QuestionI run a 350 chevy stock motor. I need to pull 17 inches of vaccum and I have a cam that should pull 19 to 20 inches in right now. I am only pulling between 10 and 14 inches. I have replaced the carb. I just dont know where else the leak may be coming from any ideas the more the better. Thanks so much!
AnswerWell, two places should be obvious, the carb gasket, and the manifold gaskets. If you've milled the heads, or if the heads are aftermarket, the manifold might simply not be seating properly against the head surfaces. A good engine machinest should be able to help you there. Also, if your piston rings are worn, it won't pull sufficient vacuum. Check your compression with a guage, it should pump around 160 PSI per cylinder and they should all be within about 10 PSI of each other. I'd suggest you also do a leak-dowon check to be sure. Leaky valves can cause low vacuum readings, especially exhaust valves.
You might look carefully at your plugs after a hard run, shut the motor off at full revs and under load, and coast to a stop. Then check the plugs before running it again. If you have several different shades of color on them, you probably do have an air leak somewhere, and the problem will be finding it. Soapy water works sometime. Squirt it around the intake runners, and if a cylinder starts running roughly, you've found it.
Once you've done those things, if you haven't found it, write back, I'll ponder it some more, and it will give me a chance to bounce the question off a pro engine builder in my neighborhood.
Thanks for writing !
Dan Liddy
Sarasota, Florida
