2004 HD sportster. bike sit while deployed for 7 months. Tried to start it and it cranks over and backfires through the exhaust and carb and will not start. Rebuilt carb and changed intake seals. Still backfires and thows flames out the exhaust and carb while making this god awful bang.

Cab or injected ?
If carb looks like an leak at the Oring manifold .Try to start and spray any carb cleaner to the manifold close to the cylinder ,.any change on idle would be an air leak!

Wasted spark system. If fuel and electrical is all good/correct it is often the case in these symptoms that motor cycle spark plugs that although looking good and firing outside of the engine are breaking down and will not fire under compression pressures on a power stroke but will fire unburned fuel on the unpressurized exhaust stroke when the plug again fires in a wasted spark ignition system and when so firing the result is a backfire, usually quite loud.

There is probably a small leak in the intake tube to that cylinder. Remove and carefully inspect and reinstall carb and induction manifold.

It's still too lean. Check the timing, too. Don't use old fuel, does this bike use high test?

By the term "backfire",are you referring to the engine spitting back through the carburetor or popping through the exhaust system? You also didn't tell me what type of carburetor the bike is equipped with. A lot of the owners of early Sportsters didn't like the original carbs and replaced them with aftermarket carbs. Since the carburetors use different starting circuit designs, it makes a difference in how the engine is started. It really makes no difference and I've heard many bikes that do either or both of these things when starting. What does make a difference is how easily it startes and how it runs AFTER it starts. If it's simply a spit or a pop, this is not unusual. Does the bike run the way it's supposed to? If not, that's what we need to look at.

Good Luck
Steve

sorry i cant help.. try harleytalking.com they have helped me .. good luck ';';';'

Is it backfiring through carb or exhaust? Has it been sitting for any period of time? We suspect carb issues. Try cleaning carb, pay special attention to the pilot jet and main jet circuits. Clean with spray carb cleaner and air, do not use wire. Make sure needle and seat are clean.

Replace the battery.

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

Sorry, could be a clogged up carb from sitting or a bad ignition box. Have you got a pal with the same bike you could change out the carb to try? If the bike sat for quite a while, it almost certainly needs a carb cleaning and service.