White? I’d be worried about white plug electrodes. That’s too lean.
If you’re certain that there’s not clogging at the petcock, then check before and after it. The worm-hole air-hole in the gas cap (all mine were blocked with corrosion or swollen fiber gaskets when I got them), and float level, although I think float level could cause running lean, I don’t think it would cause the decrease in performace oat higher speed…
If you can, try riding with a lower amount of fuel in the tank, with the cap un-latched, sitting gently closed. That will let more air in, and tell you if its the worm-hole at fault.
