Briggs engine hunts and surges at idle, runs good under load

[zxcvbob]

Any ideas?  It's a 25HP 724CC Commercial engine on a Kubota zero turn mower.  I've read that small engines have a spring in parallel with the governor linkage that looks like it doesn't do anything, and when that breaks the engine hunts.  There's no spring on this one, but there's also no slack in the linkage to take up and on one end of the linkage I don't see any place to attach a spring.  (on the other end, there is a hole the right size with nothing going to it)

Doesn't matter where we set the throttle, it does this until we engage the blades.  Then it runs just fine.  Any problems I can think of in the fuel delivery system would fail under load instead of when unloaded.

I've mentioned this mower here before.  It does have a fuel problem that I've kinda figured out is grass clippings or leaf debris in the gas tank.  When it won't run, we disconnect the fuel line at the filter and blow it out back towards the tank with an air compressor, then it's good for a few months again.

[Nick1911]

Fuel issues come to mine.  Could be a clogged jet.  Both cylinders firing?

[Brad Johnson]

My first guess would be a partially blocked idle circuit. It's a common problem due to the Green Death that oxygenated fuels tend to forms in carb bowls after sitting for extended periods. Sometimes just tapping the carb can fix it. You could also try running the idle mixture screw all the way through it's travel and see if you can clean away or dislodge whatever's causing the problem.

Edit to add - Looks like nick1911 beat me to it.

Brad

[zxcvbob]

Both cylinders firing; it sounds like it's running out of gas.  (damn modern engines set way-too-lean from the factory)  Engage the blades and you expect it to die but instead it starts running great.

[Brad Johnson]

That would also point to a fueling problem in the idle circuit.

Brad

[Jim147]

What happens when you play with the choke? Like leave it 1/4 on at idle.

[charby]

I'm guessing dirty idle jets

[230RN]

The last small engine I diddled with had a vane in the intake airflow connected to the throttle to regulate (govern) the speed (output) of the engine under varying loads

It is difficult to adjust this or any feedback mechanism including electronic, to be perfect at all levels of output from idle to full power under an operating (full) load.

This adjustment can be disrupted by things like altitude extremes and other variables, like fuel which is different from the fuel they used at the factory to make the adjustment --or even seasonal differences in the fuels you bought.

Many feedback systems are somewhat critical between negative feedback and sudden adventures into runaway positive feedback points

I don't see anything "wrong" with the feedback mechanism "hunting" at idle as long as it doesn't stall out or overrun the engine at full load.

"If it works, don't fix it."

And...

"Perfection is the enemy of production."

Terry, 230RN

NOTE Most duckduckgo references seem to pertain to biological or behavioral feedback and not to mechanical or electronic feedback and I am not disposed to hunt for those myself.  Have at it.