I don't know about removing the stuff from the backs of valves, but I know it works on pistons and combustion chambers. I've seen it. Years ago I had a Scout 185 with a premix 2 stroke Yamaha C115. I ran it for a about a year before I ever even thought of needing to use a product like ring free. By the advice of a friend I tried it, but (accidentally) added it in the "shock" treatment concentrate and not the "constant" treatment mixture. After going out in the boat, I remember when returning I flushed it on the earmuffs as usual, all of these little black "rocks" were in my driveway in the water wash behind the prop. I had never seen these before and had flushed that motor in the exact same spot every time I returned. I picked one of these "rocks" up and it crushed in my hand--it was all oily and smelled like gasoline and oil. At that point, it kind of all came together and made sense. I kept using ringfree (after figuring out I only needed to do a "constant" treatment) and noticed that the next 2-3 times of flushing, the oily black rocks got smaller and less, and by about the fourth time flushing it only looked like pepper in my driveway. When I sold that boat the motor had about 5-600 hours on it. Three of the four cylinders compression were exactly 140 psi, and the fourth was 141 psi. After seeing that on a carbureted premix two stroke, I just can't see it as a concidence.