So, I saw this MS251 chainsaw at a garage sale with a $10 price tag. I thought it probably had major engine damage given the low price and set it back down. I picked it up again and it passed the compression drop test and looked complete and pretty new. Then the owner comes over and tells me it only starts with starting fluid and the only way to stop it is by pulling the spark plug boot. I decide to take it for $10.
When I get it home I couldn't test compression because the spark plug threads are smaller than my gauge. So I pull the muffler and the condition of the piston and cylinder look brand new. I replaced the fuel and gave it a few pulls and it starts right up and seems to run great. But when I try to turn it off it continues to run. I short the ignition coil kill terminal and it still won't stop.
I removed the coil and it turns out to be aftermarket. (no Stihl part number) It also seems to be gapped too close to the flywheel. I find a used OEM coil for $25 on Ebay and swapped it out. Now it works perfect. I'm into it for $35. I'm still curious why the previous owner replaced the OEM ignition coil.