Besides the obvious possibility of a vacuum leak, what else would cause idle and cruise vacuum to be only about 10"?? Even decelerating from 80mph the vacuum only shoots up to 15"....I would expect at least 25" on deceleration (as that's what I get in the 67 Cougar with comp 270h cam) Mav engine completely rebuilt stock 302 (except pistons....have flat top now, and larger valves in the heads) runs fine otherwise, idles smooth, good acceleration, no hesetation...just low vacuum. Exerything else is stock. Note: car has the complete original 1976 exhaust system including cat and muffler.....could there be a restriction in the exhaust causing this? Also highest freeway gas mileage reading in 3000 miles (since rebuild) was 15mpg, get's 9 or 10 around town. Any thoughts??
"Proper engine operation should result in an even vacuum reading from 15 inches to 20 inches of vacuum with 15 inches of Mercury being more appropriate for high performance type engines. A vacuum reading that cycles up and down slowly could indicate a fuel mixture problem from improper carburetor or fuel injection operation. Low vacuum readings may indicate intake valve, manifold, or carb gasket leaks including internal or external manifold leaks. Spray carb cleaner around suspected areas to test for leaks. Plugged catalytic converters, stuck heat riser valves, plugged intake crossover heat passages, crushed exhaust pipes, plugged mufflers, (check out that mouse nest in the exhaust!), and similar restrictions will usually lead to very low vacuum readings. Excessively carbon in engines or those that did not get their oil changed often enough may result in sudden vacuum reading changes when valves stick. It could happen at any engine speed and may temporarily cure itself by quickly changing engine speeds. Lower vacuum readings directly related to engine misfiring may be a result of a burned exhaust valve. Constantly changing vacuum readings at low engine speeds may indicate a problem with worn valve stems, guides, or seals. Higher engine speeds tend to temporarily cancel out this problem. High-speed vacuum readings that change may be a result of broken or tired valve springs, while low engine speed vacuum readings tend to be constant. High performance or race cams tend to result in lower vacuum readings. Make sure that the cam and lifters are matched, (solid/solid, hydraulic/hydraulic, roller hydraulic/roller hydraulic, solid roller/solid roller), etc. Strange things happen when these items are mismatched, although some will actually run that way. Cam indexing/timing will also affect these readings. Also check for sloppy timing chains, and improper or changing ignition timing. Make sure you have a good vacuum gage that gives repeatable readings. Remember your high school science class experiments, where you have to go back to the original test to verify your numbers and don’t make changes until you are sure of your first set of readings. " source: http://www.tracyvette.com/enginevacuum.html Good luck!
If the acceleration is slow and power levels low then you probably have a restriction in the exhaust. At Idle your stock 302 should have a vacuum reading of between 17 and 21 inches of mercury. Does the vacuum drop to zero when you floor the throttle in gear? If not you might have a restriction in the intake between the carb and air. (air filter, snorkel, or valves for warm air)
Dang Rick...you had me going there!!! I thought they done learned you something over there in Singapore!
Ha! Not a chance, Ray. You know the saying, "Old dog"... something.. something, "new trick". I forget how the saying goes, but I could probably google it.
The acceleration is just fine. I would say Yes it does go to zero or near zero....will recover to about 10" @ 65mph, jump to about 15" when I take my foot off the gas at freeway speeds. Took the cat off...very clean inside, replaced the muffler on sunday....no restrictions in exhaust. Still idles around 13" in Park with or without the air cleaner mounted.
I am going to tell you to do something that you have to be very careful doing or you can blow your engine up - so be careful. Connect a tach and a vacuum gauge to the engine With the engine running and the park brake on and the transmission in park: slowly increase the throttle, by hand, to your maximum RPM (where you red-line the engine) If it goes to your maximum RPM then close the throttle the test is over. Do this slowly - over-revving your engine will make a big mess!!!!! If it stops gaining RPM while you keep opening the throttle then return the engine to idle. You have a restriction in the exhaust. If the vacuum gauge slowly drops as the RPM is increased then you have a restriction in the exhaust. If the vacuum rises or remains the same as you increase RPM slowly then your exhaust is probably fine. Check your timing - it may be retarded. If it seems to be correct then check to see if the balancer has slipped. If the balancer has not slipped then your cam timing may be off. Check to make sure that the timing marks on the sprockets line up correctly. If the cam timing is correct then check the firing order it may be off. If everything I have suggested checks out then pull the rag out of the carburetor or air-horn and it should run fine.
