Chainsaw Repair

Run away motor is gonna be lack of fuel or too much air.  If it has been running great and the issue is spontaneous I would look first at a torn fuel line, pulse line, intake boot.
 In my experience that particular carb in the 5100 runs really lean and any small amount of crap under the Welch plug or low speed metering circuit can compound that issue and burn up a motor quickly.  Check the basics, like the above items as well as loose cylinder bolts and make sure the piston isn't damaged from the lean out.  Granted, everything else checks out, I would suspect dirty low speed circuit on the carb or worst case crank seals...