someone on Another Site asked a similar question. Here is my response. Hope it helps here.
owed 13 or 14 661s so far. changes i've seen...
1144B vs 1144C Jug - Can't tell you the difference or if there is one even. I have an 1144B and it is one of the best running saws I have. See pics.
Intake boot - this was a problem for sure. Stihl completely changed the boot. See pics.
coil / control unit - this again was a problem. 4700C 4700D would allow the saw to go too lean...lean seize. 4700E and the newest 4701A have fixed the problem. The 4701A came out like 1.5-2 yrs ago.
Clutch covers - have seen pics of a couple of cracked cases. No one I have talked to (some very smart people) can see how the clutch cover could cause the problem. All parties asked said - user error...most likely prying on his bar. The two pictures I've seen have come out of Canada. I know many of those guys use 36" tsumura light bars. The mount area is smaller on those bars relative to stihl, cannon, oregon, etc. In my opinion, I could see how that smaller mount area and prying on the bar could focus more energy into one spot vs a wider mount where the force would be more spread out.
I had a 661 which it's serial # fell inside the recall range D.O.M was early 2013 and serial # was like 176 something. It had been gone through as it had the new 4701A coil, new boot, and an 1144C jug. One of the best running 661s I have had. So maybe they gave it extra TLC at the factory when they went through and updated it? I dunno. Me being the dummy I am, sold it!
anyway I would say check the simple stuff, change to new boot and coil if ya don't have em. Take the baffle out of the muffler. Put in a wider weave screen in. Then do a reset and you should be golden
