1973 240Z original L24 (low miles), motor stock except ZX ignition and ZTherapy carbs. Exhaust is 6-2 header with (2-1/4) to a Magnaflow. Ignition timing is set at about 17 degrees initial, getting to about 35 total with mech. and vacuum advance. Re-built the dizzy, including new breaker plate and advance unit, it definitely works. MSD Blaster 2 coil, NGK's and 8MM NGK wires.

I've got the car running about 90% right now. But I still feel that there's a hesitation at around 4,000 - 4,200 at times that wasn't there before I put in the ZX dizzy last Fall. It starts and idles way better than before. The hesitation mostly happens in the higher gears for some reason. I've been thinking it was either running lean at high rpms, or the timing advance was a problem. Actually, I still think its one of those things.

Today I was out tweaking the A/F mixture and timing after work, and I noticed that when the hesitation happens, I get a dead flat note out of the exhaust system. I should turn the stereo down more often. Not sure if this is any sort of hint as to what the problem is, but it seems to only happen when the hesitation happens. In 1st and 2nd when the car rarely if ever hesitates, the exhaust note sounds the same right up to 5,000 or so when usually I shift.

So is the "flat" exhaust note any hint as to whats keeping my Z from running at 100%....?



Edited 1 time(s). Last edit was 2009:07:02:13:47:49.

It's hard to say if the noise means anything at all, but it could be an indication of running lean.

You got the fever! It always "almost" runs like it should.. It always "almost" has enough power..

as long as you dont get a sharp note you should be good ;)

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LAS VEGAS NEVADA

1976 280ZT, MegaSquirt, Type 3 kit, Miata Seats, FMIC, MBC, 81 turbo rims, corvette tail light mod INW.
"Tony D is like a high school girl, innocent in front of authorities, but naughty with sheep"

HEI Ignitor swap info [www.zcar.com]
my own little local forum [zclv.forumotion.net]

In my 71 Z I run an L-28/E-31 with su's, I also had the problem of the engine falling flat. Not so much at a particular rpm but if I laid on the throttle for more than about 20 seconds witch would be about 4500 rpm give or take. I fixed the problem by accident when I replaced the stock fuel filter with one of those clear flow filters.I always thought they were kind of cheasy but they definatly flow beter than the stock one.Now the motor revs like crazy and will easly rev to 6k even in fourth gear.Only 10 bucks at the Zone, it's worth a try.

Sounds like you may be sucking the float bowls dry under some conditions.
4500rpms in 3rd and 4th is a far different loading parameter than in 1st and 2nd.

Fuel filters in the carb inlets are small brass screens, they can cause a restriction to flow into the carburettor.

Same goes for any filter on the suction side of the fuel pump.

Or a weak fuel pump, but that's overrated... usually it's filtration.

If you slightly lift your throttle just before it happens, does the car still accelerate but the note goes away...just acceperating slower? That changes the location of the piston and rate of fuel gulpage...and can indicate the problem is fuel related.

Also turning the mixture screws down one full turn would make this phenomenon happen EARLIER as you suck fuel down much faster, and is a companion test.

Lastly: MAKE SURE YOUR STARTER SYSTEM IS FUNCTIONING PROPERLY AND THAT YOUR JET TUBE ARE BOTH FULLY "UP" IN THE HOUSINGS! If one or the other (or both) are stuck even PARTIALLY down you may never know it. I had a 'flat spot' in my 260 that I thought was ignition related save for altering of the throttle position made it 'go away'... I could rev to 7K if I was only 7/8 throttle, but not 100%. If I lifted slightly before, or even during the 'flatness' in the powerband, the car picked up and started accelerating again! One of my starter levers had suck, and the jet was partially down. Damndest thing. Someone earlier this year experienced the same thing---this may be all it is in your case. Check those lever cables for adjustment, make sure the connecting linkage is not binding or side-loading the jet so it has a possiblity of sticking, and give a slight 'push up' from underneath when they are supposed to be all the way up, just to make sure they are where they are supposed to be.

Needless to say, if this IS the case, you will be doing some retuning as all the adjustments in regards to mixture will have been off in that carburettor.

You can have AFR's as low as 10.5:1 and NOT see black smoke out the tailpipe. You will never know you are running there other than the fumes...

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People here simply want feel good answers. Don't confuse them with FACTS, Dammit!
Tony D: "Knowledgeable but Caustic"... rationull
You simply can't call someone a F**ktard here, no matter how truthful it is.
Stupidity is contagious, and looks like it's pandemic here...

A stock factory Nissan fuel filter should handle the flow fine, some aftermarkets may or may not be good enough. It's easy to check the flow on a carb fuel filter though, just drain the fuel out of it and blow through it. It should have zero restrictions once the liquid is gone from it. The float level is critical as well.

One other thing that no one mentioned directly is..... Always check all the hose connections and the hoses themselves on the feed side of the fuel pump. If you can spin the hose while it's clamped down on a fitting, then you have a potential leak. If you don't have a completely tight system before the fuel pump, you can lose fuel delivery, especially under extreme load. It can't build a vacuum to suck the fuel if it's also sucking air.
The carb screens definently need to be checked like TD said. If you ever want to see if you're running from a lean setting on the carbs just pull back part way on the chokes and see if that solves your problem. (this is merely a test, don't run that way all the time). If pulling the choke part way solves your hesitation or flat spot then richen the carbs up a quarter turn per carb until the problem goes away.

