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Discussion Starter #1
anybody know what it means if vacuum test hooked up to manifold port reads a steady jump between 10-15? i have seen different diagnoses for vacuum tests and can't really figure out which one it is.
 

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A motor having a performance cam will develop less vacuum and flutter a bit more. However 10-15" of flutter is a sign of a valve problem. Could be an improperly lashed valve, a valve with a burned seat or similar issues. A compression test may help narrow down a valve problem to a specific cylinder.

What kind of cam do you have? That will tell us if the 10" of vacuum is a problem or normal for the cam.
 

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Discussion Starter #4
thanks dave, unfortunately i have no idea what cam is in it. i did not do any work to inner workings of engine, just replaced parts external parts so it would look good. ( i'm more of paint and body guy, not a gearhead at all). how do i go about doing compression test?
 

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Acquire/borrow/rent a screw-in compression gauge.
Remove all spark plugs after bringing engine up to operating temp.
Hold throttle to wide open position.
Unhook ignition power supply, either the power wire or unhook the wire going to dist from coil.
Make a chart 1-3-5-7 on one side, 2-4-6-8 on the other side.
Screw in gauge into spark plug hole and crank engine. Stop cranking when gauge reaches a max indication. Write reading down next to each cylinder number on chart made.
Continue for seven cylinders.

Evaluation:
1. Overall readings should be within 10% from lowest to highest.
2. One low cylinder out of range may be broken rings, misadjusted/burnt valves. Squirt oil into low cylinder, compression comes up, bad rings - oil sealed broken rings--compression stays the same - misadjusted valves or burnt valve , or maybe a worn cam lobe not opening valve as much as other valves.
3. Two adjoining cylinders same reading and lower than the rest - blown gasket between the cylinders.
4. If compression readings are good, might be an ignition problem for one cylinder, leaky manifild gasket. A quick check of exhaust manifold temps with a temp gun should show 500°F temps min. Any reading lower and you have a dead cylinder.

Be sure to release WOT when done.
Fluctuating vacuum reading may be due to loose intake, leaky hose - large hole, worn cam lobe. Might remove valve cover and witness all rockers moving the same distance during cranking. Any running engine should have a solid vacuum reading regardless of camshaft installed unless you have a 0.700" lift cam at 500 rpm idle - not quite possible.
 

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thanks dave, unfortunately i have no idea what cam is in it. i did not do any work to inner workings of engine, just replaced parts external parts so it would look good. ( i'm more of paint and body guy, not a gearhead at all). how do i go about doing compression test?
If it's a stock cam (or mild cam) 10 -15" is pretty darn low. A mild cam should give you 15-17". I wouldn't worry about those details just yet though. See what the compression test shows.

Vacuum leaks is another good point that was raised. When I aquired my car, the PO put a holley carb on an Edelbrock Performer RPM intake with a paper gasket. The accellerator pump lever was hitting the top of the #1 runner. When I pulled the carb off, you could see the witness marks all the way around except where the pump lever was clashing. A 3/8" phenolic spacer fixed the leak and it ran a heck of a lot better.
 

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Discussion Starter #8
ok, i advanced timing a few degrees yesterday and vacuum reading went up and fluctuated between 13-18. at idle it actually runs pretty good, not rough at all, it is when i put it into gear ( it's a powerglide) that it runs really rough and almost wants to stall. I thought it was combination of timing and carb needing to be adjusted...both of which i'm not very great at....or an intake leak. ran propane all around and couldn't find a leak. i'm not comfortable at all with fooling with valves, so if it is valve related, i may have to enlist some help. could there be any correlation with rear end or transmission that would cause the rough idle under load?...........if any of this involved bondo or paint, i'd be ok. thanks guys
 
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