We would recommend draining the gas and performing multiple “rinses” with B-12 Chemtool Fuel Treatment. It is widely available in 15-oz bottles (part #0116), but may be more cost effective in 1-gal cans (part #0101). After the tank is clean, use in clean fuel at a concentration 1-2 oz/gal for a few tanks in order to clean out fuel lines and carburetor. If the snowmobile is running rough, consider using B-12 Chemtool Total Fuel System Clean-Up (part #2616) at 1 oz/gal. It is our strongest detergent-based fuel additive and will thoroughly clean fuel lines, carburetor, and intake valves.