Engine dies when hot

[Wolfgangh]

Hi, I finished a complete ground-up restoration of a KR250A. I was very happy when it started almost immediately at my first attempt, and I let it run a few minutes in neutral to check for leaks etc. Then I rode it a few hundret meters around my garage, to check the gearbox, suspension, brakes etc - all fine. As soon as the temp gauge approached the mid the of scale, the engine started to miss some beats, the tach needle was jumping ( i.e. betweeb 0 and actual RPM), I was able to keep the engine alive for some more time with throttle action, but it appeard to me that 1 cylinder was first cutting out and then missing. Since the tach is driven from the front cylinder CDI, I assume it is this cylinder which makes problems when getting hot. Since I have an almost complete spare bike, I started to swap parts - one at a time: spark plug, coil, CDI. So far no change. That leaves the pickup or the exciter coil. Before I prepare everything to remove the left side engine cover to get access to these parts (which requires draining the coolant, removing the oil pump...) I wanted to ask you for any further ideas or inputs. What else could it be? If the exciter coil cuts out, wouldn't this affect both cylinders the same?