switch between them (or just use one and swap it over).
Maximum power for a petrol engine is in the range of 12.1:1 to 12.8:1 Air to Fuel (AF) ratio, depending of course on your engine, its mileage, the cam, the exhaust etc. Typically a C likes to run slightly rich.

By measuring the AF ratio whilst driving the car at various revs and engine loads, it is possible to build up a picture of whether the engine just needs adjusting but has the right needle or whether a needle of a different profile is required.

· For example, a particular needle may give 2.5% CO at 800rpm, an AF of 12.8 at 2500rpm but an AF of 11.5 at 4000rpm indicating that a needle with a thinner profile at that section of the needle is required (i.e. the section that “equates” to 4000rpm).

A thinner needle (smaller diameter) will allow more fuel through (richer); a thicker needle (larger diameter) restricts fuel from getting through (leaner).

All SU needles have one end which is thicker than the other i.e. they taper. And that taper is specific to the particular needle. There are for example 354 needles, all using a three letter code, just for the .090 jet and all of these needles have different tapers.

Each needle is divided into what are called ‘stations’. Each station appears at 1/8th inch further down the needle with station number 1 being the thickest and 12 (or higher, up to 16) being the thinnest part of the needle. Station 1 & 2 equate to the needle thickness at “Idle”. See above needle diagram.

Obviously as the main jet is wound downwards below the level of the bridge, the station on the needle that is ‘seen’ by the carburettor also moves down i.e. becomes perhaps Station 2 and also as a consequence a Station further down the needle comes into play and is also ‘seen’ by the carburettor. The important point is to establish which stations on the needle are actually ‘seen’ by the carb. It may be Station 1 to 12 or 2 to 13 or 2 to 14 etc.

Recall that the original SU needle for the MGC was ST as standard, rich SQ, weak CIW in UK:

SQ ST CIW BDL
Station 1 .100 .099 .099 .099
Station 2 .095 .095 .0955 .0955
Station 3 .0915 .0925 .093 .0924
Station 4 .088 .0895 .0905 .0893
Station 5 .0854 .087 .0875 .0865
Station 6 .083 .0852 .0856 .0834
Station 7 .0812 .0831 .0836 .079
Station 8 .0794 .0805 .0819 .0774
Station 9 .0775 .0787 .080 .0743
Station 10 .0757 .077 .078 .071
Station 11 .0738 .0753 .076 .068
Station 12 .0719 .0737 .0752 .065
Station 13 .070 .071 .0746 .062
Station 14 .068 .069 .074 .059
Station 15 - - - .056
Station 16 - - - .053

CHOOSING A DIFFERENT NEEDLE

My C currently has BDL needles in it but with a slightly modified profile. That is of course specific to my engine and its characteristics. If you do intend to change your needle there are a few points to note:

· Each needle you use in the 0.100 inch jet must have 0.099 as the first station number. This limits the vast number of needles you need to consider.
· When choosing a needle bear in mind that, even though there are hundreds of needle profiles available off the shelf, all of these are for a specific application. Modified engines either by cam or manifold or in any way non-standard are unlikely to have an exact needle for your application. Again I would recommend buying “The SU Carburettor High-Performance Manual” by Des Hammill if you want to understand how to modify a needle. His books is excellent on this aspect with lots of practical tips and photos.
· Be logically about it. If you engine is too lean in the midrange, try a needle with a thinner profile at that point. And if it is still too lean, use this knowledge to influence your next needle choice. Put simply, don’t just randomly try needles but build on knowledge gained by the each needle you try.

All of the SU needles details are set out in the SU needle chart, allowing you to potentially find the needle you need. It is worth trying http://www.teglerizer.com/suneedledb/index.html to play with station numbers or you can try http://sucarbs.com/programs.html which is just great.

BLACK GOLD MAN OF DETROIT 78 280Z 80ZX 10TH ANV 83 ZX 2+2 GL

MOTOR CITY
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post #9 of 18 (permalink) Old 11-28-2012, 01:26 AM
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Join Date: Jul 2009
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http://www.zparts.com/zptech/articles/mal_land/ml_sucarb2/images4/SUcarb_111601d.htm

OIL IN DASHPOT

What a lot there is written about this issue!

The oil in the dashpot is needed to restrict the upwards motion of the piston under acceleration so that the mixture becomes enriched. It is also dampens out fluctuations in the movement of the piston that would otherwise occur at low engine revs (the piston would move around constantly ‘fretting’ as the air fluctuated).

