This has been out for years among the cognoscenti, but I've only heard about it recently. Adding about 1.5 ounces of acetone for every 10 gallons of fuel makes the engine run better and improves mileage.
I messed up with my van and added way too much acetone. Noticed no improvement. Tried it again with the right amount and got about 10% better mileage.
Tried it with my VFR and am not yet able to ascertain if there is any difference since my riding style can vary greatly from tank to tank.
I figure that if mileage improves, then the engine must be performing better. I think the idea is that the acetone is an excellent cleaner and doesn't harm any seals.
I don't think this is snake oil or urban legend. Anybody have any knowledge on this?