When an engine is reving up and down like that, it usually is an indication that it needs more fuel, thus the "hunting" as they call it. If you can't get the carb to richen up by adjustments, then you may have to replace it. One other mechanic indicated a governor adjustment, but if it didn't get out of whack to start with, then it shouldn't be out now. Fuel starvation leading to hunting could be from partially clogged jets varnished up or some internal passage blocked or varnished over. If the carb has had a good chemical bath and blown out with compressed air and even small wires used to clean the orifices feeding into the carburetor throat near the throttle valve, then it looks like replacement time. If you think it is the fuel pump, you can do a jug test on it and hook up a smaller fuel tank above the carburetor and let it gravity feed into the carburetor and see if it is still hunting. Hope this helps.
