In my case I had sputtering happen three times. All three were after extended idling time while stopped in long traffic jams in hot weather in Houston and Memphis. When the traffic finally cleared and I accelerated from being stopped, the engine sputtered and would only take a small amount of throttle without stalling. I was only able to slowly increase my speed over half a mile or so until the engine would run normally................. After 30 minutes, I went to pull out, and noticed the bike sputtering pretty badly. It cleared up, after 1/4 mile or so. I thought it was like dirt was getting in, through the air cleaner area, but all seems good. ...................
My assumption was that the gas in the float bowl next to the very hot exhaust pipe became so hot that when when I accelerated it vaporized when it hit the low pressure of the needle jet and restricted the flow until cooler fuel entered the float bowl. I think my assumption is correct since it behaved exactly the same three times under the exact same conditions. Have you noticed the pipe glowing red hot while idling in the dark? I wrapped the exhaust pipe near the float bowl and haven't had the problem since, but I also make an effort to stay out of traffic jams in Houston.