Kohler CH20S Poor performance (Kubota)

[hukturn]

Kohler CH20S w/1bbl carb (in a Kubota GR2000) will not idle. Governor will adjust (choke out) periodically (about every 10 seconds or 15 seconds w/choke on). When this happens, engine chokes down to near stop but recovers, unless under load (blades engaged, hard turns). Some light black smoke when first starting and/or when I throttle up quickly. Engine/carburetor seems to have the most difficulty when at operating temperature and/or when under load. Runs/idles fine when cool (at startup) but has problems after about 5min of running. Runs/idles fine for longer period of time when at low idle. Seems to also have difficulty maintaining RPM when travelling up/down hills. I am not operating at high altitudes. Will run for longer at ½ throttle, but still shuts down over time and/r under load (ie; mower blade). I would not describe the overall idle as “rough” but it is also not “smooth”. Automatic choke was not opening fully (possibly 20degree butterfly), so I added a spring to allow full (90degree butterfly) opening when choke lever is disengaged. Thoughts?

Some things already done;
Changed plugs
Checked spark and has nice blue fire
Adjusted the float levels (no cracks/holes)
Adjusted external governor spring (there is tension on governor rod, so it is engaging)
Good fuel flow up to carburetor (and bowls have fuel)
Slow speed jet O-Rings are good
Oil level is good
Removed fuel filter and air filter for testing during repair (still problems)
New gas w/carb cleaner additive
Ran w/out gas cap