im having the same trouble with my ohc sprint
first time was the power valve or plunger .then a slit in vacume line
rebuilt a new carb 4 it still have to put it on lol
its like u have to feather the gas some times right.
could be a pice of dirt but doubt it . resticted fuel filter
bad fuel pump check oil make sure your not getting gas mixed in the oil to could be costly.
these r just guesses of corse just some of the trouble ive had lol good luck and message wt it was