Fouled Plug Cause

Started and idled good. Loses power and backfires after a few minutes of 3/4 throttle. Got better with less timing but still does it. Cant drive long over 45 mph.

 

This sounds like a lean-running issue. Main jet clogged in the carb?

 

Electronic distributor. Was originally advanced as far as it could go. Slowly turned it down while driving till I had better results.

I there is a scratch on the pulley and on the timing cover. But zero reference to degrees.
Most likely put there by PO.

 

If electronic distributors do that to sparkplugs, I'll stick with my points, thank you very much.

More seriously, haven't I seen here that pertronics etc, require resistor wires? What type are your plug wires?

 

Petronix wires. Not a petronix points replacement for distributor. It's a full electronic one.

 

Okay. I know nothing about that stuff. Except the couple of OEM units that have failed erratically in my trucks and drove me out of my mind trying to diagnose

 

That #2 photo with the electrode burned back is pretty telling. If the issue was caused by the timing being too far advanced, or the carb too lean, this would happen to more than one cylinder. I would check for a vacuum leak or something else that is making that cylinder run lean, or otherwise too hot.
-Donny