Sorry, one thing I didn't mention, we took the valve cover off and got play on #1 in and out rockers.
I cleaned the plugs and noticed that the connector (metal clip) that actually goes on the spark plug where deep in the rubber connector. I pushed the clip out all the way, pushed it on to the plugs and than pushed the rubber over. We gone some running, but not a good fuel supply through the carb.
Ordered an new Zenith 8 now.
Joerg