Z man of Washington

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1978 black pearl show car-retired, I bought it new
1983 zx turbo daily driver, modified, 3rd owner
1970 240 #23 being restored
1970 240 #638 driver
1973 240 roadster being built
about 70+ other z's & zx's
57 Chev Sedan Delivery
67 Jag E type
2 620 Datsun Trucks

www.zspecialties.com
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www.zman@zspecialties.com
1-800-518-5480

as long as you dont get the "brown" note, you should be fine.

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Murfreesboro,TN
'82 280zx T-tops 5-spd.Jade gray,new intake, NGK wires, new coil, complete brake rehaul all around.Diff and tranny changed to royal purple, new muffler, new clutch and other upgrades. also looking for a turbo version!

zmanofwashington Wrote:
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> If you ever want to see if you're
> running from a lean setting on the carbs just pull
> back part way on the chokes and see if that solves
> your problem. (this is merely a test, don't run
> that way all the time). If pulling the choke part
> way solves your hesitation or flat spot then
> richen the carbs up a quarter turn per carb until
> the problem goes away.

I did this on mine, and the engine smoothed out, but it started going flat earlier... The imbalance in fuel flow and having the jet stuck down can suck the fuel dry earlier. it's amazing how tolerant the engine is to rich mixtures on one carb. This test is fine [i]if the jets operate properly and aren't stuck[/u]---and likewise just like my misleading results, because the 'flat spot' moved lower in the RPM band, like it 'should have' I made the assumption it had to be fuel. But every other test said fuel delivery was correct, and so was pressure at speed.

Somehow those floats were going dry.

Because one jet was stuck partially down allowing it to go totally lean before the other carb did, and since moving the mix down is the usual fix, the other carb was so fat it ran.

Always check mechanical interactions before going internal on these carbs. There is a reason I went to EFI!

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People here simply want feel good answers. Don't confuse them with FACTS, Dammit!
Tony D: "Knowledgeable but Caustic"... rationull
You simply can't call someone a F**ktard here, no matter how truthful it is.
Stupidity is contagious, and looks like it's pandemic here...

flatblack_83 Wrote:
-------------------------------------------------------
> as long as you dont get the "brown" note, you
> should be fine.


LOL!!

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Richardson, TX

"I`ve tried nothing and I`m all out of ideas"

shifting and drifting...
mechanical music....
adrenaline surge!!!!


1977 280Z R.I.P. 6/9/96
1977 280Z stock with custom rust

A lot of great suggestions guys, thanks alot. I got plenty of things to check today on my day off.

I checked the banjo filters in the carbs, clean as a whistle (the carbs only have about 2K on them). All fuel line connections have been re-tightened, and secured with fuel-injection style clamps.

A couple things, the fuel pump is brand new OEM (Kyosendanki) installed this Winter. Fuel filter is about a year old, but it is a cheap arse Fram. I suppose spending $20 on a nice one at PepBoys is an easy and cheap test, I'll try that today.

I definitely have the car running a little bit rich at idle. When I turned the mixture screws down about 1 turn before yesterdays test ride, the hesitation in the powerband did seem to move up slightly, but I feel like they're running pretty rich now. I guess another 1/2 turn is worth a shot.

I think the choke cables are working well, and they're adjusted right. I double checked last week when I thought the A/F mixture was getting leaned out, and they seemed good. I will double check all mechanical connections on the SU's today also.

I'm thinking the fuel filter could be the weak link here. The choke stuff Tony described sounds like my problem too, but the carbs are new, so I think the chokes are working fine, but I should check that the nozzles are completely re-seating when I push push the choke off. The choke cable is original though, so I should double check nothing is hanging up. Either of those would be a great, and cheap, solution. Thanks again guys, I'll let you know how I make out.

"I definitely have the car running a little bit rich at idle. When I turned the mixture screws down about 1 turn before yesterdays test ride, the hesitation in the powerband did seem to move up slightly, but I feel like they're running pretty rich now. I guess another 1/2 turn is worth a shot."

You have just stumbled upon why the FLAT TOP carbs are much-maligned by idiots who know nothing about serious racing with SU's... The Flat Tops have a SEPARATE idle circuit dedicated to them, so you can adjust IDLE totally separately and correctly than TOP END. It's why all the flat tops in the junkyards have the needles removed---they have a much more agressive taper to them for supplying MUCH more fuel. Why? Because of that separate idle system, they don't have to compromise the idle quality and idle mixture to get power on the top end. I'm so sorry you got saddled with those crappy Round Tops, but it's the price you have to pay for using 1920's Technology, instead of the latest in SU Developments, the HIF series of Carburettors with a separate idle circuit. Outside the USA, the carbs DID NOT have the 'power valve' that blows out, and the needle is adjustable from the bottom just like earlier carbs. But I digress...

"The choke stuff Tony described sounds like my problem too, but the carbs are new, so I think the chokes are working fine, but I should check that the nozzles are completely re-seating when I push push the choke off."
NEVER ass-u-me because something is functioning properly due to it's age! Every buy a new GM Car? BIG mistake if you think it's working right just because it's new!

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People here simply want feel good answers. Don't confuse them with FACTS, Dammit!
Tony D: "Knowledgeable but Caustic"... rationull
You simply can't call someone a F**ktard here, no matter how truthful it is.
Stupidity is contagious, and looks like it's pandemic here...