My 1980 sportster back fires when started but after its running its OK.This just started what could cause this. Thanks JL

It sounds like ignition to me. Most likely coil or stater. I would start with the coil.

You didn't give me a lot of information. I'm assuming that by "back fires" you are talking about the engine spitting back out of the carb. This is the first place I'd start looking. If the bike has been allowed to sit up for an extended period of time, the gasoline would have evaporated in the float bowl. When it does this, it leaves behind the thick gummy additives that the refinery adds to the gasoline. This mess clogs up the jets and results in a very lean mixture which causes the engine to spit back through the carb and stall. I'd take the carb off the engine and completely disassemble it. Soak it in a professional grade carburetor cleaning fluid overnight and check every passageway and orifice to make sure it's clear. If you're uncomfortable in doing this, take it to a professional or at least someone with experience in dealing with the internals of the carburetor. A service manual will help if you still have the original carburetor on the engine. Also, the intake seals, called "rubber bands" on this model bike, may be leaking causing the lean mixture on one or both cylinders. While you have the carburetor off, this would be the ideal time to replace these as well.

Good Luck
Steve

Check to see that there are no exhaust leaks as this is one of the main causes of back firing.

Check your spark plugs for damage at the electrodes; state of plug wires; coil and ground connections... Could also be lean air/fuel mix and/or low float level causing it to run lean.

There is a drain plug under the back end of the primary cover. Drain the oil and refill through the primary chain inspection hole. There is a "fill level plug" on the outer primary towards the front. Remove this plug and fill until oil runs out of the hole. Should take almost a whole quart when bike is standing straight up.

Good Luck
Steve

do a basic test .try starting the bike, then take out a spark plug. is it wet or dry? if its dry, no gas getting into the combustion chamber. if its wet, place the thread of the pluged in spark plug against any engine metal, crank the engine and look for a good blue spark. no spark or an orange spark means ignition problem.

Timing is off. If the timing isn't exactly right it will turn over until the battery goes dead or the starter burns up (which ever comes first). The fuel/air mixture has to be ignited, expelled from the engine,etc. for the bike to run.

two amps and get a 2 farad cap please because if u dont u could melt wires together cause a fire or toast ur alt.