We recommend the following procedure - Take out the spark plugs and pour 1/4 can of Berryman B-12 Chemtool Fuel Injector Cleaner (Part #0116) into each cylinder.  Thread the plugs back in finger-tight and let them soak for at least an hour, longer if it's much below 65°F (overnight would be fine).  Don't worry about leak-down.  When you're ready to remove the cleaner/carbon mixture, it's ideal if you can blow a little air into the liquid immediately before suctioning the mixture back out.  This will help suspend any loosened carbon.  If there is leakage of the 0116 into the crankcase, that is not problematic.  We actually have a set of instructions for intentionally adding 0116 to the crankcase immediately prior to an oil change.   Once you've finished the piston soak, you might consider one of the other two products, B-12 Chemtool Total Fuel System Cleanup (Part #2616) and B-60 High Mileage Fuel System Rejuvenator (Part #7516).  They're very similar to each other, but the 7516 is just a splash stronger than the 2616.