RE: Walbro carb model number deciphering (GX25)
In case you didn't see all of my pics, the GX25 and the GX31 operate the pump on intake manifold pulses too. (because there is no crankcase pressure) The term blowby describes what happens when the rings are worn and combustion gases leak past the rings into the crankcase, heating and contaminating the oil. Blowby is bad, the volume of air you describe is perfectly normal, with the piston moving up and down. Obviously it will move air under it, as well as on top. Crankcase vents are typically large, (to prevent pressure buildup) The one on the valve cover of the GX25 is 1/4" and the crankcase is tiny! Ever look at the "breather" on the valve cover of a small block V8, one on each side actually, they are quite large.