[MikeK]

Let's look beyond the carburetor.

1) Burned or worn exhaust valve seats will cause backfire.

2) Intermittent electrical connections will cause backfire by causing spark misses that pump unburned fuel into the exhaust. Check:

• Upper to lower dizzy plate connection

• Armored cable internal wire and dizzy body contact

• Ground connections in multiple places

• Did you add a tranny bolt to frame ground? That will help with both cranking and intermittent issues with ignition and generator/alternator