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There is one other possibility. If the breather tube was possibly not connected to start, it may have been disconnected on purpose by a previous owner. If the engine is already worn, leaving it disconnected may reduce oil consumption.
The only reason breathers are connected to the intake (since the 1970's) is to burn blow-by fuel vapors for pollution control. The crankcase vapors often contain oil mist as well as unburnt fuel, but good engines only develop a vacuum in the crankcase, great for dissipating fuel vapors. Worn engines have a high gaseous flow rate because of blow-by oversupply, and carry excess oil mist with the flow.
Most aluminum-cylinder-wall lawn mower engines are made to run about 100 hours in their lifetime. If one mows the lawn 10 times a year at one hour each, the cheap engine will therefore last 10 years with reasonable care. These cheap mowers die quickly when used in continuous commercial service though.
The same limitations might be said of a light duty gen-set if used frequently, and/or one with poor service intervals. Continuous 3600 rpm is also a rather high duty cycle, more like a boat or aircraft, more demanding than most of our land transportation vehicles, other than HD trucks, and even then most boats, aircraft and trucks run at a lower, steady rpm.
Wes
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