spark plug fouling

[btorear]

have a 1974 f 100 with a 302. I bought it last weekend. It ran very rough, backfiring, stalling, engine quitting at stops, etc... changed the plugs and plug wires, distributor cap and rotor when i got home. one plug (for the number 3 cylinder) was fouled with caked up with a lot of black soot. Changed the plugs out and ran it a couple of days. It ran very well. pulled the new plug out from #3 just out of curiosity,and the new plug is all black with soot like the old one, just not quite as bad. Any idea what could be causing this? The other plugs look good, it's just this one. Also, the engine races while idling, could these two problems be related. I would really appreciate any help anyone can offer, thanks in advance.

[85e150six4mtod]

Old number 3.... Could be a valve seal. Stockers are crapola neoprene umbrellas inside the springs. They harden and crack on the showroom floor. (well, maybe a little longer...) But, they are easily replaced, even with the heads on the vehicle. (a recent posting told the tale, I'll try to find it and post the link) If you pull the heads and redo the motor, you can have some good seals put in there. Now this is all if you're lucky.

If you're not too lucky, you have a broken ring and a nice block ruining groove in the cylinder. Hard to say till you tear it down.

Meanwhile, find a hotter plug.

High idle: Check the high-idle dash pot or solenoid. Check for vacuum leaks. (spray some WD40 or carb cleaner around the manifold, carb base after you run down the obvious hoses.)