Thread: Manifold vaccuum question

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Hey guys, I have a SBC 350 bored .040 Professional Products airgap style intake manifold, Holley 750 double pumper, and Patriot 64cc alum. heads. Roller cam/lifter/rockers. Now the question when tuning the carb, using a vaccuum gauge I use the fully ported inlet/outlet on the front of the carb. When I pull the cap off of the stem the engine increases rpms slightly and smooths out instead of running rougher like any other vaccuum leak. What if anything does this tell you? The engine pulls 17.5" at idle (720 rpms) This engine runs great, I just think there might be a little more in there.

Thanks

That's because it's not a vacuum leak. There's no vacuum there until the throttle plates uncover the port.

Hey Henry, thanks for the reply. Do you know why it it increases the rpms slightly and seems to smooth out as if I was advancing the timing. If there is no vacuum there what am I measuring?

Originally Posted by Rgtrough

Hey guys, I have a SBC 350 bored .040 Professional Products airgap style intake manifold, Holley 750 double pumper, and Patriot 64cc alum. heads. Roller cam/lifter/rockers. Now the question when tuning the carb, using a vaccuum gauge I use the fully ported inlet/outlet on the front of the carb. When I pull the cap off of the stem the engine increases rpms slightly and smooths out instead of running rougher like any other vaccuum leak. What if anything does this tell you? The engine pulls 17.5" at idle (720 rpms) This engine runs great, I just think there might be a little more in there.

Thanks

As long as that is a manifold vacume port then I would have to bet that the carb is rich and when you pull the vacume hose off that it adds more air helping to lean it out.

17.5" at idle (720 rpms) This engine runs great

17.5" of Vacuum is not at all bad for a Street Performance Camshaft .

Thanks for the replies. I thought maybe the carb is rich. The cam is a mild one 211/221
495"/505" with 1.5 rockers.
Thanks again