[COLVERT]

As others have said, if the fuel tank breather is blocked the gradual build up of vacuum in the tank will eventually block the flow of fuel to the carburettor and the engine will slow down and stop.

It will be gradual as there will be a slight pressure generated by the fuel in the tank getting heat from the engine.

As the engine tries to slow after a while try loosening the fuel cap for a couple of seconds. If the cap vent is blocked and opening the cap increases the revs then you have located the problem.

Chuck the cap and get a new one.---