[Crankster]

I've an Ampco that works OK, but as a practical matter I don't think it makes any appreciable difference whether under load or not. The idea is to remove gum and varnish accumulations from valves and piston ring grooves in neglected or mistuned engines. Adding Marvel's directly to the fuel tank accomplishes this. This also has a benefit in that it seems to help protect alloy carb castings and rubber fuel system components. Modern gasoline tends to be corrosive or problematic in vintage fuel systems.