5.7GL one spark plug gets fouled

[Ricky_rich]

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.

 

[Bondo]

Ayuh,..... Did you do a compression test, when you did the leak-down test,..??
What were those numbers,..??
That distributor cap is beat,....

 

[Scott Danforth]

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]

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]

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]

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.

 

[Ricky_rich]

I did not do a compression test only leak down at this time. But if leak down is good, I would not have bad compression, right?

I checked the spark plug cable, and cylinder 2 have 21.7K. Cylinder 1 that is the same length have 21K. Cylinder 4 that is shorter have 16K, so I guess the cables are good.

I have ordered a new cap and rotor. Will also order and replace the valve seals. Think that is a good start right?

But do you have any idea about that BOOFF’ing sound? Air/fuel get ignited in intake or exhaust. Could it be missfire from a bad cap? Getting a spark while a valve is open?

 

[99yam40]

my only thoughts are, if it does not burn in the cylinder as it is supposed to it will burn in the exhaust when it mixes with the other cylinders exhaust