Whatever story you believe on what is best, the presence of the oil and the damper does restrict the upwards movement of the piston which in turn does mean the mixture is temporarily richer (higher velocity air movement over the jet). It also means that the mixture is not too lean as the engine vacuum increases as the throttle is opened.

I would not recommend water but the options include:
· 20W50 (in the manual)
· A mixture of 20W50 and sewing machine oil (3 engine oil to 1 sewing oil)
· Hydraulic lift oil
· Automatic Transmission Fluid (again not sure why people recommend this)

SPRINGS INSIDE THE DASHPOT

Again from what you read on the net, you would think that these springs did nothing and had no effect. They do.

· Springs hold the dashpot “down” against the vacuum. The strength of the springs will determine where the pistons sits at a given level of vacuum from the engine.
· Even more important than the strength of the spring is whether it is equal to the spring in the other carb. If the springs are different (or old such that they have different strengths now), the piston will be “held” at different positions in each carb meaning that each carb will be showing a different part of the needle profile at a given engine vaccum.
· The paint colour on the end of the spring should tell you which spring you have but may well have worn off over time.
· If you want to check a spring performance or compare two springs, then using a light-weight set of scales and a piece of cardboard folded along its length and inserted into the middle of the spring, compress the spring to that length and then read the associated spring weight on the scales. Standard springs were yellow. If damaged, replace (and I would recommend doing so as a pair for the above reason).
Yellow 8oz (load) 2.750 (length inches) (Part AUC 1167)
Light Blue 2.5oz 2.625 (Part AUC 4587)
Red 4.5oz 2.635 (Part AUC 4387)
· If you want to look at using different types of spring for modified engines, I would recommend buying “The SU Carburettor High-Performance Manual” by Des Hammill. This is a great book with a wealth of detail you can’t easily find elsewhere.

ADJUST JETS TO A STARTING POINT

There are a couple of ways of doing this.

The first is to adjust the jets so that they are just flush with the top of the jet housing (with piston removed) and then turn the adjusting nut 12 ‘flats’ clockwise. It is important to do exactly the same on the other carb, so as to keep the mixture adjustment in sync.

The second is to use a vernier measurement gauge to adjust the jet so that the main jet on both carbs are 0.050 inch / 1.25mm down from the top of the bridge within the carburettor housing. The engine will start although these settings are very unlikely to be correct. The advantage of this method is that you know the jets are in sync. It is also close enough to be able to start the engine.

Now run the engine until it reaches normal operating temperatures.

UNLOCK THROTTLE SPINDLES

Simply this means that each carb is able to act independently of the other. It is important to adjust each carb’s airflow whilst they are not linked in any way.

Now some people will tell you that you only need to undo one of the nuts / clamping bolts at one end and that there is no need to undo both ends. This is true if you are just trying to balance the air flow at idle. But once the accelerator is pressed and the throttle linkages act so that the carb comes off idle, it is important that each carb picks up equally. That means undoing both side’s nuts and setting them precisely and in sync.

BALANCE AIR FLOW

With the engine up to operating temperature, now set the air flow so that the flow is equal through both carbs at idle. There are a number of ways of doing this, all relying on turning the idle screw clockwise to adjust the airflow, achieve a balanced airflow at each carb and a sensible tickover speed. First back off the throttle stop adjusting screws so that they are both just touching their respective stops and then turn each of them exactly 1 and and a ½ full turns. Then:
· By ear using a piece of pipe placed just on the lip of the carburettor edge next to the visible end of the piston , move back and forth on each carb, compare the ‘hissing’ sound and adjust until the sound is the same.
· Use a cheap tool and get so frustrated with it that you buy a more expensive one.
· Use a tool like Unisys or Motometer to accurately balance the air flow into each carb (the middle section rotates to allow the air flow into the glass tube to be adjusted). Swapping back and forth between carbs is very quick and easy.

To adjust the flow, turn the idle screw clockwise in small increments and be sure to open / blip the throttle to ensure that it finds the new level.

Once the carbs are in balance, set the gap between the pegs on the ends of the linkage between both carbs until there is an equal amount of free play on each side – typically around 1/16th of an inch (0.15mm). I do this using a feeler gauge inserted in between the peg and the linkage and then tighten all bolts and nuts. Then check that when you pull the accelerator cable each side picks up at exactly the same time. My experience is that this takes a few times to get right.

It is also worth checking that the pegs are far enough in. This sounds odd but it is possible to have the pegs too far towards the middle of the space between the carbs meaning that when you brake or accelerate, one peg no longer operates the mechanism.

PISTON MOVEMENT IN SYNC

Balancing the above airflow only checks that the air flow is in sync at idle.

