I am stumped

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Neighbor brought a saw over with a strange problem. Chain gets very tight or very loose upon starting and running it for a very brief period.

Seems to be oiling correctly, we tighten the hell out of the bar nuts, even marked the bar against the powerhead and there is no movement of the bar. Chain is 0.063, even checked the DLs with calipers, ran feeler gauges (several 0.021+0.025+ to get to 0.063+) in the groove and it is good, there is no tight or loose spots. I checked the nose sprocket, and it turns good. The clutch sprocket looks new. The chains are in good used condition. Hell the saw is nearly brand new and is a ms271. We tried 2 of his chains, even mounted and old bar and chain of mine (which I know are good!). He's cut about 15 cords with it and this is a recent problem.

He seems to be aware of the saw, and there is nothing strange that suddenly happened, so there is no obvious clue in his usage, i.e no abusive use.

What is the cause, anybody?
 
could be bearings going down on the crank. I ran into this with a 562 and the problem showed itself eventually.
+1
I have seen several bearing cages fail on that model.
Usually causes running problems though.
I have seen ones that seemed fine, but caused enough wobble at high speed to cause the seal to leak, which, in turn, scorched the piston.
 
Sounds like old damaged chains on a new sprocket.

Sprocket and chains where purchased new together, and the chains are in like new condition. The bar shows some signs of overheating, when he ran the saw with the chains experiencing the overly tight phenomenon vs. the excessively loose condition.

Unless I am going crazy, it is NOT the chains nor the bars. I feel that I have completely ruled that out in mounting my old bar'n'chain which got loose just as fast.
 
Tight then loose chain often points to a badly worn spur or rim drive, Harley is on the right track.

I agree. That does seem the most likely culprit. For sanity purposes I am going to walk over there later today and take a second look at the Spur drive. It is smaller then the spare one off my ms290, else I would swap it out. I will take some pictures also.
 
FWIW, the problem seems to be fixed. Another thing I did yesterday, was take the cover off and cleaned the air filter. The owner had never cleaned the filter -- GASP!!!. I shamed him a little and he has vowed to be more attentive to maintenance. We think the saw has some technology that compensated for the dirty clogged filter, because it seemed to run fine even with the nasty dirty filter. We have theorized that the saw compensated for the dirty filter at the expense of a hotter then normal engine temp, which may have contributed to the excessively tight chain (possible, grasping for straws).

I just ran it, something I didn't do yesterday, and it pulled itself thru the test log fine. And then I ran it at WOT in the air, the engine ran smooth, at a consistent RPM and oil was dripping from the bottom beneath the chain.

Basically everything seems fine this morning, and until things change -- problem solved.
 
Pieces are now coming together. The guy use to own a Husky and after twice having oiling issues got rid of it. Months ago I told him that it was probably the worm gear, a cheap plastic piece, that I could easily have replaced at the cost of the part. He twice spent over a $100 to have the shop do a fix, and after encountering the problem again decided to junk the saw as the cost of repair was exceeding the cost of a new Stihl.

I am now thinking he never cleaned the air filter on the Husky, over heated the engine and the plastic worm gear was the 1st failure point. Hmmmmmm... sounds likely.
 

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