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struggles with SU mixture... (long)

579 Views 13 Replies 8 Participants Last post by  roboto
Okay, please forgive my ignorance (that's why I'm here, right?) ...

In 6 months I still haven't been able to get the mixture right on my car (round top SUs, e88, L28). I've got it running okay, but it's still pretty down on power. THe funny thing is that as long as I've had it, no matter what I do the plugs get fouled (dry and sooty black stuff) unless I lean it out to the point where there's absolutely no power. I've messed with the floats and stuff... and that's another issue because I can't see how you can have any sort of precision on the measurement. I've read volumes on this subject, but I can't get it right.
I guess I'll start w/ the floats:
- so I did the measurement from the lid to the top of the float, but since the float isn't necessarily flat (i.e. it could be slanted depending on when it hits the valve), where do you measure from? I think the better way to do it is to actually see how much gas is in the chamber...
- i haven't done the thing with attaching a tube to the float chamber drain because I don't really get it... I just stick a tube on there and hold it up and see where the level of gas is in the tube? But then doesn't it matter how far up you hold it? i.e. how far down you let it sag? I'm a bit rusty on my fluids... =P
- I have tried taking off the domes and pistons and looking inside to see the gas, and I can mess with the floats till I get the gas right at the top of the jets, (is that where they're supposed to be? like you should be able to see the gas w/o a flashlight and stuff right?)... but it seems like this adjustment gets lost after some time or something. like... the gas doesn't come to a consistent level (maybe I'm wrong about this), so any adjustment you do is meaningless...

So assuming I can get my floats at the right place, I should then be able to go through the carb tune... the thing about that is that when I lean it out to what seems reasonable, when the weather gets cold (bay area, so "cold" is 50F degrees or so, compared to 60-70) there's absolutely no power below 3k rpm and it runs really really rough... so i can just open up the screws and it's a bit better, but then you can definitely tell it's too rich (more backfire, smells like gas). I know SUs like to run rich, but this seems like a bit much. I think the power drop off in the higher revs is also a lot more severe than it should be. ... but yeah it seems like this engine is just all over the map, and i can't quite figure it out... i'm almost ready to take it to a mech, but then it's not like i really need the car to be running or anything, it's just for me to mess around with. =P lucky me.

btw, COTM this month looks awesome

Sorry about the length, thanks for your help.
I've done quite a few searches and things, I just find that the directions and suggestions aren't quite idiot (me) proof. So if you're going to describe a procedure, just pretend I'm a kid who doesn't know anything at all, my intelligence won't be insulted. Thanks.
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just buy the video from ztherapy. i think it saved me a lot of time with tuning and such.
Haha, I don't have a VCR! =P
But I guess I'll go ahead and get it and try to watch it somewhere.
our Z Therapy SU's run rich also. there is nothing i can do short of changing the needles im told and we have checked and tried everything.

right now i have the mixture screws turned out on 3/4 a turn on both carbs. If i go in anymore the car wont run good at all. the car is fast runs excellant, but the plugs always show rich and black smoke out of exhaust when running it hard. we have also tried everything from atf, 5 and 10 weight damper oil also.

They would probably be spot on for a 280 engine, but just too rich for the stock cammed 240.
Is it possible the wires on your coil are backwards??? Sounds like a weak spark.

Yeah, I actually just recently started suspecting the ignition as well. I think it's a combination of multiple problems though. I suppose I ought to fix the ignition first, but that's just a whole 'nother story. I need to decide if I want to go ahead and do the ZX dizzy swap or not or just keep fooling w/ the points, etc.
This may or may not help at all, but FWIW as far as I can remember when I had SU's. These are gross adjustment ideas to get you in the ballpark of where you want to be. If your carb is well worn, fine adjustment is probably impossible.

With regard to determining the level of fluid/fuel in the bowl - connect a tube to the bottom of the carb bowl and hold the other end loose. The level of fluid in the tube will be exactly level with the fluid/fuel level in the bowl. If you cannot see thru the tube (to see the fluid/fuel level), then slowly lower it. When fluid/fuel starts to dribble out the top, you have found the level in the carb bowl.

My recollection in my SU's was that the hole in which the fuel needle seats got worn over time. This wear kept the needle from closing off fuel as it is supposed to, making the carb run rich. In other words, at any RPM, the open area around the needle was larger than it was supposed to be, allowing more fuel than necessary to flow up the needle and into the carb throat/venturi. This was true even at idle. The fix was to replace the needle seat.

The other gross problem my SU's had was that the needle valve and seat connected to the bowl floats were either worn or had trash preventing the needle from seating properly. A good (but careful) cleaning fixed this.

Another thing that happened was that the floats did not float properly (they were not as buoyant as they were supposed to be). Cleaning fixed mine. Also check for the pistons moving freely up and down in the throat of the carb.

Also look into each carb throat to make sure the butterflies are adjusted close to the same (they should both close to idle at the same time, of course). If one is jacked way out of proportion, half your engine will run super rich with the other super lean. You can also place your hand (briefly) over each carb inlet. The suction on the palm of your hand should feel the same on each carb.

I can't think of anything else right now. Good luck.

Post Edited (Nov 8, 1:35pm)
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I bet your timing is off, same thing happened to me on 2 different cars. black plugs and little power, it's the timing, and I'm not confident about most things mechanical.

good luck!

I highly recommend the electronic ignition over points and condensor any day.

I don't have one yet, but I heard a great tool for setting the mix on SUs is the Colourtune. I think Victoria British sells it. It is basically a spark plug that lets you see the color of the flame in the combustion chamber. Adjust the mix until you have the right color flame in the front 3 and rear 3 cylinders-and your mix should be set perfectly. However, it is important that you fix any problems with ignition first--I also suspect that timing may be an issue for you. Also, you should completely disengage or disattach your choke just to make sure that it is not a factor. My choke got stuck open once and fouled all my plugs.
I also opted for the 79 280ZX dizzy conversion--works great and no more points!
I have the wiring diagram for the swap if you need it-pretty simple stuff!
the colortune can work well, but if you adjust it at idle and its fine, sometimes at speed it is off. alot depends on other factors of the carbs and not just the idle mixture screws and that is what i think the colortune is adjusting for is basically idle mixture at idle rpms.

i havnt used one yet so im not sure if it covers the whole rpm range using it.
Perform a complete tune-up FIRST, new plugs, cap rotor, fuel filter, valve adj, timming. Get rid of the points, and make sure you have a good coil.
If you have never had your SU's rebuilt, the throttle shafts leak, after 30 years there is no doubting that, this makes carb adjustments impossible, I have been through this too many times. You must make sure the engine
is in perfect order before you can diagnose a carb problem.
Great, thanks a lot guys...
Yeah, I guess I'll try to chase down my ignition problem first.
I was just hesitant to do the ZX dizzy swap because of the immediate cost. (even tho it'll probably save me money/headache in the long run)
Briann510 is right. You must first set your mix with the colortune, then synch and adjust your carbs so that everything flows correctly at high speed. Do you have a UniSyn?
Save yourself time and money--go to ZTherapy.com--they have instructional videos for complete SU overhaul and tuning, and general Zcar tuneup.
Also, fri71240 is correct. You must have everything else working perfectly--ignition, etc. before you can mess with the carbs.
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