Drain all the gas out of the fuel tank.
Using a screwdriver, take off the tank clamps. Also, open the straps, if any, holding the gas tank in place.
Now remove the tank from the vehicle. Removing the gas tank from car or motorcycle will make cleaning much easier. Make sure to take your time and follow any guidelines that might help you easily remove the gas tank. You might also want to get the help from a friend as this step can take some extra effort.
Place the tank inside a large plastic container and pour some gas into it. Drain the excess gas into the container.
Transfer the gas from the container into a gas can and put it on a side to dispose it of properly after completing the tank cleaning.
Fill the gas tank halfway with hot water and add half gallon of muriatic acid to it. Plug the openings of the tank using any plastic cork that fits the opening and let the acid mixture stay in the tank for two hours.
After two hours, empty the tank into the plastic container. Pour the acid mixture from the container into an empty gas can and set it aside to dispose of it later at the proper facility. Never throw this down the drain as it is extremely toxic and very harmful for the environment.
Rinse the gas tank with tap water at least thrice to make sure that no residue is left behind. Do not forget to empty the tank into the plastic container when rinsing. Store the water in the same gas can in which you put the acid mixture.
Let the tank dry for several hours, then rinse it out with clean gasoline.
Once your gas tank is clean, put it back in place using the same clamps or straps.