number of flush draw combos on flop
On the flop = number of flush outs x 4 = (13-4)x 4 = 36%. On the turn = number of flush outs x 2 = (13-4)x 2 = 18%. Obviously a naked flush draw is not a very good hand to move all-in with on the flop since you are most likely to be up against a much stronger hand if your opponent raises you.