Motorcycle Repair: motor scooter runs poorly

Question
I have a Schwinn Newport 150. Schwinn is a brand of Pacific Cycles. Going down hill at speed (above 40 or so) the motor feels like it stops for a second and immediately runs again. This happens over and over. Many times each minute. Feels like some bad fuel, but it has been with me for over a year. Local mechanics have looked at the carb and reported no problems. They said that both jets in the carb are ok and they could find no other problem. It works ok when the motor is pulling the machine forward, but the failure is when the motor may have to provide some braking - if that makes any sense.

Thanks for your help.

Ned

Answer
To me it sounds electrical, maybe a short in a wire.
How does it idle?   It may be the air jet adjustment is to rich.
Change the spark plug it could be fouling out.
Check the fuel level, it maybe to high. I wish I could help more, but its tough not seeing or
listening to what its doing...