Berryman B-12 Chemtool Fuel Injector Cleaner (part #0116) is often strong enough to dissolve such gum and varnish, but you might also consider using a solvent- and detergent-based combination additive like Berryman B-60 High Mileage Fuel System Rejuvenator (part #7516) for maximum cleaning. That said, if varnish buildup is exceptionally severe, you may need to employee the use of an immersion cleaner, such as Berryman Professional Chem-Dip Carburetor & Parts Cleaner (part #0905).