Have you adjusted the valves? That almost always needs to be done after the first hour or so of operation.

Another possible answer is that you've got the engine under-propped. It will turn a Graupner 14x7 with authority, and you don't want anything less than a 13x6. If the engine is overspeeding (surging is one of the indicative sounds), the valves are floating.

Just my opinion from running three of these. Around here, anyway, guys seem insistent on under-propping four strokes. I have no idea why.

Steve