My 2004 sportster back fires when im down shifting or going down the road

If your bike is equipped with a carb and you can get to the idle mixture screw, turn the screw counterclockwise about a half turn. Also, check your exhaust system. If the gaskets at the heads or the crossover are leaking, they will "**** air" inside and cause the backfires though the exhaust.

If the engine is spitting out of the carb, your carb is jetted too lean. If the carb has a #42 jet in it, go up to a #45. You can also raise the needle in the carb by removing the needle and adding a single 3mm flat washer under the needle stop. The small flat washer is approx. 0.023" thick. The stop is the 'swelled' portion of the needle that keeps if from falling through the slide. Put a little grease on the washer and thread the long end of the needle though the washer. Then install the needle in the slide. This "raises" the needle just a bit enriching the mid-range mixture ever so slightly. You're bike will run much better.

Good Luck
Steve