when and how does it backfire?
at shut down
gear shifts
engine braking
is it a poof or a pop or a KAPOW?  

backfires are usually cause it's lean or exhaust leak.

I got a supertrapp that's so tight... (how tight is it) it refuses to backfire even when the idle screw is completely closed off.  On shut down I get a little poof, on engine braking I get a nice gurgle and an ocasional pop on upshift.  

I got mine off Ebay.  The sound difference is night and day.  My bike finally sounds like a real bike.  As far as the back fire sound it sounds like a lound POP!!  Every time I lay off the throttle or decelarate.  At first it was fun to scare little children in the neighborhood with the loud backfire but now it's just annoying.

maybe someone mentioned this already. Have you tried rejetting? I used a stage one dyno jet kit. Fixed my leak and put the 146 main jet it. Presto, sometimes a pop on shut down, the rare crack on upshift.

That was my next step.  I see a lot of different "High Performance" jets out there.  Some say Stage (or Level) I, II, III.  What does that mean and what should I get?

Lancer, regarding the "where you live" portion of your post...  

I realize it isn't an exact science, but take a guess at how much change in altitude is needed to start noticing a difference in how the bike runs.  500 feet?  5000 feet?  I'm planning a trip this weekend that could take me from altitudes of 1500 feet to 7000 feet.  Just wondering what effect this will have on the bike.  I'm already running rich and can't seem to cure the problem.  I'm afraid if I get way up in the mountains, I may have to pull over and put in a smaller pilot jet.  That would be a pain.

One last question.  Where exactly is the idle mixture located?

On topic, I'd vote for an air leak at the head to head pipe or head pipe to muffler as this allows air (with 20.8 % oxygen usually) in the system and burn rapidly (very rapidly) any fuel left there from the closed throttle or shut off ignition.  On altitude re-jet ? I've run mine from sea level to over a 9000 ft pass and it is set for 1200 ft which is the altitude where we live. It ran a little lean in Seattle area and I'm sure rich over the pass, but just kept on thumpin' in the snow in July. Years ago when I went to Auto Tech school in Klamath Falls OR, we used the 5000 ft altitude there as the cut point to rejet, we did many cars in the Tune-Up class using a chassis dyno to verify we didn't go too lean, this was before the days of Emission controls except for a PCV valve. Modern stuff, cars truck and now bikes with Fuel injection and O2 sensors check the barometric pressure on start up and at specified RPM, throttle setting, & load, check it again to use a different look up table in the computer to correct the A/F ratio on the fly. The O2 sensor(s) fine tune it from there like the scales of justice, out of balance lean add a little more injector on time, a little rich cut them back by a couple ms (milla-seconds) always varying so little you cannot feel it in the operation of the rig, but observable with a DVOM on the O2 sensor, GMM (graphing Multimeter), or scanner. Max