cylinder misfiring

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?
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type of truck? year?

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The truck is a 1981 Chevy 4x4 K10 350 engine.Sorry, I quess I left out some of the most important info. Any ideas?

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Run a compression test on #8 yet?
Doc

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No, running a compression check is the only thing left that I haven't tried.

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Thanks.Tomorrow I'll run a compression check.I'll let you know if that solves the problem.Thanks again!

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the compression test wont solve the problem ....but it will help u isolate it. 1. vacuum leak will not cause plug fouling 2. i doubt the carb is your problem since you stated that the plugs are white 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 problems. 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
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Mad-Dog
'79 Chevy K-10
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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
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Mad-Dog
'79 Chevy K-10
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Ran into a problem like this on an Old Buick. Was the transmission modulator. Killed only one cylinder.
Al
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Yep, early gm's had the vacuum modulator and brake booster plumbed n2 the same intake runner...... but i can,t r'member whether it was #7 or #8
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Mad-Dog
'79 Chevy K-10
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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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