my '78 motobecane mobylette runs awesome until about 10 min. of riding then slows and cuts off in a matter of a few seconds, sounds like it could be a spark plug problem, but I am just getting into mopeds and am not really sure what the problem is, I have experimented with gas mixtures and different brands of oil, I switched from a champion spark plug (L86C) to a B5HS but no improvement, someone recommended a B6HS so I will try that also I'm goanna put on a fuel filter because it looks like ther could be a little rist in my tank. It has been doing this ever since I'v had it, which is about a year. any help would be help. thanks andrew.