The tires spin because of the centrifugal clutch. The clutch-shoes are held in by a spring and when the engine revs up it makes the shoes move outwards and make contact with the clutchbell which drives the wheels.

In regards to the fuel it is not a good idea to leave fuel in the tank for too long. Nitrofuel is hygroscopic (it attract moisture from the air) therefore when you leave it for a longer period of time it will have become bad leaving you with poor performance and more wear on your engine because of poor lubrication.