I have 2 mopeds and they both have the same problem. After a long drive full out, 15 to 20min. They starts to chug allot and drive at half the speed. If I let the ped sit for 10 min it drives fine, but after another 20min of driving it starts up again. I was using supreme gas and synthetic oil. Since then I changed to reg gas and reg 2stroke oil. The peugeot did the same choking thing after 15min, but cleared itself and didnt lose half its speed. I find the spark plugs were black and dry when i used the synthetic oil and super gas. I havent done a plug chop on this new "regular" mixture im using.
p.s. the other moped is a motobecane 50v 1970, if that makes any difference. It takes longer to act up...the chugging...the motobecane also backfires when the chugging goes on