Help me fix my SU's!!!

[Pir0San]

Ok, I put a rebuilt my carburetors, and now I'm having problems getting them just right. I can't get the mixture just right. I can set the air flow with my unisyn, and I can drop the idle down to under 1000, but the mixture I can not get right. I have set the mixture nuts to 2 1/2 turns, but the car will bog out when I lift up on the lift pins, like it is going to die. No matter how much I turn the nut out, the car backfires out of the carburetors, which I am assuming is a lean situation, since it is mixed with the bogging out when I push the lift pins. My carburetor dampers are good, I have SU fluid from MSA in them, and I might end up using Marvel Mystery Oil to see if it makes any difference. When I have my choke on, the car runs good, but when it is off and the car is warmed up, it stumbles when going from idle, and can die if the throttle is not feathered just right. The nozzle and needle are not stuck, because when I lift the piston/dashpot, and I drop it, it slides down fast and makes a distinct clunk noise.

What could cause this issue? I don't believe it to be my float settings, because I replaced them with another float that I have from another set of carburetors, and it didn't make a difference.

If you want more information behind the matter, and what I've tried, see my thread at Zcar.com

Any help is appreciated, I just finished rebuilding the engine, and I'm eager to get her back on the road and driving again!!

[Arne]

Mixture setting with SUs is dependent on several things - among them the level of the fuel in the floats. If the float level is too low, a lean mixture will result - and adjusting the mixture nuts may not help much unless you screw them way out. The fact that it runs better on choke supports this theory, as the choke simply drops the nozzles way down. Replacing the float itself may not be enough to know that the level is close, you need to actually measure it properly. So in your situation, first thing I'd want to verify is that the float level is where it should be.

[Pir0San]

how would I check to see if the float level is where it should be?

[Arne]

This is the best way:

ZTherapy's sight glass

[Pir0San]

when rebuilding the carburetors, I set the floats to 9/16 as advised by victoria british (who I got my rebuild kit from). The reason I just replaced the float is because the float I replaced it with came off of an old dirty pair of carburetors, but the PO said the car ran with them...I'm lost as to what to do.

[Pir0San]

could my problem also be related to the amount of air the butterflies allow to enter the cylinder/pull through the carburetor? If this is too far off, will it cause my mixture to be hard to set as well? I'm just trying to find the best idea possible before I break down and try to take it to a mechanic.

[Arne]

Previous floats may not match up with replacement needle valves, which is why trying the old float(s) may not tell you anything. All your symptoms point to float level being too low. The only other thing I can think of would be wrong or improperly installed needles in the pistons, but I can't really envision needle problems making the car run this lean.

[Arne]

could my problem also be related to the amount of air the butterflies allow to enter the cylinder/pull through the carburetor? If this is too far off, will it cause my mixture to be hard to set as well? I'm just trying to find the best idea possible before I break down and try to take it to a mechanic.

No, I can't see this as relating to your issue. The carb is supposed to meter fuel based on the amount of air passing through. But again, the mixture is adjusted primarily by the float level, and then fine-tuned by the mixture thumbscrews on the nozzles.

[Pir0San]

oh, good point, I see what you're saying. So, if I were to do some trial and error with the floats, would I want the float to be closer to the float lid (such as instead of 9/16, like 7/16), or would it be the other way around?

Thanks for your help, I can honestly say that there is not a part on my Z motor that I have not touched.

[Pir0San]

I found this site, and this is how the guy recommends I check my float bowl settings, does this sound right?

SU Tech Pages

[Arne]

oh, good point, I see what you're saying. So, if I were to do some trial and error with the floats, would I want the float to be closer to the float lid (such as instead of 9/16, like 7/16), or would it be the other way around?

You want to raise the level in the float bowl, so you want the float closer to the lid. I'd go in smaller increments, probably 1/2" as a first step.

You can try to judge the amount needed by opening the choke slowly until the engine starts to run decently, then look to see how far the nozzles had to drop. If the nozzles moved 1/8" raise the float 1/8" closer to the lid. If the choke pulled the nozzles down 1/4", raise the float 1/4" and so on. That should get you in the right ballpark.

I found this site, and this is how the guy recommends I check my float bowl settings, does this sound right?

That will work fine, provided your SUs are not the early models (like mine) without float drain plugs. It allows you to make an accurate actual measurement, and to get it the same for each side.

[Pir0San]

yeah, mine are the stock 3 screw carbs from my 240Z. Why do the drain plugs make a difference, out of curiousity?

Just to make sure I read what he said right, put the clear line on the bottom of the float bowl, turn the motor over until the fuel inside of the plastic line stabilizes (air is bled out), and the put the line up to the mark I made on the float bowl, and adjust the float setting until the fuel in the plastic line is at the same height as the fuel in the float bowl?