I also think that the stinger size of the pipe is one part of the problem. I also see when I ride pretty fast is that the temprature of the engine goes way past the 70 Celcius. I also took the head of the top barrel and the patern of the piston looked perfect en the inner chamber was perfectly clean.

And the spark plug of the lower barrel looked also pretty dark brown so I dont think ist a jetting problem.

But I find it pretty strange that the problem is only on the lower barrel. I realy want to solve the problem before the nice wheater begins.