I am having difficulty with the idle/run of my Kohler CH20S w/1bbl carb (in a Kubota GR2000). 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. 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)