Thats what I get for posting while someone is talking to me at work. Completely missed the gear drive part.
Still sounds like a timing problem, but since gears aren't exactly prone to do that......
When you say backfire, I assume you mean back through the carb.
If so, then it sounds like crossfire. You need to closely check the dist. cap and rotor, and make sure the plug wires have not come loose from their holder.
If, however, you mean through the exhaust, then it sounds like the ignition is cutting off briefly and then firing, in which case you need to check the same thing, but also suspect the ignition module.
BTW, I once, and only once, saw a gear drive jump time without massive amounts of wear or abuse.
I had just helped bolt everything up, got it fired off and set the timing. He blipped the throttle about 3 times, and it started backfiring and running like.....well, you know.
First, we thought the dist. had spun, but it was tight.
Checked the obvious, then the not so obvious. Finally realized the timing was way off. More than it should have be able to do.
Looked like a timing chain, sounded like a timing chain, heck it almost smelled like a timing chain.
Only one thing to do.
All the while we were pulling the timing cover, we kept telling ourselves that it could not be, we were just doing that to eliminate that possibility once and for all.
Brand new Woodruff key had sheered off and the gear had turned on the shaft.
Some days you just can't win.........
