Yanmar L100 problem

I have a 5 kw diesel generator with a Yanmar L100 clone in it that is bogging down to very low revs when on a high load it should be able to handle, the compression is very good, fuel is flowing to the pump, the same fault occurs with the air filter off and exhaust manifold off, anyone any ideas ?
 
Injector spray pattern ?
Pump worn out ?
Governor not responding ?