Three weeks doesn't really seem like long enough for gas to go bad, though maybe the ethanol stuff is different.
Some STA-BIL might help prevent the fuel from going bad if you don't ride it all that much. The stabilized fuel in my Honda generator works just fine, even a year after I put the stabilizer in.
If it's a fuel issue, you might consider getting some Chevron Techron Fuel System Cleaner -- it's available at pretty much any auto parts store. The bottles are normally sized for 15-20 gallon car gas tanks, so be sure to put in proportionally less additive for the smaller scooter fuel tank. That should help clean out a bunch of gunk.
