I've got a problem with my Qingqi scoot with the 139QMB motor. Here's the story:
I was riding it along and suddenly it made a horrible sound and died. Wouldn't restart. Got it home (finally) and suspecting an engine issue, I tore down the engine. Nothing. So, I put it back together. Next, I checked the drive belt, and there it was; the belt was broken and bunched up under the driving pulley. I pulled it out and lo, it started and idled! So, I get a new belt and put it on. Now the motor won't start unless I give it full throttle, or put it up on the work stand so the rear wheel can turn freely--if I hit the rear brake while it's idling, it stalls. I'm thinking it's a clutch problem but I'm not sure how to go about dealing with it. Nothing seems tight, and everything seems to turn smoothly without sticking. So, it might be a timing or mixture problem. I'm at the end of my rope here. Help!