Stihl MS261 mtronic

[BlueMax]

An update, thanks for all the help and suggestions. The machine is now stripped completely. Turns out than the worm gear for the oiler was frozen solid. Possibly the mating gear that rotates with the clutch was stripped, as it’s completely smooth and has no teeth to rotate the worm .The bearing behind it was all shot, it actually was missing at least one ball and the plastic ball spacers and the seal were non existent. no wonder it did not run well or idle, it’s like having a swine carb to suck air through!
 

Difficult to tell which caused what to fail first, the frozen oil pump or the busted bearing. The oils tank had a lot of crap inside as well-possibly old oil was used at some point rather than chain oil.

 

the crank appears to be fine, as I think the lack of a ball or two was making it run eccentrically, as shown by a bit of wear in the inner race of the case.the pistons, rings and cylinder all appear visually fine with no obvious signs of wear.

Now I face a rebuild and should be good as new. Thanks for your help.

( and by the way tying 3 knots in a small bit of cord and inserting via the plug hole to use as a piston stop is a brilliant hack!)

[Dan Maynard]

Crikey that is bad! At least you can point at what's wrong and know which bit to fix though.

[spudulike]

Looks like the crank lobe has been catching the inside of the crankcase as well. I would check the surface where the drum and oil pinion run on the crank shaft VERY carefully. The earlier ones had extremely ropy hardening that broke down very quickly scrapping many a machine. Get a magnifying glass on it and inspect for pitting before rebuilding.