Spark plugs

[Hottrodder427]

Hi all, I need spark plug help. I am running auto lite 3924 with a gap of .040 two of the plugs are covered in black soot, the rest look good. Also my gap seemed to have changed gotten smaller. I used a bore scope and don't see any damage to the pistons. Any help is appreciated. 390 Fe bored .030 over aluminum heads and intake zero decked SummitRacing distributor summit racing ignition box Taylor 8 mm wires Holley 850 dbl pumper mech. Secondary

[NAZ]

Hard to troubleshoot spark plug issues by just a description. Good photos of your plugs (good and bad) would help. The heat range you're using is about middle of the range. The last number indicates the heat range: 3927 is hot (long insulator) & 3922 is cold (short insulator). There is a wealth of information on line now days to help you read plugs. It used to be easier to tune your carb by reading the plugs when we had leaded fuels but dry looking soot may be a rich condition while wet black may be oil fouling. Try a Google search and you should find many photos of plugs with various problems and match as close to what you see on yours and you'll have a good idea where to start looking for a problem.

[Hottrodder427]

I have been googling, not one looks like mine lol. My luck. It runs good I just want the most efficient burn

[Bob Cowan]

Soot usually means a rich condition. But if it's just two pugs it could also be oil from a leaky valve seal.

[Hottrodder427]

Brand new heads, and held compression

[NAZ]

Without photos it's hard to say but it's not unusual for one or more cylinders to be rich and others to be leaner or on the money with A/F ratio when running a carb. If that's the issue here and you can't find a happy medium between all cylinders you may have to go to a hotter plug to prevent fouling. Also, don't dismiss Bob's comment above. If it appears wet it's likely oil and the prime suspects are valve seals or rings. Yes I read that the part about new heads but you have to follow the evidence trail regardless if you have new parts. Not sure what "held compression" means but the best way to check cylinder health is with a leak-down test. I always get a baseline reading when an engine is fresh so I have a point of reference down the road. For your two cylinders that have sooty plugs you can compare leak-down on all the cylinders with these two to determine if you may have a sealing issue on these two.

[AC Bill]

Well I sure hope that the plug gap looking smaller, is just the soot build up..:confused:

Or were those two plugs not gapped correctly from the get go? If the spark is weak because of an incorrect gap, it may cause those cylinders to run cooler. Thus the carbon build up.
Just a guess..