I recently purchased a 1978 Peugeot 103. When it's running properly, it will hit 60 - 65km/h (37.5 - 40mph). A few days after I got it, it started to miss. It sounds a bit like a WWII fighter plane going down in flames (runs for a bit, misses for a beat, runs for a second, misses, etc.). This problem does not happen at idle. It does happen regardless of whether the moped is on the stand or being ridden. Compression is 120lbs/square inch.
Here's what I messed around with before the problem occured (I was trying to get it to idle smoother and eliminate a rattle when it idled):
1. I cleaned the carb. Twice. There was no gasket between the carb and the engine so I put some of that Insta-Gasket stuff on it to create a better seal. I made sure the gasket material is not blocking the fuel/air passage.
2. I changed the spark plug. I used an NGK BPR6HS which, I believe, is the correct plug. Gapped at .030.
3. I took the variator apart and cleaned it.
I originally thought that the problem might be a slipping clutch in the variator. I've eliminated this, though, because I've cleaned the frikkin' thing about ten times now. I've degreased it thoroughly with brake parts cleaner until paper towels wipe clean. I've scuffed all the mating surfaces with 100 grit sandpaper until their shine is gone.
I do know that the original petcock is shot. It will not close. There is a lawn mower fuel valve placed in the fuel line to allow fuel shutoff. Fuel seems to flow at a pretty good rate. The tank doesn't seem very rusty. Sometimes when the ped is on the stand (not running), it will drip fuel (very slowly), sometimes it doesn't. I notice that the occasional, small, air bubble works its way into the fuel line when the engine is running.
Are you still reading? If so, do you have any suggestions?