You need to test the injectors and then if that does not fix the issue you will need to have the injection pump rebuilt.I've got a similar problem, and mine doesn't respond to the throttle. It was having trouble starting over the winter and I just replaced the stop solenoid and glow plugs and it was running great. Then yesterday it started chugging but was responding to the throttle, so I put it back in the shed and turned it off. Today, after starting it, it started at very high RPM with heavy black smoke and wouldn't shut off. I had to put it under heavy load up a hill to kill it.