Never heard of a Zenoah 40, but your problem is either old stiff diaphragms in the carb or a leak in the pulse from the crankcase to the carb..Check the small hole in the insulator block under the carb.....
The cover with one screw in the center is the pump side, the diaphragm goes against the carb with the gasket on top..The other side is the inlet needle side, 4 screws, gasket against the carb and the diaphragm on top...