De-carboning engine.
 

In the past I have misted cold water into the carb to remove some of the carbon from the intake valves and pistons. You get some black crud out the exhuast but I never new how well it really worked. I usually do it in the spring before I put the new plugs and oil in. I just did a tune up on my motorcycle and I could see the buildup on the intake valves. I got out the spray bottle and gave each cylinder a good misting. I would 75% of it is gone. I guess my only concern is the carbon chunks getting caught in the valve seats. Anyway, it works better than any fuel system cleaner could.