If you don’t drive a vehicle and you let gas sit in a tank for awhile, it will become old. Old gas looks dark and smells sour. After a couple months or so, the gas will evaporate and leave a varnish residue in the tank. Varnish is a sticky, caramel-color substance. It will gum up the fuel system in a vehicle. You can easily clean the varnish out of a gas tank with a few materials and some time.
Remove the gas tank from the vehicle (see resources). Basically, you’ll need to jack the vehicle up and remove the straps that hold it in place. Put the tank in a well-ventilated area away from sparks and flames.
Use a siphon to empty out any gas that may be in the tank. Spray the inside of the tank out thoroughly with a garden hose. Dump out the water.