I am getting a backfire from the pipes about one every couple seconds. My plugs are dark like I am getting too much fuel. This bike was sitting for many years... Here are the knowns... New cylinders and pistons, valve adjustment had no efect on the backfire. Compresson is 121, 123. Timing change had no effect on the backfire. The carb has been rebuilt though I think it may be the sorce of the problem. Any Ideas?