Reverse Manifold Vacuum? What?

[jbarraclough]

Help me figure this one out. I posted recently a small block Chevy crankcase pressure problem. Good compression (130-150) all around. PCV valves functioning properly. No noticeable smoke from the exhaust (maybe a little more than the other engine). Major blow-by. I tested the manifold vacuum at the base of the carb. Results? 15-16 PSI at idle, 20 PSI at 2000 RPM. How is that possible? If it's a stupid mistake on my part, I apologize but something is causing this blow-by and I'm going nuts trying to figure it out. Any ideas? Thanks.

 

[echozulu]

Re: Reverse Manifold Vacuum? What?

how do you fiqure 130-150 lbs is "good compression "? its ragged out tear it down

 

[snapperbait]

Re: Reverse Manifold Vacuum? What?

Jbarraclough, when you checked the vacuum you read positive pressure in the intake manifold in PSI, correct? Or, did you get a reading from a vacuum gauge in in.HG (vacuum) in which case 15-16 in.HG would be about right...<br /><br />Have you done a leakdown test with compresed air on the engine? That'll help nail it down to a piston, rings, valves not closing, ect...

 

[conimicut1]

Re: Reverse Manifold Vacuum? What?

if its driving you crazy get a leakdown test, much better than a compression. Finding a tester may be a little harder, check a rebuild shop and promise to take the engine to them for a rebuild if it needs it later.

 

[mabbott]

Re: Reverse Manifold Vacuum? What?

Compression of 130-150 is fine. echozulu, what makes this so bad? The brand new spec. is probably 150 right. Vacuum of 15-16 isn't bad. Try ajusting the carb to reach the optimum maybe 18. I would expect the vacuum to increase at higher rpm since it's drawing more air.