Carbon Buildup on intake valves
Carbon build-up on intake valves is fairly common. Though there are some products that claim it will remove this when mixed with gas, I have never seen any that actually work. The air flow cone around the valve becomes disrupted with this buildup on the valve, resulting in decreased performance. Not a problem other than slight loss of power. That’s why racers freshen up their motors on regular basis. The only practical way to get it off is to remove the head and take the valves out and scrub it off. I usually place the valve stem in a drill and spin it in a piece of scotchbrite held in my hand. While you’r at it you might as well do a valve job.