dies

ok, i've been having trouble with my motor. i got it about a week and a half ago, i've been through a total of about 5 tanks (my tank is a half gallon size). and i've maybe been topping at 17MPH, and then it slowly loses power until it maybe maxes 10 mph without me pedaling. i've been thinking...

This has recently happened to my '82 Hobbit were it will only idle with the choke on, and then quickly high rev and die with the choke off unless your giving it a bunch of gas. Is there a fix? I've taken it into a shop cleaned the carb twice, just put in a new crank so I highly doubt there is a...