Speed Variation Problem in Waukesha 12V275GL+ Gas Engines

Have you done any testing of the electrical supply of the system to see if it picking up any electrical spikes on the DC power supply side of things?

Oscilloscope monitoring may show you any voltage spikes that could be affecting the control system.

No we haven't done yet but we will checks. Thanks for the comment.

Bad fuel?

What are your readouts? What are your fuel specs?

https://powergen.gepower.com/products/reciprocating-engines/waukesha-275glplus.html

I think you need to first find out what you are dealing with:

How to Tell if an Engine Hesitation is From Loss of Spark or Loss of Fuel, cont. When the complaint is that the engine hesitates, the hesitations can be caused by either a temporary (intermittent) loss of either spark or fuel. To verify which one it is, do this test: 1. Use an RPM inductive clamp that plugs into a DVOM (digital volt ohm meter). 2. Hook it if possible around the coil wire, if not accessible, around a spark plug wire. 3. Set the meter to read either AC or DC voltage – it does not matter. 4. If possible, bring the lead, with an extension, from the RPM inductive clamp into the vehicle front seat at the meter. 5. Once you start the engine, you will see a varying voltage, it will be either the AC or DC you selected, it does not matter. All you are using the meter for is to tell you there is a constant signal on the meter as a result of a constant spark. Typical values with be 2 volts or higher at idle, and will increase with engine RPM and load such as applying TCC (torque converter clutch). What you are looking for: does the voltage when the hesitation occurs remain the same as when it did not hesitate? If the voltage did not change, yet the vehicle hesitated, the problem is fuel related. If the hesitation caused the voltage to go to zero, than you should concentrate on the ignition system.

Joe Glassford The Hands On Vehicle Testing Reference

Thanks for your detailed comment.

We will check the voltages and let you know the results.

It could be control related if the engine is at light load. The tuning of the speed control may be set hot for heavy load pickup, which may then overshoot at light load. Below 20% rated might be a good guess at light load.

Wish my late father was around to ask. He was the national service manager for Waukesha Engine starting in the early 70’s until his retirement. While in college I even co-op drafting for Waukesha Power Systems adding the generator and switchgear.