About 2 or 3 years ago I picked up a mower someone threw out, it was a 2019 MTD 21" FWD Deck, but it had a very old Quantum on it with the style of gas tank where most of the beauty cover was part of the gas tank, and it only has a small little circle piece that popped in that Identified it as a 4HP quantum, the Starter housing was definitely replaced because it was from 2012 so i don't have the original date and engine model, but the Gas tank had the date molded into it and I'm pretty sure it's from 89 as opposed to 99 or 09. I only mowed with it a couple times, then it completely lost compression, I figured either the valve seats were worn or the cylinder walls finally wore enough, it didn't run particularly great when I was using it, but it didn't really go out with a bang either, it just causally lost compression and died while mowing and wouldn't restart. Not until I took it apart recently did I see the rod exploded. The camshaft for being ancient nylon still appears to be in good shape despite the one gouge that's probably from me turning it over and it catching debris. Lots of ware, the crank journal completely scored, shmoo all in the bottom of the sump, there was shmoo in the rings when I popped the piston out too. Lots of scoring in the bore and along the piston skirt