The Stovebolt Site Forums The Shop Area Engine Shop Idle vacume

Any ideas out there about what Idle vacume should be?? We have what appears to be a 261 in a 1950 C3100 compression seems ok 90-140 psi but the Idle vacume is around 12 in.

My 250 will idle around 17 in. 12 seems a little low.

Hello Bob and welcome to the site. Your compression is between 90 and 140? Please tell us more...you can write in German if you need to do that. Your compression seems to be a large range (between high and low).

My 235 idles at about 21 inches of vacuum, on my gauge. I’ve never checked my gauge against another to see about accuracy.

Please post readings of compression for each cylinder. Jon is right, that is a very wide range.

Ignition timing will have a large effect on vacuum.

Compression readings for all the cylinders should be within 10% of each other. It sounds like you have some valves that aren't sealing well, or worn piston rings. Either problem will cause low vacuum. Is the vacuum gauge steady, or does it fluctuate while the engine is idling, and does the gauge stabilize when the RPM is increased? Try advancing the ignition timing a few degrees and see if the vacuum rises.
Jerry

Hi, Thanks for the responses!!!
I am a Expat living here in Germany and have a small shop that only deals in American cars.

The compression is 90,90,90,100,140,100
I opened up the valve lash to 12/14 hot and the truck still only has 12in vacume.
Timing is at the ball.
Thanks again , Bob

Hi Bob,

my 261 has a similar vacuum reading, but it's not a stock cam - are you sure you have a stock cam?

Greetings from Augsburg, Frank

Elevation factors into manifold vacuum as well.
If compression test was done with engine at operating temp I'd next do a 'wet' compression test or a cylinder leakage test .

For those ‘bolters that don’t know what a wet compression test is, after your initial readings, inject about a teaspoon of oil into the cylinder the repeat the test. Don’t inject more that a teaspoon of you may get false results. If the compression comes up significantly (over 10%) it’s the rings. If no increase, it’s the valves.

The 140 reading is way above normal. Try putting a bore scope into that spark plug hole and look for lots of carbon deposits. A tablespoon of extra carbon in a combustion chamber can raise the compression ratio by up to about half a point, depending on the size of the chamber.
Jerry

My 261 with 848 head off a 235 reads 145-155lbs. hot at 2500 ft. altitude. It also has a hotter cam that should drop the cranking pressure some. The 235 with the same head and cam was about 135-145 hot. Idle vacuum on both engines is/was ~ 12in.Hg at ~700RPM Less if pulling the A/C compressor or if the alternator is loaded.