Yep there is always Light holiday reading FAST GRASS haha ..Its all good fun working out how things work...The way I look at it is like a river and muddy water with gold maybe? haha..Well where flow drops it give's the mud (fuel) time to drop out and settle and you can see that on the piston in two spots at the back and over time it leaves carbon...Here is a pic before I cleaned the combustion chamber and the decompression valve its open in this pic but even closed you can see it upsets the flame front and fuel will drop out settle behind it,you can clearly see the way carbon has formed behind it... [Linked Image]