I'm thinking I have an air leak. The timing checks out, I tried to run it with every possible needle and regulator set up and it still make that noise and I can see big bubbles when running at slow speed and very small bubbles in the line at higher speed that the line that goes from the front of the engine to the carb.I think that might be making it go lean and backfire.When it backfires there are sparks between the nuts that hold the prop on!