AUEngineer
I have a 13kW model 4390-3 running on NG, unknown hours, as it was inherited.  The unit was running on LP, now converted to NG.  I removed the jet from the idle circuit port on the fuel regulator before hooking up NG.  When I inherited the unit running on LP, the adjustments on the fuel regulator were almost opposite each other, meaning one cylinder's adjustment was against the pin stop in the closed direction, and one was almost against the pin stop in the open direction.

The unit runs about 60.5Hz/240V with the lid closed under no load.  I'm taking frequency measurements with a Fluke true RMS meter.  Under ~60-70% load, the unit drops as low as 57Hz to 57.5Hz, though it fluctuates some.  This is an old genny, so I'm really not comfortable taking her up to 100% load as the service manual suggests for governor and fuel regulator adjustment (plus the practicality of hitting 100% and not 110% accidentally with just house loads I'm not real sure of, and I don't have a load bank).

Do I adjust the governor first to get the unit's hertz a little higher under no load?  Then look at adjusting the regulator with a load applied?  I have the regulator adjusted midway between the stops for both cylinders right now.  Unit starts and runs nicely--valves have been adjusted as well.

Also, turning the fuel regulator adjustments doesn't seem to make much difference to the frequency while the unit is under load.  I am a bit baffled why the service manual warns of engine overspeed when adjusting the regulator?  I really don't see much difference while making adjustments while the generator is under load.  Having two different adjustments seems confusing as well--close both adjustments or open both adjustment completely and do one then the other watching for the RPMs (frequency) to drop?  Perhaps I'm doing something wrong?

I have the factory service manual, but some feedback from the experts would be great.  Thanks to everyone for sharing their knowledge and experience on this forum.
CARRIER 15kW Model ASPAX1CCA015 on NG (re-stickered Generac 4390-3)
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Peddler
That should be a 15 KW unit but only on LP.  I set the gov. for 64 HZ and about 245 volts AC line to line no load.  Don't worry about loaded adjustment.  The fuel taps should be about the same opening kind of an art.  I would set both about 1/2 a turn off wide open pin. 
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78buckshot
I'm a little concerned that you don't get much change in RPM while adjusting the fuel mixture. Can you describe the fuel supply, meter, piping, diameter, length, elbows, etc.? Have you checked the fuel pressure? I'm with Peddler on 245 VAC and 63 - 64 Hz. no load top open, it will slow down a little when you close the lid. Adjust the fuel mixture from side to side to achieve the highest speed then 1/4 turn counterclockwise then adjust the loaded governor screw under the air box then adjust the voltage.
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AUEngineer
I will see if I can get a photo of the gas plumbing.  I have 2psi service from the gas company.  The step-down regulator is a Sensus 143-80-2, 3/4" connections, 1/2" orifice with blue spring.  The generator is quite close to the meter.  I'd estimate about 2 to 3' of 3/4" pipe from the meter to my step-down regulator.  Downstream of the step-down regulator, I'd estimate about 2' of 3/4" pipe with a couple elbows and then I had a 2" "reservoir" pipe of about 3 feet installed just before the flex connection to the generator.  I have not measured the gas pressure.

Could you offer a recommended governor adjustment procedure?  The service manual recommends properly resetting the lever to the governor shaft, then removing the idle spring and adjusting full load speed to 58Hz, followed by returning the idle spring and setting the no load frequency.  Peddler said no need to worry about the loaded adjustment--so I'm interested in best method to get this thing set correctly.  Thank you!
CARRIER 15kW Model ASPAX1CCA015 on NG (re-stickered Generac 4390-3)
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78buckshot
Sounds like the gas supply piping is fine, it would be good to know the static pressure and running pressure at the generator if you get time to check it. The manual is good for the governor adjustment, adjust the shaft to lever first as outlined in the text, then the screw under the air box-it's hard to see and tricky to get to. I use a flexible 1/4 inch drill extension with a bit holder and a #2 phillips, you will see a black plastic plug in the steel bulkhead divider above the battery - feed the flex through that. I shoot for 62.5 - 63.0 Hz. no load cover open.
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Peddler
I would not mess with the mechanical linkage just adjust the spring.  Normally if it is close the fine adjustment spring is enough to get your desired speed of 64-64 HZ.  The voltage should be 245-246 no load and that is an adjustment pot on the voltage regulator.  Adjust Hz first then set the no load voltage and then don't worry about it.  With a mechanical govenor you are shooting for an average of around 60 Hz with medium to heavy load. 
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