I have a john deere LT160 mower. I replaced the fuel pump but it does not seem to be pumping as I crank the engine. It acts as though the actuator arm on the pump is not engaging inside the engine. I have tried to find exploded views of the engine with no luck. What drives the pump? Is it the cam shaft, the balancer shaft, or something else? I hate to tear into the engine if I don't have to. Any help would be greatly appreciated.