Engine Misfire And Carbon Fouling Plug On Cyl. 4


New Member
May 13, 2012
Lincoln, Nebraska
Been trying to figure out a carbon fouling plug issue mainly on #4 and a bit on #8. All other plugs look fine. First will fill everyone in on what the engine is.
Block: 408w (351w stroker)
Heads: AFR 220
Intake: Super Vic EFI
Cam: Compcams XFI 248 hr
Ecu: PiMP (microsqirt basically)
I'll stop there for now as the list goes on. Checked compression, all cylinders are the same. Performed cyl. leak down test, was less than 10% leakage. Changed injectors to new Bosch injectors. Put new plugs in it (autolite 3922). Compression ratio on the engine is 10.1:1 so not real high. Running 91 pump gas. Been running the engine for about 2 years with no noticable misfires. Does run rough under 2500 rpm but is believed to be normal characteristics of the large cam. Ecu is batch fire on the injectors so i have a hard time believing that is the issue. Anyone have any similar experiences withthw two rear plugs carbon fouling? Or any other ideas would be nice. Getting to the point to where i may have to pull the head.
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5 Year Member
Aug 11, 2013
Cleveland OH Area
Are you sure they're carbon fouling? I've run synthetic oil for a number of years and I find I get dry deposits more often than oily residue on the plugs so it may be oil unless you're certain. 4 and 8 are at the back of the engine so the tilt and oil drain-back can cause some oil to run down worn valve seals, especially if there's some issue with the drain path. I've also had oil get into cylinders across from one another when an intake manifold gasket failed (though not on a 351w), but checking the intake torque never hurts. Could also have a problem with an oil control ring, again if it's oil.

Do you get smoke out the tail-pipe at start-up, or under other scenarios? I imagine you're using aftermarket fuel rails, is it possible their design is causing more fuel pressure at the back-side of the rail (if your fuel pressure is maybe low or uneven and the computer is compensating for the leaner cylinders causing richness in the back two). Just a complete guess but if it's been doing this ever since you installed the intake / fuel rails it may be worth considering.

If your leakdown test and compression test were good, I'm not sure what you'll find by pulling the head.


New Member
May 13, 2012
Lincoln, Nebraska
Well, the plug soots up after running the engine for just 15-20 minutes so I don't think it's oil as it would still be wet and would not have been hot enough to turn the oil to a powder. I looked down the intake runner with a bore scope and they all look good. Looked into the cylinder through the spark plug hole and noticed the top of the piston is more grey, like the surface of the moon, and there was a score mark all the way down the cylinder wall. Hard to say if its anything deep or carbon. The #3 cylinder is more black and carbon looking with some of the metal surface of the piston seen. This looks more normal to me. Afraid the heads are gonna have to come off to check the piston more closely.
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