General Category > Quadrajet Carb Talk and Tips
Idle Bypass Air ??
Cliff Ruggles:
Most likely a vacuum leak and/or the throttle plates aren't fully indexed and seated at idle speed. .086' idle bypass air isn't enough to supply the engine at 1300rpms with all 4 throttle plates fully closed....Cliff
beertracker:
Thanks Kenn & Cliff for the replies. Cliff you already answered my question. I removed a vacuum hose and the engine sped up. I will increase the idle bypass air holes from .086 to .090 or as close as I can get with a numbered drill and re-test. If you think this is too much of a increment let me know. bt
beertracker:
With idle bypass air holes at .086" I do not see fuel drips at the primary nozzles. However I see a coating of fuel on the primary nozzles as the engine idles. When I shut the engine off and touch the nozzles they are wet with fuel.
Does this mean I am close to the correct idle bypass air hole size? Should I add more bypass air?
omaha:
I am agreeing with Cliff on this one for sure. That engine has to be getting the air from somewhere 'cause .086 holes are not that big at all. Even a lot of the factory carbs I have seen have at least .100 holes and some bigger. And that is with no big cam or anything like that. I don't think that your cam is excessively large (...did you degree the cam to see if it is indeed the one you bought...just a thought). And especially because it is ground on the 114 centerline it would seem that it should idle "fairly" decent. With that said, I would be looking for something major in the way of air getting past the primaries and secondaries. Maybe a warped baseplate or the plug missing for the big vacuum port. I've seen cracked baseplates and incorrect gaskets cause all kinds of problems. It could even be a cracked intake manifold. As far as testing for vacuum leaks, there seems to be several methods, unlit propane torch, cleaning solvent sprayed from a squirt bottle even wd-40 sprayed around suspected areas. My favorite was lighting up a cigar and blowing smoke through the top of the carb. Tape up the top of the carb except stick a long straw through the middle of the primary area. Open the throttle a bit and hold it open, let fuel dry first. Now take a puff and blow the smoke right through the straw. (engine NOT running ). the smoke should appear where you have your vacuum leak. Let me remind anyone trying any of these methods that none of them are 100 percent safe, use your best judgement and take the precautionary measures. BE safe FWIW.
beertracker:
Well, I am totally confused now. I thought idle bypass air and idle speed are two separate issues? My idle speed now is down to 1000rpm & I haven't been adjusting the idle mix screws. I have been adjusting the idle bypass air holes to make primary drips go away. As I stated in post 7 the drips are gone but not I see the primary nozzles are wet with fuel.
So I assume I need to add more idle bypass air? :-\
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