My 77 400 motor is oil fouling the number 3 spark plug. This normally take about 3-4 weeks before is starts running on 7 cylinders. I then remove the 1 and 3 spark plug and clean number 3 and switch the plugs. What would cause this? Bad rings or a bad valve seal. The motor has just turned over 100,000 miles. It still passes the smog check every year, but I need to fix the problem.
have you done a compression check, its probably leaking by the rings though. If you can get a scope look down through the spark plug hole and see if the cylinder walls are scratched up. You're sure its oil and not just carbon build up with some gas on it. Is it hard residue or can you wipe it right off when you take it out?