I have an 06 660 that I bought with an intake backfire. I rebuilt the top end with all new except for a cam. After all the work and retaining 3 or 4 times I have still not fixed the backfire. I have tried changing jets and haven't had any luck. Engine had 920 hrs prior to rebuild. I am starting to think maybe my issue could be with the old cam being worn throwing off the valve timing? Anybody think that could be a possibility?