what else besides a clogged muffler makes a moped backfire?
The timing being off.
Check and set your points.
Cleaning them wouldn't hurt either.
I'm no expert, but I've got some experience with my own backfiring.
Once it was caused by too rich a fuel/air mixture. More gas was going through the cylinder than could be efficiently burned, so it collected in the exhaust until it blew up with a most convincing racket. It ruined my exhaust baffle.
Another possible source is bad gas. I tried using some gasoline from my two-stroke snowthrower in my moped. I normally use 32:1 to 40:1 gas/oil ratio in my bike, but the snowthrower gas was 50:1, using some kind of synthetic lubricant that came in a little plastic packet. Further, the gas was a year old.
The moped backfired repeatedly, but stopped when I drained out that fuel and refilled with the normal stuff. I never did determine if the backfiring was due to (1) using a mixture that was too light on the oil, (2) using a lubricant that my Minarelli engine just didn't like, or (3) the gas had gone bad.
Re: Some ideas
It could also be a gap between the tailpipe and the exhaust manifold, or a hole in the tailpipe.
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