Birken Vogt
Here is a fun one.

Called out for a governor fault. Verified problem, unit runs for a minute, then begins to hunt occasionally and shut off on governor fault. Replaced throttle body. Unit exhibits same behavior but does not shut down any longer.

Surmise all the aggressive throttle movements wore out the throttle body. They don't seem to last too terribly long when they run a lot.

More specifically, the problem is that the unit runs pretty steady (although you can tell it is hunting inside the throttle body by the sound of the gears in there working, slight variations in the exhaust sound, sound of the gas metering through the regulator in time with the surges, and slight variations on the water manometer) and then eventually it goes into a mode where the throttle completely closes for a half second, engine loses significant speed, then it catches itself, and continues running.

Checked gas pressure throughout the system. Changed spark plugs. Set depth on MPU on flywheel. 11-13VAC there at 4800 Hz.

Interestingly, the cam MPU has only 600 mV AC but not sure what a good one should have. I believe this is not picking up teeth but rather a magnet once per cam revolution.

No codes for anything MPU related logged in the controller at any time.

I was thinking cam MPU but after reading how it works, leaning toward Nexus controller at this time. Nothing else left.

???
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Peddler
If it was the cam MPU then it would be backfiring I think and running really bad.  Is it natural or LP?  I have had a few LP units where the regulator was vented back, underground to the side of the house and somehow water gets trapped in the line and and starts to bounce with the movement of the diaphragm.  It starts off very smooth almost unnoticeable but then gets progressively worse until a fault occurs.  I have seen it nat gas lines as well that run underground.  In the case of the LP you just loosen the cap over the spring so the regulator breaths as normal and hunting will go away.  The other thing I have seen is a bad first stage (10 psi) regulator on LP cause surging but this was on a 70 KW where the regulator might not have been big enough, in that case you could hear the surging at the regulator, replaced it and the problem went away.
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Birken Vogt
It is LPG. I did measure the gas pressure at the regulator and after the electric solenoid and both places the pressure was 11" +/- a few tenths. Measured with a water manometer. Neither regulator has a vent extension but in any case, if that was the problem I should have seen the pressure doing something. The sound is variable but I think it is in response to what the engine is doing.

I did measure pressure (vacuum) after the demand regulator where it goes up to the mixer/carb. It is pretty steady there about -3" or so right up until the problem occurs. It seems the throttle actuator is slamming shut causing the air flow and thus demand vacuum to go to zero, then it catches itself and opens the the throttle wider causing a burst of flow and a burst of vacuum which I can't measure but it is a lot and makes the water bounce. Then it steadies out for a few seconds and does it again.
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Peddler
If that unit has a governor driver board then I would replace that and see what happens.  I have solved a lot of unexplained loss of speed signal and or over-cranks by replacing this board that would work fine every time I was on site otherwise.  Good Luck
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Birken Vogt
No other boards, it is all down inside the Nexus control panel.
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Peddler
I don't recall where you are located so this may not be relevant.  Does your unit have an Oxygen Sensor and control solenoid?
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Birken Vogt
No, open loop fuel system, strictly a demand regulator into a carburetor, the carburetor has no moving parts.
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78buckshot
As Peddler mentioned, take the adjustment cap off of each LP regulator, look into the regulator for signs of rusty springs, leave the caps off for your next round of tests as this will vent the top side of the diaphragm to atmospheric pressure.
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