Carbon fouling, other problems in cylinders even a damaged gasket "fire ring."
Timing retarded
Lean idle mixture
Too fast idle speed
Too hot spark plugs
Cooling problem, either system running hot, or something restricting flow causing block hot spots, IE rust/ sludge/ corrosion/ deposits/ etc
Something "odd" like an exhaust restriction can cause odd problems and is off the radar. Does it seem to have the "power" it should? I know slants are an "impressive powerhouse," so ??
