I may as well throw my 2 cents out there from what I have seen. IF timing is off you can get backfire through intake, (was mentioned). If intake valve is incorrectly adjusted to too tight, you might get backfire through intake. If you (and I have had this)have water in gas you will get bucking engine, backfire in intake and an engine that hardly runs and sounds like its about to come apart. You describe my truck after I sucked in a ton of water that was in teh fuel tank, I really thought I had a mechanical engine problem, sounded like pistons or valves hitting either each other or something elese inside.