Hi, Tim, and thanks for your interest in Berryman.  You’ll want to drain the tank as much as possible.  Then, your best bet is to add straight B-12 Chemtool Fuel System Cleaner (part #0101) and move the tank around so that the liquid can clean as much as the inside of the tank as possible.  The product is very aggressive, so be sure to keep it off any sensitive or delicate surfaces, especially paint.  You will probably need to soak the tank for at least 30 minutes and maybe even a few hours.  Afterwards, drain the B-12 Chemtool solution out and repeat until the liquid comes out clear.  Be cautious with the spent B-12 as it is extremely flammable.   Once you’re satisfied with the condition of the tank, you should use a strong fuel additive to help clean out the carb and intake valves.  B-12 Chemtool at 2-4 oz/gal will help, but a detergent-based product would be even better.  We recommend either Berryman B-60 High-Mileage Fuel System Rejuvenator (part #7516) or Berryman B-12 Chemtool Total Fuel System Clean-Up (part #2616) at 1-2 oz/gal in order to thoroughly clean the carb(s), intake valves, and even combustion chambers.   Hope this helps.  Let us know if you have any additional questions.