50cc 139QMB 4 Stroke dies at full throttle  By  thebean (Read 2,799 times)

Hi all! Just bought my first scoot (Jonway 50cc YY50QT-6) a couple days ago and it came with an unexpected problem. Everything is fine from 0-60% throttle or so, I can get up to 30mph on flat ground. If I open it to full throttle, the engine loses power and if I hold full throttle long enough it dies.

I am really new to troubleshooting engines and am still earning, so please forgive my beginner lingo and questions.

I have:

1) Replaced the spark plug. The old one was a rich rust brown color around the top. I can post a picture if you'd like.

2) Opened up the carb. Cleaned it out a bit. It was in pretty good shape, but I cleaned the jets and float pin just to make sure.

3) Replaced the in line fuel filter, just to be sure.

My carburetor only has an idle screw, no adjustment screw. I'm wondering if it's running rich or lean but since I can't adjust it I'm wondering how that could be the issue (except for an air leak or gas delivery problem.)

Should I get a new carburetor for $20 or $30 with an adjustment screw? I ordered one on Amazon for the heck of it. If the adjustment screw came plugged by the manufacturer I can't imagine it's a good carb to begin with.

Any tips on what I could check next? I've had suggestions to check for an air leak, but I don't know how to do that exactly.

Thanks for all your help. I'm ready and willing to learn, just at a loss for what direction to take in my troubleshooting steps right now. Ready and willing to take pictures of any part of the bike or video of any of its behavior too.