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-colour 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.
Place a hand full of nuts and bolts into the gas tank to clean varnish out. Roll the fasteners around in the tank for five to 10 minutes. Dump the fasteners out and spray the gas tank out again. Repeat if necessary.
Dress in old clothing. Put safety goggles on and rubber gloves to protect yourself. Fill the gas tank with a couple inches of water to clean out varnish. Pour a cup of muriatic acid into the tank. Move the tank around for a few minutes so the solution covers the inside completely.
Pour three or four cups of lime into a bucket. Pour a gallon of water into the bucket and stir it with a long stick. Slowly pour the used muriatic acid that's in the gas tank into the lime to neutralise it. Add more lime as needed until the fizzing stops. It's now safe to pour the acid down a drain.
Fill the tank with water once more. Pour a half gallon of muriatic acid into the tank. Move the tank around a few minutes. Let the gas tank sit overnight to clean varnish out.
Repeat Step #5 to neutralise and dispose of the muriatic acid. Spray the inside of the tank out thoroughly with a garden hose. Allow it to air dry before you use it again. Otherwise you'll have water mixed in with the gas.
Read and follow the instructions on the muriatic acid container. Add muriatic acid to water. Never add water to the acid. Use caution when using muriatic acid. Don't splash it on you or breathe the fumes.