Kohler CH20S Poor performance (Kubota)

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

 

Kind of sounds like the float sticking periodically, or possibly a clogged bowl vent causing excess fuel to burp into the carb periodically.

 

Thanks, Cheese! Is it reasonable to rule out ignition problems, considering my symptoms? I've thought about pulling the carb and cleaning. But, hesitating as I try to rule out other things.

 

I don't know that I'd completely rule ignition out as a possibility, but it's not where I'd start looking.