I've been running YZ timing with the following setup all summer long with no problems until now: Current setup: Main 172, YZ needle at 4th clip, 48 pilot and fuel screw 1 3/4 out. This setup was run from 6000 to 13,000 feet. However, I recently began to experience popping on deceleration. So I set the fuel screw to 1 1/2 out. This seemed to help. Then on my last two rides (6000 to 8500 feet), I have been experiencing backfiring on deceleration. Last night I changed my main from 172 to 170. Am I going in the right direction here? Pulled my plug and it appeared to be tan (if not leaning a smidge towards white). It was black around the bottom of the rings though. Any thoughts on this?