My daughter could really do with a patrol lawnmower as her electric one is woeful. As it happened, I discovered that my neighbour had a "broken" 16" lawnmower sitting in his garage (that "he thought" was a two stroke). Consequently I offered to buy it off him if I could fix it.
It has a B&S 450 series engine and quick google confirmed my suspicion that it wasn't a two stroke.
It turned over alright on the recoil starter but not a hint of ignition so I had a look at the air filter that was absolutely saturated in oil. Oil spatters were also on the deck by the exhaust. The dipstick confirmed that it was well over-filled.
After cleaning the oily spark plug and with the air filter off I could get it to start easily but not keep running unless I pumped the priming bulb continuously. Clearly it wasn't drawing fresh fuel from the tank so it was off with the carburettor, a quick clean, drained the engine oil and refilled with the right amount and lastly drained the fuel tank which had reddish petrol in it, presumably some sort of two stroke mix.
After reassembly it started and ran on the first pull, emitting clouds of smoke for the first five minutes as excess oil residues were presumably burnt out of the exhaust & silencer.
Best of all, my neighbour refuses to take any money for it. ;-)
Tim