This is because as the pilot goes bad people will adjust up the idle to get the bike to idle. What you have done is set the idle on the needle. Thats why the bike will idle but as you rev the bike the engine doesn't want to return to idle. If I was a betting man I would put my money on the jet. If the bike is that old seafoam probably will not work. Its a good product but if you could see some of the crude I have pulled out of neglected bikes carbs you would understand.