stihl 391

Just bought a used Stihl 391 on Craigslist for $400 w/ 20" bar and new chain. Looked ok, started ok, seller told a pretty good story. Bucked thru 4 or 5 30" dia American Elm logs, ran strong. Then it stalls while cutting. Starts right up on 1st pull, runs about 15 seconds, then stalls...

I've been running Stihl's for 20+ years. Usually find used ones to buy but used prices are within $150- 200 of a new one and those that aren't are beat to death. Currently running a 290, 310 and an 041(only when I have to because it beats the hell out of me). I'm looking at a new 391 with a 20"...