I have a runaway NH TC18 engine after having electrical problems, which turned out to be a burned out regulator.
I have the regulator replaced and everything seems to be operating well, except that the engine runs full out with no power and lots of black exhaust all the time. (Too much fuel to the cylinders???)
In my efforts to fix the problem, I have had the cam-driven injector pump off a couple of times, and did so again today thinking that perhaps I had stressed the linkage so that it could not work. When I put it back together, I checked the pump/solenoid pressure point, and it moved freely. After starting the engine with no improvement in operation, I removed the solenoid. The pump/solenoid pressure point on the back of the pump was fully depressed (in the off position) and would not return.
The engine cannot be stopped with the ignition switch, although the cut-off solenoid works when powered with 12v juice, something in the pump (or elsewhere) keeps the fuel supply to the engine from being affected by the solenoid. So I have to turn off the fuel and let it run down.
My thoughts about the electrical system possibly affecting fuel delivery is due to the fact that the problem began with a burned out regulator.
Any help would be appreciated.