Arch has the right idea but using Seafoam is not the only way to use "his" procedure. For an F-53/80 gallon chassis purchase 4 quarts of cheap ATF. Add it to your fuel tank, top off your tank and head on down the road. The detergents in the ATF will do a great job of cleaning the injectors, back side of the intake valves and piston crowns. It is one of the oldest fuel system / upper end cleaners known to mechanics and it is cheap. $20.00 buys your ATF. The Seafoam would cost around $70.00.
Randy
p.s. Marvel Mystery oil was just a thinned ATF and it was purchased by millions of people as a fuel system cleaner and air tool lube.