It is also worth checking to see if the pistons rise at an equal rate and do not stick or otherwise travel unequally as the engine revs rise. It is possible to use a homemade tool to do this but I would recommend the cheap but effective SU Toolkit which consists of two rods that are inserted where the (removed) dampers used to be with two perpendicular lengths of thin wire which act as pointers. As the engine revs rise, the pointers should rise equally; as the revs fall they should fall equally.

If it doesn’t rise and fall freely, this may be either that the jet is not centred correctly, the dashpot or piston needs cleaning or the suction chamber screws are not equally tight, assuming of course that the pick up by the accelerator mechanism is in sync as noted above. Just possibly you or someone might have swapped the pistons over (they are made to match the dashpot) in which case it might be worth trying them the other way around. Also check that your needles are not bent. (Note that the shaft of the SU Tool can also be used to check the gap in the fuel chambers between the hinged level and the underside face of the fuel chamber lid.)

ADJUST MIXTURE AT TICKOVER

Looking down from the top of each carb, turn the adjusting nut on the underside of the HS6 clockwise to richen the mixture (jet moves down exposing a thinner part of the needle) or anticlockwise to weaken the mixture.

Again there are a number of ways of checking the adjustment of the mixture at tickover. The principle used here and for all of these and the standard tuning methods is the implicit assumption that if the mixture is adjusted correctly at tickover, then it will automatically mean that the mixture is correct across the whole rev range. This is true if the engine is standard and unmodified and using the standard recommended needles.

So by way of example, using ST needles in an unmodified C engine with original air filters and adjusting the mixture to be correct at idle should mean that the mixture is correct right the way to the rev limit. Obviously where you have made mods such as, in my case, Kent AH2 cam, K&Ns, unleaded head, fast flow exhaust, modified inlet manifold, 123 ignition, the original standard needles will not be the best needles for my set up, even if they can be made to idle nicely.

· The ‘By ear’ method. Sounds imprecise but in the hands of an experienced tuner, often very close to perfect.
· ‘The method in the manuals’ – where the mixture is adjusted until the engine RPM just begins to drop. If you did the 12 flats clockwise originally, then adjust by turning anticlockwise (weaken) until the engine speed just starts to drop. Then turn clockwise until engine speed starts to rise, followed by one more “flat” (on the nut) clockwise. Then, using the lift pin on the underside of the dashpot mounting flange, press up until you feel contact with the piston and then lift slightly.
· If the engine RPM increases when the pin is lifted, and remains elevated, the mixture is rich – weaken the mixture by turning the adjusting nut anticlockwise.
· If the engine RPM drops when the pin is lifted your mixture is too lean - richen the mixture by turning the adjusting nut clockwise.
· If the engine RPM initially raises as the pin is lifted and then settles back to the original RPM or very slightly above, the mixture is right.
· ‘Tailpipe Emissions’ – it is possible to use emissions testing equipment to adjust the mixture. If you have one exhaust pipe at the rear then obviously by the time the exhaust gases reach this point they will have mixed completely. So you could have a front carb very rich and a rear carb very lean and it appear to be correct. The way to avoid this is to keep them in sync. Don’t turn the front carb without turning the rear carb the same amount (if you have twin tail pipes work out which one relates to which carb). The ideal air fuel ratio is between 12.8(4.3% CO):1 and 12.1 (6.3%CO):1 but at tickover it is recommended to go leaner than this i.e. the leaner the better for the engine whilst this allows for a smooth even idle. With only a visual test for emissions, the level of CO emissions is not important for the MOT but it is for your bank balance (wasting unburnt fuel) and the environment:
· 2.5% CO – Lower end of ‘ideal’at tickover
· 3.5% CO – Upper end of ‘ideal’ at tickover
· ‘O2 Sensor’ – See below for more details

ADJUST TICKOVER

With the mixture adjusted correctly, the tickover may need to be adjusted. Again, it is important to undo all of the bolts and to turn each idle screw exactly the same amount to keep the carbs in sync.

TEST DRIVE & REPEAT LAST TWO

Take the car for a test drive and if necessary adjust the mixture. Cs have a tendency to like to run slightly rich.

ADJUST CHOKE

Pull the choke until the jets are just at the point of moving but have not moved. Screw both fast idle screws in until they just touch the fast idle cam. Alternately turn each fast idle screw until the idle speed is increased to 1,000 RPM with the engine warm.

