Problem...

the guys hit it on the head, it's the squirter size. first off with the motor OFF move the carb to full open and hold it there with something or someone. now take a .016" feeler gauge and see if it slides between the pump lever and pump cam easy. if it does you good to go. if it doesn't adjust the pump spring until it does. next start by installing larger squirters until the stumble goes away rapping it by hand. next take it out to some back road and try it out, if it still stumbles increase the squirter size until it's gone.