I got from scooterdogs an 80 cc.cylinder kit for my 50 cc. Rocketa scooter.It‘s aQMB139 moter.All went well runs .Had 2 rides only notticed it needed to break in.Now I cant get it beyond 3/4 throtle without it wantting to die,unless I lower the throtle quickly. The timming   is allright,spark continues, carberettor is fine .I gave it a spray shot of gas when it was dieing but no saving it.Ignition and carb. don’t seem at fault. Do you think a valve spring is broke? even though it seems fine up to 3/4 throtle,or about 3000 RPM.


Me again. I adjusted the valves and it still didn’t work past 3/4 throtle. I tried the spray gas in the carb. idea and this time it made a diferance’ for the better. so I guess it is carburetion after all. Funny it rode well the first 2 times’ but maybe since it wasent broke in yet and now is more so the piston is pulling in more gas now. Carb. is a CVK with # 78 h maine fule jet and # 34 h low speed jet. low speed seems o.k.

What do I do now scrappin dog? Hope you can help! Thanks

Without being able to see what has been changed or modified, it’s difficult to tell. I’m inferring by your statement that your spraying gas in that you’ve removed the stock airbox? This might explain some things. These airboxes are very restrictive to pass DOT noise regulations. When this restriction is removed in the pursuit of suppossedly more power, the vaccuum is grealy reduced at the venturi nozzle point, which leads to a lean-below stoic condition. This is very common for scooters to just not want to run when airboxes are removed. It’s even more common to hear all the subjective analysis that everthing was working fine “until” this or that was done, or it worked like that before eg. “it’s not the carb because I spuirted gas into the carb with no change”. The piston seal will improve with proper break-in, but not to that degree to make that kind of difference. I’d recommend replacing the stock airbox for baseline test. I normally install a #85 main whenever I remove the airbox, and even then, because of resonance differences, a substantial loss in bottom end power results.

Last of all, when all else fails, seek a professional that can tactilly repair your vehicle. (hard to find I know) If it was possible I’d invite you to Arlen Ness motorcycles where I consult part-time for a challenge fix. Was this CVK a stock carb or a later addition? MO!

Me again.You really know your stuff, I’m glad your there man.I put the air filter box back on like original.And it runs great now! I had taken it off in place of a new 3″round x 2″tall chrome air filter.Looked great,but I guess it’s running right is better than looks. I’ll bet you get in operation vs.looks with Arlen Ness sometimes.ha.ha.

Your not scrap to me man. Have a good dayand thanks