Well, I checked the timing and that doesn't seem to be the problem, engine still backfires. With the vacuum advance disconnected, it is set at 10 degrees at 1,000 rpm, and at 2600 rpm the total timing is 30 degrees. At 2600 rpm with the advance connected, total timing is 52 degrees, which all fall within the reasonable range for the engine. I guess my next step is going to involve taking apart the motor...