Berryman B-12 Chemtool Fuel System Cleaner (part #0116) contains 100% active solvents and can be used to clean gum, varnish, and carbon from the gas tank all the way to the combustion chambers. It can be used in all 2-stroke and 4-stroke gas engines at 1 oz/gallon and is safe on oxygen sensors and catalytic converters. Use 0116 every 3rd fill-up for routine maintenance. B-12 Chemtool Total Fuel System Clean-Up (part #2616) takes the same strong solvents used in 0116 and adds our strongest detergent package for the most thorough cleaning of neglected fuel systems and high-mileage engines. Like 0116, 2616 is safe in all 2-stroke and 4-stroke gas engines at 1 oz/gallon and won’t harm oxygen sensors or catalytic converters. Use 2616 when your vehicle is not running properly (hard starting, rough idle, hesitation, engine knocking, etc.) or after you have purchased a pre-owned that you do not know the maintenance history of.