A coil will fire fine until the engine is warmed and the heat causes the coil to expand internally and quit firing. However, I have not seen a bad coil that would continue to fire after warming up. The ones I have been around would cause the engine to die(or quit firing). Of course after you sit on the side of the road long enough for the coil to cool off, the engine would fire up and run great until the coil heated up again and the engine would die. If you rev the engine a little while parked until it warms up, does it start to lose power or is it only under a load? The backfiring through the carb indicates a change in timing. At first I thought it was the dist cap, but now I am leaning towards a bad coil wire or coil although it would be the first bad coil I have heard of that would hold current while warmed up. Joe
