Motorcycle Repair: engine backfire, 883 sportster, palm springs ca

Question
QUESTION: I have a 95 883 sportster with a S & S carb. when I turn the throttle, rpm's increase and then it backfires then shuts down and a little vabor come out of the carb.  Thank You

ANSWER: Did you just install the carb or has it been ont he bike for a while? Have you made any changes to the bike recently? Pull the plugs, what color are they?
Mike

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QUESTION: The carb has been on the bike for awhile, the only work I have done is change the iginition switch. I did have the bike in Williams, AZ.
(6000 ft.) and the mechanic there may have adjusted the carb for altitude but I do not know, now I am back in Palm Springs,CA (300 ft).

I have been riding the bike short distances since I've been back and the problem just started.  The plugs are new (500 miles) and  I got them from Harley.  Thank you for your quick response.

ANSWER: You never told me what color the plugs are. I am wanting to know if they are fouled
Mike

---------- FOLLOW-UP ----------

QUESTION: Mike, both plugs are pretty black and kind of sooty  Thanks, Jim

Answer
I would start by check the OHMs on the coil and plug wires. That might be why you are fouling plugs. If they check out fine then check the intake manifolffor leaks. If that is ok then I would check the jetting on the carb
Good luck and happy riding
Mike