My first thought was that the spark plug is broken. So today I replaced all spark plugs and did a test drive. Unfortunately, the problem was not completely fixed. The cylinder seems to get out arbitrarily. If I press the gas train about 1/4 during the drive, I feel as the cylinder runs normally. If I now very LANGSAM continues to span the gas train or give gas the cylinder remains with it. If there is a sudden gas, the cylinder will be left with it. When I have suspended the cooling circuit, some cooling water has run into the spark plug depth (or as it is called = D). After about 30 km the water must have evaporated again, so that no short circuit can arise or? The speed range does not seem to play such a big role, whereby it switches on on the top (approx. 5000) more easily. Turns out, even up to 1100 rpm no discoloration of the exhaust fumes fuel would also have to go to the cylinder anger valves were re-set about half a year ago. Carburettors were cleaned (not synchronised) A spark plug was heavily rusted, maybe that helps.