Hello. Im new here. I have bought lifan 125cc p54fmi engine. And i was wondering about its top speed. Bore x stroke= 54mm x 55.5mm. Gearing is 15/35. 18inch wheels. 1st 2nd and 3rd gear i can hit MAX rpms... but when i shift to 4th gear i cant get pass 5500rpm and 75km/h. Some of my friends can hit 90km/h with 110cc engines. So is it CDI.. is it carburetor ( i have stock 19mm carb) or is it simply that the engine is not strong enough. My weight is 75kg. I would be very happy if some one could help me out. Im planning on buying bigger carb cuz maybe it is lacking air/fuel. And maybe racing CDI cuz "maybe" it is limiting my 4th gear RPM's.