Berryman B-12 Chemtool Fuel Treatment (part #0116) can be used to remove sludge and varnish buildup from the gas tank. If the gas tank is small and can be removed, you’ll want to do so, pour 0116 in, and either let it soak or better yet swirl it around before draining it. Repeat until the B-12 comes out clear and colorless. If the gas tank must remain installed, you can use a combination of B-12 Chemtool Carburetor, Choke & Throttle Body Cleaner aerosol (part #0117) to cover as much of the sides of the gas tank as possible. After draining, follow that up with 0116 soaks to ensure the bottom of the tank is clean. Lastly, you should run a premium detergent-based fuel additive like Berryman B-12 Chemtool Total Fuel System Clean-Up (part #2616) to clean up the fuel pump, fuel lines, fuel injectors or carburetor, intake valves, and combustion chambers. This will eliminate hard starting, rough idle, hesitation, and poor fuel economy. Good luck!