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Afternoon,

John Deere z235e and the engine is a Briggs and Stratton 40N877. It runs fine at high RPM's but hunts at the lower. I've found this thread but doesn't seem to help (https://www.lawnmowerforum.com/threads/z335e-engine-surge.71692/).

Here's a video of it running:
I've had the carb apart twice and have cleaned all the holes with carb cleaner and wire set. It's still hunting and I can't seem to isolate. Any advice?

Alright i'm back.

I took the carb off one more time (getting fast at this point). I then took an air compressor at 110psi and blew out the holes on the top part of the carb. I then used some carb cleaner, then back to the air hose. Put it all back together and it's fixed.

Video of engine purring:

The plastic manifolds are prone to warping & leaking air .
The rubber seals also can leak
The carb solenoid can fail to retract fully so the float bowl fills then drains then refills then drains
Get some liquid WD 40 ( or similar ) and saturate the manifold region using a trigger pack
If you get white smoke or the engine speed changes then you have an air leak
Carb cleaner can attack the rubber O-rings that hold the jets in place
Full instructions for properly cleaning the carb are available at https://outdoorpowerinfo.com/