My h22a will backfire sometimes when i shift.No cell codes.i checked the plugs,and they look fine,no excessive back soot on them,they look grey in color,im running type s pistons with a milled head.any ideas??possibly lean backfire??

 

normally, a backfire is due to too much fuel entering the chamber. I would have to think that there is a spot in your RPM range where the fuel is a little rich and when you down shift it causes the fuel to "pool" in the chamber, your tip in fuel comes in and mixes with that causing a rich condition which then creates a back fire when it exits into the exhaust.

I know run-on sentence...

 

not a worry, most cars have a mild backfire due to the rapid opening and closing of the tb. the higher output your motor has, and thus higher fuel demands, will increase the likelihood of a "pop" coming from the exhaust. keep building the motor and soon you'll shoot a small flame out when you shift.