I thought that the vent to the float bowl in a carburetor went out to the clean side of the air filter, and the input side of the choke valve so that the choke can still work. This means that the pressure in the fuel bowl is the same as the clean side of the air filter. So a dirty air filter causes same result as operation at high altitude, which is slight rich running and reduced power.
Looks like they changed it on a 4 year old Honda GCV160 lawn mower engine. The vent for the float bowl goes directly to the outside. Yet another bad design on a new product. Removal of the rotating screen on the air cooling intake was another, now grass can clog up the cooling fins on the engine. Fixing the float bowl vent may be as simple as changing the seal between the carburetor and the plastic air filter box so that the little vent hole is directed inside instead of out the bottom to outside air.
I cleaned the grass that was built up behind the foam part of the filter and now it works right again. No more running rich with black smoke. Once the throttle would open up under load, it would start to run very rich and slow down, so the governor would hold the throttle open and the problem wouldn’t ever correct.
I assume vehicle carburetors always had the vent going to the clean side of the air filter? I wouldn’t know since those are before my time.