Number eight cylinder fouls out.Replaced all vacuum lines,replaced intake
manifold gaskets,new plug wires, and distributor, runs day or two the fouls
out.When running good the spark plug is white,when not running good the plug
turns black.Professional mechanics think this is a vacuum leak but cannot
pinpoint trouble.The carburetor is a 750 holly with aluminum intake.Any
ideas what I should try next?
the compression test wont solve the problem ....but it will help u isolate
1. vacuum leak will not cause plug fouling
2. i doubt the carb is your problem since you stated that the plugs are
when running good (LEAN)....?
3.sounds like you are oil fouling the #8 plug VIA a broken oil control ring
or worn valve guide or maybe the valve guide seals are riding the stem and
letting oil run down thru the guides.
4. run a dry compression test (per DOC) and record the reading, then inject
a few cc's
of your favorite petroleum based lubricant into the cylinder and run a
second compression test
and compare the readings, if the wet test is higher than the dry test than
you have ring seal
Test all 8 cylinders and compare readings amongst them...they should be
within 10% of each
other on a well used engine.......much less variance on a rebuild
5. Hope this helps.
6.hello Doc, still runnin the big roads.....got a 4" lift kit waiting on me
at the house! Ye Ha
we can also rule out that the intake is sucking oil outta the lifter valley
cause this affect all cylinders.....not just #8.
The only time that becomes an issue is when the heads have been milled
excessively creating a gap at the bottom of each intake port.
angle milling remedies this phenomenom.
for a moment i was thinking that maybe the vacuum line for the power brake
booster could be plumbed into the #8
intake runner and when combined with a
defective brake booster/master cylinder
would draw brake fluid into the #8 cylinder causind said fouling and loss of
brake fluid as well.........Christ almighty
The vacuum line comes out the bottom front of the carburetor and goes
directly to the transmission.That doesn't appear to be the problem,it just
affects the shifting.The brake booster line is plumed into the back bottom
of the carburetor.The T goes to the other side of valve cover with line
attached to PVC valve, into valve cover above number8 cylinder.It seems like
I have way too much vacuum. The vacuum advance line to the distributor has a
separate outlet to distributor plumed into intake manifold.I tightened a
hose on line to modulator going into tranni, replummed brake booster line
making the inline filter vertical.It has been running fairly good all day.No
cylinder fouling.Still getting more vacuum to back of carburetor and brake
booster.Could it be too much vacuum.It seems to be sucking lots of internal
gases from valve cover into carburetor.The hose from valve cover PVC valve
seems to be more wet than normal.Still working on it--thanks for all the
info.Helps to have new ideas.I'm not giving up. What I think I need now is a
drawing of vacuum pluming to 81 Chevy 350.
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