This is looking down into the intake tract of a 2006 Suzuki GSXR750 with 66,000 km. As you can see there is considerable carbon deposits built up on the back of the intake valve. This is a fuel injected engine and the injector sprays on the back on the hot intake valve to help vaporize the fuel. Higher than normal intake valve temperatures and poor quality gas can aggravate this condition. Another issue is Ethanol in fuel. This can cause a variety of issues especially in vintage bikes and 2 strokes. Most gas has up to 10% ethanol in it and is best to use gas with the lowest amount or none at all. Some stations sell ethanol free Shell 91. I personally do not believe fuel additives can remove these deposits once they have formed but there maybe a case for using them from new to help prevent the buildup in the first place. There is a product we carry that has many good customer reviews and also acts as a fuel stabilizer (see picture below). Keep in mind that carbon deposits act like a sponge and when built up severly enough on the back of an intake valve like this can actually have a profound effect on how much fuel is being delivered. This can cause your engine run leaner then intended, reducing both performance, mileage and drive ability.
I have been involved in mechanics and motorcycling from a young age. I formed Cycle Improvements in 1981 and still have the same passion to learn today as when I started. Hope you find this blog interesting and educational.