RPM Surging

[FALKLAN]

Howdy!

I have a 2018 ZT 6000 w/the KT745 engine.

At full throttle w/blades engaged, the engine runs smoothly. I do believe even at 3/4 throttle w/o engaging the blades, the engine is surging. I haven't worked on carbureted engines in many many years, so I'm here asking for some troubleshooting help.

Yes, I did search the forum for surging, but the resolution was a warranty replacement of the entire 2024 model mower... I don't have that option... I've worked on automobiles all my life, so I am able to troubleshoot most things. It runs fine when you put a load on the engine.

For starters, it does have a brand new fuel filter. The old original fuel filter was fine, but I changed it any way. Fresh fuel is at 3/4 tank. I'm not sure where to go from here, since it is running.

Can anyone assist me with further troubleshooting this problem?

Thanks in advance for any suggestions and feedback!

 

[ILENGINE]

Carb needs a thorough cleaning or a little more technical explanation. A carb has two fuel circuits. The idle circuit and the high speed or load circuit. When an engine is running without any load it is running on the idle circuit even at full throttle. When you engage the deck the load causes the throttle plate to open further which exposes the load/high speed circuit and it transfers to getting fuel from the high speed jet circuit. So the issue is the idle circuit of the carb is dirty. The engine is trying to run on the idle circuit but because it can't deliver enough full the engine starts slowing down which in turn causes the governor spring to try to increase the throttle, but then the governor tries slowing the engine back down, and the cycle continues which is the up and down surging that you are experiencing.