Question on SU operation and possible cause of a problem

[BTF/PTM]

OK, here's one for Bruce and our other resident SU experts.

First the preface; today I was finally able to take the z to meet a fellow forum member (Motorman7) for some carb tuning lessons. Well, the good news is we didn't find anything wrong with the float levels, didn't find any damage to needles or seats and we didn't have to do much adjusting at all to get both carbs idling smoothly. Bad news is, we found an air leak at the back carburetor that was audible when the Uni-Syn tool was on that carb.

Now the problem; the back carb is running quite rich and is fouling the three back spark plugs in very short intervals.

Now the hypothesis; it is supposed that, since the carb is leaking air, the slide isn't getting full vacuum and thus the slide/needle aren't rising fully for a given air flow and this is causing the carb to run rich due to reduced air flow.

Now the question; is this a plausible theory? Are there ways of finding leaks on the carb bodies? I don't imagine soapy water would work well since it's a suck rather than a blow. Any ideas are welcome. Thanks, everyone.

[Bruce Palmer]

Spray carb cleaner around all mounting surfaces and throttle shafts.

Lean out that back carb. May be idling fine but sounds like it's fat.

[beandip]

If you can hear a vacuum leak it is a major leak. Like Bruce said buy some carb cleaner and with the engine running and warmed up, spray the cleaner on the throttle shafts and on the insulators and the gaskets . If you see a change in RPM when you shoot the cleaner then you have found the leak. These carbs are vary much Dependant on vacuum , so what you said about the piston not lifting could be correct, causing the richness on the back carb. At least this is a possibility.

Gary

[BTF/PTM]

Thanks, Bruce! Am I applying said carb spray with the engine running? Should I block the air way to amplify it? Am I watching for bubbles, or perhaps listening for a change in engine pitch?

*edit* beandip beat me to my own question. Thanks guys!

Edited June 1, 2010 by BTF/PTM

[Zs-ondabrain]

exactly. The carb cleaner doesn't burn like fuel. Spraying it in will cause the engine to dip down (lower RPM) So if you spray it on the outer throttle shaft and the engine slows or stumbles, then the shafts are severely worn.