Hello i have a linhai 300 of the 2004+- era. I had a cluch problem so we changed it. After changing it has a hard time to start going. When you pop it into gear and press the gas it barely drives but when it starts going about 20km/h it start driving well the acceleration is normal and everything do you have any idea of what could be the problem

 

Firstly is it a bike with a clutch or is it belt drive? Admittedly some belts double as a clutch but it is far less confusing to call a belt drive a belt drive. If you have changed the belt, did you take the pulleys apart and clean them. Grease in the bit that slides on the shaft is best left alone, but nearly all roller assemblies should be clean and assembled dry, as should sheave faces. If your machine does have a separate, lawnmower style, centrifugal clutch, this may be worn out. If it doesn't, you are supposed to shim the pulley assembly so it grips the belt at the right revs, when you fit a new belt. Never done one, but Kawasakis have this system, so there should be U Tube videos of how to do it. However far more likely to be dirty or stuck sheaves, or the wrong belt, than needing re-shimming.