The Stovebolt Site Forums The Shop Area Engine Shop Vacuum pressure in my Rochester B

I purchased a rebuilt Rochester B carburetor and installed it on a rebuilt '59 235. The engine has new valves, seats, springs, etc; was recently timed, and the valves were adjusted. The engine runs and idles smoothly. At idle speed, the vacuum pressure at the carburetor is only 4 inches of mercury. Specs in my '58 shop manual call for 17 to 21 inches. Any idea on what's wrong?

The gauge is new and seems to work correctly. The vacuum hoses to the windshield wipers and the distributor are removed and plugged. I do not currently have an air filter installed on the carb, but I don't know that it matters. I could not find a spec on how tight to torque the nuts holding the carburetor to the manifold, so I torqued them to 20 ft-lbs. Finally, adjusting the air/fuel needle valve does affect the vacuum pressure. It stays constant at 4 inches. At higher RPM (approximately 2000), the vacuum pressure increases to 15 inches.

Thoughts?

Correction: adjusting the air/fuel needle valve does NOT affect the vacuum pressure. It stays constant at 4 inches.

Where are you tied in to get the vacuum reading? Should not be hooked up to the carb, instead needs to be hooked up to a main port on the intake manifold,like where the vacuum source for the wipers would be.

Are you reading a ported or non-ported vacuum? It sounds like you are taking the reading in the ported port for the distributor. Move the gauge to the intake like Roy mentioned to get engine vacuum. Ported vacuum only has high vacuum at higher RPM as it works on a venturi principle like a pitot tube. As the incoming air rushes past the venturi orifice, it creates a mild vacuum.

the engine vacuum (non-ported) is affected by the engine compression ratio, quality of rings, quality of valve seals, rpm, displacement, cam shaft profile, ignition timing and intake tract sealing. It will not be affected by the idle fuel/air mixture.

Ah! That would explain things. I'm definitely measuring vacuum at the wrong location. I'm connected to the carburetor port which leads to the distributor vacuum advance. Can you tell I'm new at this??

I'll check it again tonight. Much thanks!

I doubt it you will get 21". I think 17 to 18 would be normal, at least in my expderience. As long as it idles smoothly that is the main thing. You can play with the idle adjustment by ear probably as good as with a gauge.

I get 21-inches right on the mark. No doubt this is because the engine was recently rebuilt from top to bottom. Thanks guys.

My tired 216 pulls 21 inches at idle. It was rebuilt 25 years ago...

Then agin, my vacuum gauge is from China....

Crabbyappleton is right, as long as it runs smoothly.

My vacuum gauge is from China also. I wouldn't be surprised if yours read a little high and mine read a little low. As long as it runs good, I guess that is the main thing.