| If you’ve got a sick engine that allows oil to get past the valve guides or piston rings, or you’re running a two-stroke that is lubricated with oil mixed-in with its fuel, carbon build-up can be an issue. Also, burning a too-high octane fuel in an engine designed to operate at a lower octane rating will produce carbon deposits secondary to partially unburned (higher octane = slower burning) fuel. However, in a healthy V-twin the carbon deposit control additives/detergents contained in ALL gasolines will keep your engine clean.
The main concern with carbon deposits is that they reduce combustion chamber volume and thereby raise the combustion ratio of the cylinder, the octane requirements of the engine, and the NOx emissions of the vehicle. In the real world it doesn't happen in a healthy engine. Carbon deposits tend to break or burn off when they reach a thickness above 0.5mm. So low mileage and high mileage healthy engines have essentially the same amount of deposits.
One thing that contributes to carbon build-up is running at a light throttle for extended periods because flow velocities aid in the removal of the deposits. That’s why old-timers developed a low-tech (more fun) approach to this problem called Blowing Out The Carbon. If you’re not convinced that it works, put on a white shirt and go down to your local dyno shop. Stand behind a bike as it makes it’s run.  | 
Shazaam!
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