Ax100 won't rev over 4000rpm problem

My Suzuki ax100 was running perfectly when got it then I replaced the fuel lines and it ran like shit and it wouldn't go over 3500-4000 rpm under load but in neutral it revs fine so I bought a new fuel line and new spark plug and sanded the coil connections and cleaned the carb and a whole heap of stuff and it still runs like shit, any suggestions that might help

Ax100 won't rev over 4000rpm problem