Motorcycle Repair: Rich mixture 2004 1200XL, octane value, rich mixture

Question
QUESTION: My 1200XL was in storage for over a year. After moving to a new location I Started up with no problems and ran it for a few miles. Upon returning from short trip my bike started to loose power. From that point on I have problem to run the bike. It starts up, but when I take off, I am to able to accelerate. Spark plugs fouled up and black. I replaced them, just to find out that new plugs got fouled up again.
I took carburetor apart - all jets are clean, needle valve and fuel level OK, air filter OK. Is there anything else I should check?

ANSWER: Could your problem be old gas?

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

QUESTION: Old gas drained and purged from lines. Obviously.

Answer
I can't think of anything that would cause this that you haven't already checked. During the winter your oil may have drained down into your lower end from your oil tank then pumped into the air cleaner when you rode the bike for the first time this season causing your plugs to foul and lowering the octane value of your fuel. But this would have been obvious when you took apart the carb. Could the problem be a weakening ignition?