Mazdaman, I tried to e-mail you but it wouldn't go through. I upjetted the Main Jet to 135, Pilot Jet to 45, and moved the e-clip to the fourth position. The bike uses more gas, but the backfire is 90% eliminated. I have been told that backfiring is a common problem with the 300's.
If you want to e-mail me, I will type out the procedure for adjusting the e-clip. (bill.mcculloch@shaw.ca). It is pretty simple, the carb can stay right in the bike.