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Ahhhhh, carb jets and emulsion tube probably need a going over. The pilot jet will give surging conditions like you've described if it's so much as a few degrees out of line - some engines are very...

Or the heater is trying to drag 4.1 on crank up until it settles in to it's work. Some things drag a scary amount of power in the first few moments of switching on, it's normally considered a motor...