I have an 80cc unbranded Chinese bicycle motor. When I first got it it was working fine. I eventually upgraded the carburetor to a racing one which seemed to work fine. Later, it started seriously lacking power to the point where it wouldn't go up the smallest of hills. I even hold the clutch in and rev the engine and it doesn't rev nearly as high as it should. I've cleaned the carburetor, adjusted the throttle, cleaned out the exhaust (even tried running it without an exhaust and still had the problem,) replaced the spark plug, and even replaced the engine. But I'm still having the problem. It's also worth noting that now it won't even idle even with the idle set as high as it goes. What could be my problem?