smelly z

[5thhorsemann]

I will take some pic's and post them. can you get a larger jet or would it be different needles that I would need?

I never shopped jets for round tops, as I have a 2.4 L. I'm not sure about needles either, maby a call to ZTherapy is in order. But if the 2.8 is fresh, it will definetly want more gas at speed than a stock 2.4 L. I don't pretend to be a Z motor guru, but I have built ALOT of hot rods in the past using Kein side drafts, which are similar, and I always installed the DynaJet kits before installing the carbs.

[racer88]

This engine is a 2.8 bottom with a e31 cylinder head with large valves and a mild porting job. Middle of the road cam, light flywheel, header with 2 1/2 stainless exhaust. ZX dizzy converted to a gm hei. my only complaint is the rich smell at idle and low speeds and the missing and sputtering at wide open throttle

[5thhorsemann]

You have something of a hot rod there. It's gonna smell somewhat, it's a Z.I still think you should look into the jetting.

[LeonV]

If you have carbs jetted for a 2.4 L on a 2.8L motor, it must run rich at low R's to feed the car at high R's. Sounds like you need to go up a size or two on the jets which will lean the car out at low R's (because you can adjust the car properly now) and still feed the motor at hammer down hold on tight time.

To be blunt, you don't understand the fundamentals of SU carbs.

Racer88, use a sniffer or WB02 to really see what's going on. I've been fairly certain the you're running rich. Follow the suggestions on Hybridz. You either have leaky, mal-adjusted carbs, or one (or both) of the jets is stuck in the down position.

[5thhorsemann]

To be blunt, you don't understand the fundamentals of SU carbs.

You may be right about the SU. But mechanically they seem to be roughly the same as a KEHIN CV where the diaphram lifts the needel out of the emulision tube, which contains the main jet, based on vacume demand. That same jet contributes to idle and low end fuel supply in the CV, but supplies all idle and low end fuel in the SU (I think?). Again, I don't claim to be an expert, but based on past experience with OTHER carbs, my logic says a bigger jet, adjusted leaner for low end and idle, will provide proper fuel supply at high rpm and eliminate rich exhaust at low end. If I'm wrong on this, please enlighten me, cause I think I will need it in the future when I finish building the motor of my cars future.

[LeonV]

There is no emulsion tube or anything else besides one orifice, which you can call a jet, and a tapered needle. The needle adjusts orifice area when engine vacuum causes the piston holding the needle to move in its bore. You can't "change the jet" on SUs unless you want to get into major (unneeded) modifications. What you can change is the needle geometry. A narrower needle equals a richer mixture. There are different needle profiles available to fine tune SUs. I don't think this is necessary in this case.

The round tops also don't have a dedicated idle and low speed circuit as "typical" carburetors would. It's all one circuit, so if you richen up the low end, you are richening up the high end as well. Vice-versa for leaning the mixture.

My hypothesis is that there is a vacuum leak, likely in the throttle bushings which tend to wear out and leak. Since there is a vacuum leak, the tuner compensates by making the mixture richer. This causes idle and low speed cruise AFR to be richer than it needs to be and there's your smell. You won't see black smoke until 10:1 AFR.

Here is an explanation of SU functionality: SU Principles of Operation

[racer88]

Leonv. It may be an adjustment issue. I can assure you there is no vacuum leak either at the throttle bushings, intake manifold or lines. i have totally dissasembled the carbs right down to removing the throttle shafts measuring bushing to shaft clearance the nozzles operate smoothly. In short the carbs are in perfict shape. I have set the needles flush with the bottom of the suction piston. I wonder if I pulled the needles slightly out of the suction piston would that tend to lean it out a bit. Because I assumed it was a lean misfire at wide open throttle i have been trying to richen it up to clear up this miss.

[LeonV]

To experiment, try to lean it out enough to make the smell go away and then drive it. Also make sure that your ignition system is in proper tune: clean plugs, timed correctly, cap is seated on the dizzy (yes that was a problem for me before), etc. An 02 sensor or gas analyzer will tell all.

[5thhorsemann]

Good stuff Leon, I didn't know about the needle change VS. jet change thing. I guess I learned my new thing for today. Question; could the damper fluid level, or vescosity being too high in one or both carbs cause that top end lean condition? I was told that when I went to a K&N dual element that I might need to use a lighter oil in the damper to compensate for the higher flow / less vaccume in the throat to get the carbs to work rite off idle. Turned out the car ran fine and didn't need it, just want to pick your brain.

[Racer_Z_]

Good stuff Leon, I didn't know about the needle change VS. jet change thing. I guess I learned my new thing for today. Question; could the damper fluid level, or vescosity being too high in one or both carbs cause that top end lean condition? I was told that when I went to a K&N dual element that I might need to use a lighter oil in the damper to compensate for the higher flow / less vaccume in the throat to get the carbs to work rite off idle. Turned out the car ran fine and didn't need it, just want to pick your brain.

The oil does not effect the lean/rich mixture nor does it effect how far the throttle opens. What it does effect is how fast the carb responds to your control (foot action). The oil is a "damper". A thinner oil should make for a crisper response. Out on the track, we haven't been able to notice much difference.

The S/U (Hitachi) is a constant velocity carburettor. Unlike an American four barrel carb, you can't force the carb to open any wider than it needs to be. With a four barrel, you can (at low RPM) open the carb so wide that the air flow drops, the fuel mixture leans out and the engine stalls. The Z's S/U won't instantly GO BIG. If there isn't enough air flow to support WIDE OPEN, it won't fully open. When adjusted and working properly, it is a wonderful carburettor and has won many a race.

Is it the best carb? No.

Is it the worst carb? No.

It is a good carb and there is nothing wrong with it.

Sorry, but I can't explain it's workings any better.

[LeonV]

Good stuff Leon, I didn't know about the needle change VS. jet change thing. I guess I learned my new thing for today. Question; could the damper fluid level, or vescosity being too high in one or both carbs cause that top end lean condition? I was told that when I went to a K&N dual element that I might need to use a lighter oil in the damper to compensate for the higher flow / less vaccume in the throat to get the carbs to work rite off idle. Turned out the car ran fine and didn't need it, just want to pick your brain.

Damper fluid does not affect the steady state mixture whatsoever, what it affects is transients (acceleration). If you read the link I posted, you would see that the lower the piston is, the smaller the cross sectional area, therefore the higher the velocity is at the venturi. Higher velocity equals higher pressure drop, so more fuel is draw out and your mixture is richer.

A lightweight oil will let the piston move quicker, thus it will tend to lean out transients. Thus, a more viscous (thicker) oil will richen transient mixtures. If your oil is too light it may cause piston flutter which leads to an unsteady mixture.

RacerZ says a lighter oil will make for "crisper response." I don't buy that generalization because the only way response will improve by going to a lighter oil is if your transient mixtures are too rich. That's like saying that decreasing the size of the pump jet on a typical carb will improve response.

[5thhorsemann]

Understood, but if you overfill, could that effect (restrict) the piston travel?