You haven't clearly stated under what conditions it backfires. When I had the louder, straight through slipons on my bike, it would pop a bit at about 4500 rpms when decelerating if I cracked the throttle just slightly. I almost had to go out of my way to make it happen (okay, I often went out of my way to make it happen 
). I've never been able to get any other sorts of backfires from it. My bike has an open airbox and the largest of the three FactoryPro jets with the needles one position leaner than the middle position. It's a bit rich on hot days and at high altitudes, but otherwise it runs just great.
Now that I have lightly modified stock cans on the bike, with a little more back pressure than the slipons, but less than stock cans, the bike no longer pops at all.
So please give us more details about the conditions under which your bike backfires.
If it were me, I would tune the main jets so that it runs right at full throttle, then adjust the needles for part throttle operation and set the idle mixture screws for the best behavior when going from throttle closed to a cracked throttle. Then I would stop worrying about the backfires.
If it really bugs you, you could try to get a little back pressure from the pipes.
). I've never been able to get any other sorts of backfires from it. My bike has an open airbox and the largest of the three FactoryPro jets with the needles one position leaner than the middle position. It's a bit rich on hot days and at high altitudes, but otherwise it runs just great.Now that I have lightly modified stock cans on the bike, with a little more back pressure than the slipons, but less than stock cans, the bike no longer pops at all.
So please give us more details about the conditions under which your bike backfires.
If it were me, I would tune the main jets so that it runs right at full throttle, then adjust the needles for part throttle operation and set the idle mixture screws for the best behavior when going from throttle closed to a cracked throttle. Then I would stop worrying about the backfires.
If it really bugs you, you could try to get a little back pressure from the pipes.
