1991 Sportster wont start backfires

Check The V O E S

Vaccum
Opereated
Electrical
Switch

Where do I find this ?

Should be under the tank...has a vacuum line from the carb to it...here's what one looks like...

1991 Harley-Davidson Sportster 883 Hugger XLH883 Vacuum Operated Electrical Switches (VOES) | J&P Cycles

Some info about what it does....

http://www.wildwestcycle.com/f_voes.html

Thanks for the help everyone, I give up.

Like said before...Fuel, timed spark and compression and it'll run...It's not difficult to rule out what it isn't....Drain the tank and put fresh fuel in...rules out fuel as the problem...If you don't have a compression tester,put your thumb over the spark plug hole and crank it,if it tries to blow your thumb outta the hole,you've got enough compression to start it,then onto the ignition system(spark) with a multimeter you can test the components...I don't think it's your alternator or VOES...I think from what you posted it's in your ignition system(intermittent spark)a little patience and a multimeter and a few guys here could walk you through the steps to check the components...
If you want to work on your own bike Service Manual and Parts catalog are essential... Trouble shooting procedures are all in there...

Didn't you say your carb had just been rebuilt? Don't mean the guy you had do it didn't make a mistake. Happens to everyone sometimes.

I will bet that its the pick up coil behind the points cover is melted or gthe hall effect wheel has to be broken