Make sure the gas cap vent is open and the gas line is not pinching closed from vacuum.
You can try heating the coil with a hair drier or heat heat gun to see if you can stop the motor from running in less than 15 minutes... being careful not to over heat the coil.
Check the inside of the tank to see that it's clean.
Change the gas if it's been in it awhile, some states don't require their gas stations to have signs on their pumps stating their gas has a percentage of ethanol in it.
Less likely but If the engine has a low oil engine stop switch and you don't have enough oil in the crank case going up or down hills could shut the motor down.
I'm not a mechanic, just throwing out ideas.