Chainsaw Repair

I went out and fired it up today made a couple or three cuts to warm it up then put the tach to it, the most I could get was 12100 at about 2 1/2 turns out.
  When I bought it I looked in the plug hole with a lite and could see a mark about 5/16" wide above the exhaust port, but the owner put his comp. tester on it and it blew a little over 160, so I decided to take it for $90.
 
I took it home and ran it a small amount, then I got my own comp tester and the saw was down to 148, after running it today it's at 130,I think I gotta pull it down and look at it, looks like a new slug & jug is less than $50
  This will be my first clamshell.

   John