5.7GL one spark plug gets fouled

Ricky_rich

Petty Officer 2nd Class
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Mar 4, 2014
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Volvo Penta 5,7GL PLKR
Holley two port carburettor.
Pertronix ignition module
Pertronix high voltage ignition coil.

I’m having some problems here. One spark plug is getting a lot of soot on it. (Cylinder no 2.) And after a couple of hours, it’s not giving any spark. If I put a new plug in engine runs fine again. A few years ago I could run a whole summer on a set of spark plugs, now I'm down to a few hours. So what ever it is, it's getting worse.

Sometimes when accelerating I hear a BOFF sound, like if the air/gas mixture was ignited in the intake or the exhaust.

I thought it could be a bad valve so I did a cylinder leak test. Cylinder 2 have about 5% leak, mostly down past the piston from what I can hear. I did the test on a couple of other cylinders and the all show about the same leak %. Also reset the preload on the valve/lifter, but it didn't make any difference.
 

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Scott Danforth

Grumpy Vintage Moderator still playing with boats
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Plug 2 is oil fowled. Could be as simple as a bad valve seal

The cap is about 100 hours past its replacement time (it's toast.... Burnt toast). You see the corrosion AND the carbon tracking
 

Lou C

Supreme Mariner
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also check the resistance of all the spark plug cables, usually is 1000 ohms per foot of length, if that one is way higher than the others, might be your problem.
Many years ago (1990) my new wife had a 1988 Toyota Camry 2.0 liter twin cam. One day it starts running like a 2 stroke. So just on a hunch I ohm out the plug wires and 2 are like infinite! New set of wires and it was all good again. It was literally running on 2 cylinders.
 

99yam40

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also check the resistance of all the spark plug cables, usually is 1000 ohms per foot of length, if that one is way higher than the others, might be your problem.
Many years ago (1990) my new wife had a 1988 Toyota Camry 2.0 liter twin cam. One day it starts running like a 2 stroke. So just on a hunch I ohm out the plug wires and 2 are like infinite! New set of wires and it was all good again. It was literally running on 2 cylinders.
did that cause the plugs to carbon foul like the OP is showing?
I don't think so.

the plug he shows is burning oil is my guess.
rings or valve seal
 

Lou C

Supreme Mariner
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Nov 10, 2002
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14,516
Don't know.
it was 36 years ago lol.
yep could be that as you say but the distributor cap looks bad, a weak spark due to bad cap/wire will result in a weak spark/misfire/not burning off anything in the cylinder.
 
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