Does it have a fuel filter of any sort?It could be any number of things but
it sounds like its starving for gas after a short period of running.
Could easily be something else,but i'd start there.
Also,it could be the float needle sticking closed in the carb bowl.Has it been sitting idle for a long time?
digger2
P.S........Check for water in the fuel tank.This "new" gas draws moisture from the air real bad.I have problems with water in my fuel in my snowmobile every year even though i drain it at the end of riding season.