IMO its either an air leak or running lean they are the onlt 2 things
I can think of atm as to why a motor would back fire Ill add timing as well
so theres 3 things to look at.
Get the timing just of pinging and you'll be close to the ball park .
are you blowing the air cleaner box off yet??
Have you Y pieces instaed of Tees that will aid in water flow and less cavitation
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