Motorcycle Repair: Engine performance, high octain, shell v power

Question
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Followup To
Question -
I own a 2003 Sportster 1200. It has a little over 4,000 miles.  I have had the Phase I download which was done approximatley 6 or 7 months ago.  The problem which I am experiencing now started approximatley 3 weeks ago. When I throttle, the bike periodically, misses and/or backfires.  I thought it was bad gas, so I ciphered out the old and refilled with Shell V Power High Octain. It ran better for awhile and I thought the problem was solved, but, it begun missing and backfiring again.  Last week I changed spark plugs from factory to Gold with 40 gap. The bike ran fine for a day and then it began missing and backfiring again.  The bike is still under warranty, but the Harley shop will not be able to fix the problem until sometime in November.  Can you advise?  P.S. I have not had any other problem with the bike.  Thanks
Answer -
Pull a spark plugand let me know what color it is or if it looks fouled.
Mike

I pulled both sparks plugs and both looked burned (Kind of a meduim black) but clean.

Answer
I would first look for a exhaust leak then a intake leak. Both will cause your problem. Start the bike and run your fingers around the exhaust where they attach to the heads. Rev the bike up and you should be able to feel it if its leaking. On the intake, spray a little carb cleaneer around the intake. If the RPM's go up then you have a leak.
Once they have been checked I would next test the ignition for problems. You might have a ignition going bad.
Good luck and happy riding
Mike