Yep, exactly! I fought with this issue all summer long, only to finally find out it was due to a weak spark, caused by the dwell code in the MS Extra firmware. I havent tried out the version that was supposed to fix it, but I ultimately installed a Crane ignition CDI box, which completely solved the misfire/backfire. A rather expensive solution,but it works great. I still use the Mitsu ignitor to fire the Crane box.