Eslenk,
Does the engine sound like it's running differently (missing, spitting, sputtering, backfiring) when it does this or does it just sort of bog down like it's not getting the gas it needs while engine is still running smoothly.
You mentioned a puff of smoke. Are you sure it smells like gas or could it be oil that your smelling?
Remove your air filter, is there a puddle of oil inside the filter area by the carburetor opening and is the filter soaked with oil?
I do maintenance/repairs on my friends 2084, 20hp kohler magnum. I just had a situation where the Crankcase Breather gasket (RTV sealant) had failed and was allowing pressurized oil to fill the breather area then get sucked up into the air filter area and into the carb. If the engine was running at a constant speed eventually it would gather enough oil by the carb opening and suck some in. The engine would bog briefly, a puff of smoke and then run fine again until it rested the cycle. If you have an oil saturated filter and pure oil poured/sucked into the carb it might be enough to cause this issue. I replaced the Crankcase Breather gasket and my problem was solved.
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