I do have one thing for you to try,, it drove me nuts TWICE,, because I forgot it had happened a decade earlier on a different tractor,,

Remove the air filter, remove the air filter bracket (the round thing that screws to the inlet of the carb)

Now, look at the air inlet of the carb,, at the 6 o'clock position of the air inlet is a hole.
That hole is in a hump that sticks up into the air inlet,,

Well,, it turns out, that the hole must be clear, there is a passage from that hole into the carb bowl/float area of the carb.

With the bowl removed, shoot some carb cleaner into that hole, the carb cleaner MUST shoot out through a hole into the bowl area.

If the hole is clogged, it is a TOUGH passage to clean.

I have only successfully cleaned it one way.
I put a 3 to 1 solution of Pine Sol bathroom cleaner and water in an old crock pot, and submerged the carb,, an plugged in the crock pot.

If I remember correctly, it took about 12 hours of "cooking" to clear that passage,,
I am sure the time will vary, with each carb situation. Check it every hour or so, I checked it by blowing in the hole with an air compressor and blow nozzle.

Once that passage is clear,, the carb will again idle as it should.
If it does not correct the carb, at least you will have a GREAT smelling shop!! :bigeyes:

Actually, I set the crock pot outside, I do not know if Pine Sol fumes are bad for you,,,:bigthink:

The other possibility is that the condenser is bad,,,

Good luck

I installed the new carburetor and adjusted the governor.

The timing was originally set to the top dead center mark and I readjusted that to the spark mark. That cured the glowing red muffler.

The tractor now runs better but still surges a bit randomly. It still runs rougher than it should although it’s much better than it was.

I’m afraid that it will run worst once it’s under load. We will see tomorrow, the snow is stacking up and needs to be plowed.

Did you check your fuel tank, sediment bowl (or fuel filter) and fuel line to make sure they are clean and unobstructed? Is your petcock opened all of the way? How is the linkage to your carburetor? Is it tight or loose and sloppy?
You might also try running some Marvel Mystery Oil in your fuel to it and see how it behaves after that? I have had some really good results with that fuel additive and like how it helps lubricate the upper chambers of the engine when used.

Just some suggestions that I haven't seen yet.

Cub Cadet 123