261 autopsy

[Daniël Bos]

I'm in Australia helping a friend on their farm and their 2020 261 died whilst cutting firewood.

I was fairly surprised they had a 261, farmers tend to go bigger but cheaper...

Anyway, it got hot and lost compression. I advised them to dump the fuel from the other saws (026 and 211) using the same can just in case and there were no further issues with these two other saws in the rest of the two days of use.

The 261 got hot (very hot) and lost compression.

I tore it apart and found the piston had a hole melted through... The bore is scored beyond repair.

I can't feel any play in the crank, the carb boot is supple and without splits or tears etc. I've not yet taken the flywheel or clutch off to inspect the seals but i've not seen seals fail without the bearing failing first in a saw so young? 

Spark plug looks a nice colour, maybe even a little dark?

 

So how did it get so hot? 

 

I think I'll rebuild it it with an aftermarket p+c (a Stihl p+c is more than half the cost of a new saw here)

 

Edited April 22 by Daniël Bos

[Dan Maynard]

I'm thinking lean can be caused by too much air, or not enough fuel - maybe carb blocked with fines? Carrying on with blunt chain produces a lot of dust, how is their sharpening?

[GarethM]

Lack of oil in the mix would be my guess.

[Daniël Bos]

56 minutes ago, Dan Maynard said:

I'm thinking lean can be caused by too much air, or not enough fuel - maybe carb blocked with fines? Carrying on with blunt chain produces a lot of dust, how is their sharpening?

Air filter is clean, carb is clear.

They're using a carbide tipped chain that isn't blunt 

[Daniël Bos]

56 minutes ago, GarethM said:

Lack of oil in the mix would be my guess.

It's what I'd guess if I didn't know better.

They have almost clear petrol here and red oil, there was definitely oil (though nobody knows how much oil...)

[Muddy42]

Lack of two stroke oil, not warming up the saw, operator pushing too hard, fins/fuel filter/air filter/carb clogged. Broken ring or metal in the cylinder?  The intake side of the cylinder also looks bad - so debris could have got in there.  Who knows. 

 

Pressure and Vac test is a must. Put the cylinder back on, clean the saw and do it now, before you waste money on parts. And watch out for debris falling into the bottom end (one of your photos shows this).

[GarethM]

Daft secondary thought, what about wrong spark plug ?.

 

As that looks suspiciously like it's been hitting it and looks to be in around the exact same location.

Edited April 22 by GarethM

[openspaceman]

Strangely the piston crown away from the exhaust side looks a nice tan brown. It looks like it melted rather than impact but over protruding plug could explain the hot spot.

 

I wonder if the use of carbide chain is significant, from my limited experience of it it provides less resistance so revs go up and you tend to press harder. If the saw is running lean it would tend to rev higher than it should anyway.

[spudulike]

Typical seizes are using old fuel oil mix where the oils lubrication has been degraded, lack of or no two stroke oil in the fuel, air leak or running lean. Running lean can also include split fuel lines, blocked fuel filters, blocked gauze strainers etc 

The piston crown damage is interesting. It is possible that someone has damaged it using a poor choice of piston stop or it has caved due to the heat.

The damage looks like a bad case of pre ignition but that could only happen if the coil had developed a weird fault or the flywheel managed to shear it's key and stop a few degrees in advance. 

Never seen one that bad before.

[Mark_Skyland]

Ive never seen a piston with that type of damage before. As suggested it looks like a plug fault/ timing issue due to the damage on the crown and intake side of the piston. Any chance something has broken away and been sucked into the engine?