REATTACH AIR FILTERS

CHECK FOR FUEL LEAKS

TUNING USING A GAS ANALYSER – O2 SENSOR

The huge difference here is that you are no longer simply tuning at tickover. By mounting an onboard Air / Fuel monitor (such as http://www.ngk.com/afx/ - I would of course recommend only doing this in such a way as to be able to remove all wiring once tuned i.e. temporary mounted), it is possible to see the air fuel (AF) ratio across the entire engine rev range. These work by using an O2 sensor inserted into a welded in boss in your exhaust pipe connected to a monitor. ‘Wideband’ monitors are the only way to go. It is also possible to wire two sensors and switch between them (or just use one and swap it over).
Maximum power for a petrol engine is in the range of 12.1:1 to 12.8:1 Air to Fuel (AF) ratio, depending of course on your engine, its mileage, the cam, the exhaust etc. Typically a C likes to run slightly rich.

By measuring the AF ratio whilst driving the car at various revs and engine loads, it is possible to build up a picture of whether the engine just needs adjusting but has the right needle or whether a needle of a different profile is required.

· For example, a particular needle may give 2.5% CO at 800rpm, an AF of 12.8 at 2500rpm but an AF of 11.5 at 4000rpm indicating that a needle with a thinner profile at that section of the needle is required (i.e. the section that “equates” to 4000rpm).

A thinner needle (smaller diameter) will allow more fuel through (richer); a thicker needle (larger diameter) restricts fuel from getting through (leaner).

All SU needles have one end which is thicker than the other i.e. they taper. And that taper is specific to the particular needle. There are for example 354 needles, all using a three letter code, just for the .090 jet and all of these needles have different tapers.

Each needle is divided into what are called ‘stations’. Each station appears at 1/8th inch further down the needle with station number 1 being the thickest and 12 (or higher, up to 16) being the thinnest part of the needle. Station 1 & 2 equate to the needle thickness at “Idle”. See above needle diagram.

Obviously as the main jet is wound downwards below the level of the bridge, the station on the needle that is ‘seen’ by the carburettor also moves down i.e. becomes perhaps Station 2 and also as a consequence a Station further down the needle comes into play and is also ‘seen’ by the carburettor. The important point is to establish which stations on the needle are actually ‘seen’ by the carb. It may be Station 1 to 12 or 2 to 13 or 2 to 14 etc.

Recall that the original SU needle for the MGC was ST as standard, rich SQ, weak CIW in UK:

SQ ST CIW BDL
Station 1 .100 .099 .099 .099
Station 2 .095 .095 .0955 .0955
Station 3 .0915 .0925 .093 .0924
Station 4 .088 .0895 .0905 .0893
Station 5 .0854 .087 .0875 .0865
Station 6 .083 .0852 .0856 .0834
Station 7 .0812 .0831 .0836 .079
Station 8 .0794 .0805 .0819 .0774
Station 9 .0775 .0787 .080 .0743
Station 10 .0757 .077 .078 .071
Station 11 .0738 .0753 .076 .068
Station 12 .0719 .0737 .0752 .065
Station 13 .070 .071 .0746 .062
Station 14 .068 .069 .074 .059
Station 15 - - - .056
Station 16 - - - .053

CHOOSING A DIFFERENT NEEDLE

My C currently has BDL needles in it but with a slightly modified profile. That is of course specific to my engine and its characteristics. If you do intend to change your needle there are a few points to note:

· Each needle you use in the 0.100 inch jet must have 0.099 as the first station number. This limits the vast number of needles you need to consider.
· When choosing a needle bear in mind that, even though there are hundreds of needle profiles available off the shelf, all of these are for a specific application. Modified engines either by cam or manifold or in any way non-standard are unlikely to have an exact needle for your application. Again I would recommend buying “The SU Carburettor High-Performance Manual” by Des Hammill if you want to understand how to modify a needle. His books is excellent on this aspect with lots of practical tips and photos.
· Be logically about it. If you engine is too lean in the midrange, try a needle with a thinner profile at that point. And if it is still too lean, use this knowledge to influence your next needle choice. Put simply, don’t just randomly try needles but build on knowledge gained by the each needle you try.

All of the SU needles details are set out in the SU needle chart, allowing you to potentially find the needle you need. It is worth trying http://www.teglerizer.com/suneedledb/index.html to play with station numbers or you can try http://sucarbs.com/programs.html which is just great.

BLACK GOLD MAN OF DETROIT 78 280Z 80ZX 10TH ANV 83 ZX 2+2 GL

MOTOR CITY
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post #10 of 18 (permalink) Old 11-28-2012, 01:47 PM Thread Starter
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YOU ARE A GOD OF INFINITE KINDNESS AND KNOWLEDGE.
Seriously, I can't thank you enough for taking the time to compile that.
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