Chainsaw Repair

if the little screen in the carb is getting blocked (either from a passing fuel filter or fuel gumming) it will act somewhat like an air leak.
best to check the easy and most obvious first.

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...

Did you try and richen up the high speed?

+1 and no decomp on a 5100

Here in europe they had the Makita 5001(I forgot what the exact Dolmar counterpart was) which came standard with a decomp. I know because I had one.

7

My 5105 is the hardest to pull over saw I have.

However it fires if you can get it past Top dead center and has never back fired or kicked back.