I obtained this bike used. I thought it had a carburator issue and would be a quick adjustment. Cleaned the carburator (completely dissasembled) no improvement, even purchased new carburator. No difference- actually, a little worse. Bike has to be chocked at all times to run, is very hard to start and idles rough. When starting, it appears to periodicaly have a point in the cycle that it will have an event like a backfire, but it will actually kill the bike completely. The intake manifold looks fine, O rings look fine. No obvious air leaks. The air line connected to the intake manifold generally has suction when attempting to start, but will periodically have positive pressure during the cranking process. With my limited knowledge, I believe the bike has a timing issue.

My plan is to evaluate and adjust the valves. I don't have a manual and cannot find one online. Does anyone know the appropriate spacing? 250 cc VBIKE 4S

Just so you know, it has a new spark plug, new carb, good spark, new air filter. If this fails, I will see what I can do to have the compression checked. Or change my plan if someone parts some other wisdom upon me.

Thanks

 

this video might help, the engine is different but the process is the same.....
YouTube - Chinese motorbike adjust valve clearance 1

The valve clearance is: 0.04~0.06mm(cooled) for most 250cc quads.