I have a single cylinder, 200cc, carbureted, 5 speed, 12V, 2006 Lifan LF 200 motorcycle that typically runs fine. It will sometimes randomly lose power and I cannot get it to go above 25 mph. When this happens, my bike appears to lose all torque above 4000 rpm and I cannot get the engine to spin any faster in any gear unless I am going down hill or down shifting. When this happens, even in first gear, I can feel my bike weakly accelerating then once I hit 4000 rpm it loses all power. When I open the throttle and it is maxed out at 4000 rpm, the bike shakes as if it regains power for a split second and then loses it again. It feels like it is trying to pull away. This happens randomly both on cold starts and in the middle of a ride. After 5-20 minutes of being bogged down, it will suddenly regain power and pull away (I will hold the throttle open for 10 minutes and suddenly the revs go up again and I have full power).
If I disengage the clutch, I can fully rev the engine, therefore I don't think it is a problem with fuel delivery. My fuel tank cannot have negative pressure so that cannot be the problem. I checked my brakes and they are not engaging by themselves so that is not the cause. If it were a slipping clutch I should still be able to rev higher, just not transmit the power. I replaced my spark plug. I cleaned the air filter but I may try just removing it and see how it runs. I checked my oil and it is fine (I am considering loose shavings or something jamming the transmission as a possible cause though).
Any ideas would be greatly appreciated. I haven't been able to find any answers online and I asked two mechanics and they could not give me an answer (although they did not do any in-depth checks on the bike).