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Question: Vacuum leaks

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Sorry, I don't have time to read the whole post but 15-16" vacuum at idle is too low for a mild or stock cam. If you have a quadrajet, then sunk floats or leaking well plugs come to mind but that should also cause sooty plugs. For vacuum leaks I like to use propane, with a small hose attached to the end of the bottle. With a small enough hose you can really pinpoint the leak, plus it is safe to use, easy to modulate how much to dispense, and propane is heavier than air and goes down not up. Any time you spray an area with a propellant you never know for sure if the residual just got sucked up and into the throttle opening or that there actually is a leak, plus most propellants are flammable.

Thats what i suspected. It is the original e4me quadrajet. Float level is fine, and still has fuel in the float bowl after sitting for over a week, plus plugs seem fine.
Have checked for vacuum leaks a few times and not found anything, but not using propane. I do have some so wouldn't be hard to try.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Ok, Just finished the check with propane, seems to be all clear of vacuum leaks. Must be something else causing it to be low.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Decided to do a leak down test to see what that shows up. Unfortunately the tester I used was a piece of crap so the results probably aren't worth the paper they are written on. Highest leakage was 26%, so if correct no problem there. But I did notice that the intake valve seem to close very close (IMO) to TDC. This lead me to do a few other checks.

1st: I checked how far a can rotate the crankshaft before I see any movement at the rotor in the distributor, This was About 7* (+/-1/2*). Is this too high?

2nd: I checked the valve positions at exhaust TDC and found the the intake valve was starting to open, but the exhaust valve had already closed. That to me also seems wrong. Am I correct in my assumption? (I thought both should be open partially at this point)
 

LLC5

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Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
Decided to do a leak down test to see what that shows up. Unfortunately the tester I used was a piece of crap so the results probably aren't worth the paper they are written on. Highest leakage was 26%, so if correct no problem there. But I did notice that the intake valve seem to close very close (IMO) to TDC. This lead me to do a few other checks.

1st: I checked how far a can rotate the crankshaft before I see any movement at the rotor in the distributor, This was About 7* (+/-1/2*). Is this too high?

2nd: I checked the valve positions at exhaust TDC and found the the intake valve was starting to open, but the exhaust valve had already closed. That to me also seems wrong. Am I correct in my assumption? (I thought both should be open partially at this point)



26% is high in my opinion, anything over 15-20% and I try to figure out why. Did you lightly tap the valves while they were fully closed to loosen any carbon deposits while testing? Did you test with a hot engine?

7* is at my limit for timing chain stretch, 8* and I would consider replacing it, but I don't think this is your vacuum issue.

I have never checked valve position as you have so I cannot answer that, but a flat cam could cause low vacuum.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
26% is high in my opinion, anything over 15-20% and I try to figure out why. Did you lightly tap the valves while they were fully closed to loosen any carbon deposits while testing? Did you test with a hot engine?

7* is at my limit for timing chain stretch, 8* and I would consider replacing it, but I don't think this is your vacuum issue.

I have never checked valve position as you have so I cannot answer that, but a flat cam could cause low vacuum.

The tester seemed unreliable so I don't really trust the results it gave me anyway. I didn't tap the valves. But the only place any leakage could be heard was at the oil cap, so I don't see what difference tapping the valves would have made.

so if the chain is at it's limit then I may as well go ahead and replace it. Worst that could happen is it won't solve the vacuum problem.

from what what I can tell there is not a lot of overlap (if any) but not sure if this position is correct. I have checked for flat cams, while I have not measured lift there was no valve the had obviously less than any other, but still could be some wear on all of them.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
OK, new chain, water pump, and fan clutch. Also shimmed the distributor to removed some play there. No change in vacuum. the new chain sure was a lot tighter than the old chain though. It was good to get it running again after so long though.

Just one thing I noticed, Does anyone know what the timing should be at idle with the 4 pin plug connected? mine appeared to be about 18* I would have thought there should be more advance than this at idle. Or am I overthinking this?
 

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