It could be that your fuel tank is being sucked of what ventilation it has in supply over that 20 minute period. Happens often actually.
Be sure your fuel tank vent tube on the cap is not restricted.
Then take the cap off the tank and verify that it is venting by blowing the hose.
Blowing hose never sounded more sociably acceptable huh?

 

check to see if the vibration shook the screws off the piston in the carb, might not be getting air, happend to me