i would check to make sure that the exhaust is not clogged. take the mufler off and try to run it that way. Dont do it for too long maybe 3 minutes. if you do it too long you could seize the engine. so take the muffler off try to run the moped like that, if it helps that mean you probably have a clogged muffler otherwise it might be your fuel mixture. let me know if its the exhaust?