I think you're on the right track wih the heat riser, & if the fuel pump is leaking, of course it needs replaced as well. But, the fuel pump can't cause it to stumble "when you step on the gas" he carburator has a float bowl that acts as a holding reserve, so if the pump doesn't work @ all & the carb. is full, it will still run a couple minutes...I'm just mentioning this because its probably some other problem w the getting up the driveway issue. (How's the choke setting ? )
