Westerbeke 30B Deceleration Stumble/Stalling
Suspect that I have a governor problem and it's a lot of disassembly work to get at those parts to fully examine them. But, from what I can see with the top cover off, the governor parts all move and springs seem to work.
The engine has 600 hours and runs fine in every respect but one. I have a stumble/stall when rapidly decelerating from 2000+RPM to idle set at 1100RPM, with and without load. This happens only when the engine temp gets above 120 degs and then always happens. Injection pump and injectors have been professionally tested, serviced to spec and reinstalled -- no improvement. Boat's fuel delivery system has been eliminated as cause by isolation testing using a separate lift pump separately powered, inline filter, separate fuel supply and return lines to injector pump, (return line into bucket), and fresh fuel in a carry jug.
The engine was over-reved (runaway due to starting fluid) once and this may have affected the governor. I can't pinpoint when this problem began. It seems like there's an overshoot, causing fuel starvation, when the throttle is cut rapidly to idle. RPM drops to 500 range, engine stumbles badly, giving more throttle has no effect, sometimes recovers to idle on its own, sometimes stalls. Again, only when the engine is warmed up a little. . .not when cold, load and no-load.
Lab test of crankcase 15W-40 oil shows nothing unusual. Viscosity and everything else within spec.
What might cause this, other than a governor problem?
If the governor is the likely culprit, what governor parts are the main suspects for the problem I have?