have you replaced the vacuum line ?
have you vacuum gauged it to make sure no air leak?
my MS260 had the vaccum tube crack and suck air , they run great , too great when reved up because they are running lean.
also pull your muffler and look to see if the piston is scored that is what happened to mine , I rebuilt it and ran it 2 more years and it did it again.
Not to hijack a post GCP I'm just curious if the vacuum leak was fixed when you rebuilt the saw, reving up a cold saw can sure cause a scored piston.
Sage .
the vacuum leak was the little hose to the carb I replaced it but too late the piston was already scored. everything was vacuum tested and held when I put it back together but at that point it was a decade old saw and something else let go after another 2 years I wasn't easy on it in those 2 years either , bucking 30 inch rounds till the gas ran out then fuel up and do it again and again
after the rebuild it ran great would sit and idle for a half hour if you let it.
I sold it for parts and bought a bigger saw to do what I really need to do , rip through 30 inch rounds over and over