Ms260 Rebuild by a beginner

[Tinkerer Carver]

Hello gentlemen, I am about to undertake my first saw rebuild and Mastermind told me I should post it, so here I am. For some background, I work at a woodcarving shop and run saws all week long and know how to tune and maintenance them, but have yet to rebuild until now. Included are some pics for why I’m rebuilding.

 

[Tinkerer Carver]

Parts are here! Rebuild should start this weekend once pesky finals are over

 

[ElevatorGuy]

@drf256

 

[Mastermind]

I'm gonna go ahead and get in here with you. I'm glad you posted this thread.

 

[Spladle160]

Cool, looks like a great project. Top end only or are there new bearings hiding around there somewhere? OEM top end or trying something else? Welcome and thanks for sharing.

 

[farminkarman]

Thanks for taking the time to document & share. I usually learn something new from these threads.

 

[RI Chevy]

Did You find out the exact reason for why the piston scored?
Cool thread. Going back to basics here at OPE.

 

[Mastermind]

Did You find out the exact reason for why the piston scored?
Cool thread. Going back to basics here at OPE.

This.....

Gotta determine why it scored in the first place.

 

[FergusonTO35]

Eagerly watching this thread since I have very little experience rebuilding 2 strokes also.

 

[pwheel]

Did You find out the exact reason for why the piston scored? ...

FWIW, I had to remove the tab on the high speed limiter cap on my ms260 because it was running too lean.

 

[farminkarman]

FWIW, I had to remove the tab on the high speed limiter cap on my ms260 because it was running too lean.

I’m pretty sure that was the cause of scoring on two ms271’s I rebuilt recently. Checked out fine other than being too lean on the high jet.

 

[FergusonTO35]

So, being lean can cause little strips of scoring like that?

 

[Mastermind]

So, being lean can cause little strips of scoring like that?

It looks like it really just scored to begin with on the edge of the exhaust port.....then the piston became overly tight in the bore because of the transfer, and forced it to be too tight on the opposite side, lost it's oil film, and then that spot began to transfer. So yes.....I'd say it was lean (because of an air leak, or tune I can't say with any confidence) and began scoring next to the exhaust port where the heat is the highest.

 

[Tinkerer Carver]

Cool, looks like a great project. Top end only or are there new bearings hiding around there somewhere? OEM top end or trying something else? Welcome and thanks for sharing.

Top end was the plan for now. I’ve been turning the crank over by hand and looking and listening for anything wrong and it seems to rotate like it should. That said, I’m having my brother who is a welder/machinist help me determine the condition of the bearings

 

[Tinkerer Carver]

FWIW, I had to remove the tab on the high speed limiter cap on my ms260 because it was running too lean.

I had planned to remove the limiter tabs anyway because I plan to warm it up, but also since mastermind had said running too lean was likely the cause of the scoring

 

[Mastermind]

I had planned to remove the limiter tabs anyway because I plan to warm it up, but also since mastermind had said running too lean was likely the cause of the scoring

Too lean can be caused by a lot more than tune alone though.

 

[Tinkerer Carver]

Too lean can be caused by a lot more than tune alone though.

Huh, I hadn’t really ever thought of too lean as anything other than a tuning issue. What could be the other causes so I know what to look for?

 

[Tinkerer Carver]

Too lean can be caused by a lot more than tune alone though.

I read back and saw that you said an air leak was a possibility. What signs could I look for on the saw to determine that?

 

[Chainmale]

I read back and saw that you said an air leak was a possibility. What signs could I look for on the saw to determine that?

Fuel hoses, impulse hose, intake boot, crank seals, cylinder and case gaskets

 

[drf256]

We are here to help. Personally have done a few of them 026/260’s.

Which parts do you have? Looks like a 44mm top end to me. I’d use that jug cleaned up, you won’t get the same performance from an AM P&C set. Meteor pistons are fine. That broken fin won’t do a thing.

Do you definitely have a carb with 2 adjustment screws? They came both ways, and IIRC only the Zama has an adjustment limiter. Never seen one on a Walbro on the 1121 model.

If you plan on working on saws, a vacuum and pressure testing tool is a must. In general. A small air leak will result in issues with keeping idle stable. You’ll chase it forever.

You are really forced to go through everything because the saw scored in the first place. Invest in a new oem fuel line and filter and a new oem impulse line. Carb needs to be gone through as well, may as well rebuild it in case there is an issue with fuel supply with the carb itself. The screen may be caked with the debris or there may be a metering issue.

The 1121 is one tough little saw. The bearings are oversized and usually last a very long time. If there is no radial play and they look good, you can probably get by with leaving them alone. A case split may be more than you’re up for at this point.

I know it sounds like a lot, but you can do it.