MS 261 RPM Maxed out @ 12,600

[Crispexx]

Hey all, I have an MS 261 non-mtronic that was given to me in pieces. Top end was removed and the saw had a lot of screws missing but everything else was pretty much there. I have only recently started getting into saws and small engines in general so i figured this would be an awesome project. Purchased a Hyway brand Nikasil plated top end rebuild kit from HL Supply and went to work. While going to town I saw the carrier for the oiler was broken so i found a whole oiler assembly kit on eBay and got that fixed as well. What I did NOT do after fully assembling was pressure or vacuum test it. YET. the saw starts and runs but when tuning the carb it simply will not lean out and rev above 12,500-12,900 RPM. I found another OEM carb on ebay and swapped them out thinking it might be the carb, but it does the exact same thing. Both carbs were cleaned prior to running. Fuel is ethanol free 92 octane mixed 50:1. I am going to pressure and vacuum test it this afternoon so check the seals and all but I was wondering if you guys have any other idea what might possibly be wrong with it if not a leak? I am almost leaning towards me messing up the top end install and breaking a piece of a ring or something so the compression sucks? Looking through the muffler and intake there is no visible scoring on the piston or cylinder walls. I do not have the ability to test compression at this time but it does start and run. just not to its potential. Thanks all!

 

[RED-85-Z51]

Some aftermarket top ends don't have ideal port timing and can restrict top speed

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[Andyshine77]

Like said above most of the aftermarket top ends have not so great let timing numbers, plus other things that make them run weak. This isn't the first time something like this has come up. Most cylinder can be saved with a little work, so often a new piston and rings and off you go. How bad was the OEM top end?

 

[drf255]

Check the spark screen. It could be caked up with carbon.

Even though it could be the AM top end, saws need to breathe to rev. I’d try a run without an air filter as well.

A small muff mod doesnt hurt these at all, and would still be fine if you go back to an OEM top end.

 

[Crispexx]

Like said above most of the aftermarket top ends have not so great let timing numbers, plus other things that make them run weak. This isn't the first time something like this has come up. Most cylinder can be saved with a little work, so often a new piston and rings and off you go. How bad was the OEM top end?

The piston was missing when he gave the box over but this is what the cylinder looks like, we were discussing how some of those marks might just be remnants of the rings. But honing a cylinder is a bit above my experience level [emoji2371]


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[Crispexx]

Check the spark screen. It could be caked up with carbon.

Even though it could be the AM top end, saws need to breathe to rev. I’d try a run without an air filter as well.

A small muff mod doesnt hurt these at all, and would still be fine if you go back to an OEM top end.

The screen has been removed, but it wasn’t bad at all. The filter is brand new but it was also ran without and same results. We actually did open up the muffler yesterday evening and I ran it this morning and still no change.

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[RED-85-Z51]

Like said above most of the aftermarket top ends have not so great let timing numbers, plus other things that make them run weak. This isn't the first time something like this has come up. Most cylinder can be saved with a little work, so often a new piston and rings and off you go. How bad was the OEM top end?

Is there such a thing as a limited 261 coil?

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[CR888]

That saw should have an unlimited coil, my 2015 MS261 is runs out to 15,300rpm. I think you could sand off that transfer in the cylinder with fine grit paper and Scotch Brite. I really can't comment on Hyway kits, some are probably good others not so much, consistency is the problem with AM compared to OEM. If I was you, I'd pressure/vac test the saw. Compression test the saw. Then I'd dial I. The timing, you can net close to 1k of rpm dialling in the timing, I don't shoot for absolute max rpm but as close as I can without any poor running characteristics. If your still not happy which I wouldn't be with what you have I'd clean up the OEM saw and put a meteor piston or OEM piston in it. Sell off the AM cylinder and spare carb. With a bit of work you could have a good saw.

 

[BradSt]

That OEM cylinder doesn't look that bad. As long as that metal is only transferred from the piston, and the plating isn't damaged, it should be able to be cleaned up. I use muriatic acid on a q-tip, and gently rub the transferred aluminum, until it is totally removed. It will eat it quickly, and give off nasty fumes. Be careful about where any acid runs, as it WILL eat bare aluminum anywhere it touches, until it is neutralized. I then wash the cylinder thoroughly with hot soapy water. I may "deglaze" the cylinder with a scotch brite pad as well, depending on the condition of the plating.

 

[RED-85-Z51]

Will it go above that rpm with the chain off?

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[Crispexx]

Will it go above that rpm with the chain off?

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I'm not sure, i can try that later in the day though

JavaScript must be enabled.

That OEM cylinder doesn't look that bad. As long as that metal is only transferred from the piston, and the plating isn't damaged, it should be able to be cleaned up. I use muriatic acid on a q-tip, and gently rub the transferred aluminum, until it is totally removed. It will eat it quickly, and give off nasty fumes. Be careful about where any acid runs, as it WILL eat bare aluminum anywhere it touches, until it is neutralized. I then wash the cylinder thoroughly with hot soapy water. I may "deglaze" the cylinder with a scotch brite pad as well, depending on the condition of the plating.

I'm going to give it a shot at cleaning that cylinder up and see how it looks.

 

[RED-85-Z51]

If it hits 12,900 no chain...then id be looking at the coil maybe. If it pegs out...that cylinder is where id look

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[Justsaws]

Not at all familiar with the 261s. However I would also check these items...

Make sure that the carb linkage is allowing the throttle butterfly to open all the way and holding it tight.

Make sure the choke butterfly is not slightly closed and being held tight.

Do your check with out the air cleaner.

Do you get any change messing with the carb at all?

Make sure that any passages from intake of the carb through the muffler are not misaligned or obstructed.

 

[drf255]

The oem pistons for 261 are sub-$50 at dealers. I’d clean up that jug as well, it sure looks usable.

I use a hybrid method of acid on a Q-tip and sanding as well, as mentioned above.

 

[Crispexx]

Not at all familiar with the 261s. However I would also check these items...

Make sure that the carb linkage is allowing the throttle butterfly to open all the way and holding it tight.

Make sure the choke butterfly is not slightly closed and being held tight.

Do your check with out the air cleaner.

Do you get any change messing with the carb at all?

Make sure that any passages from intake of the carb through the muffler are not misaligned or obstructed.

The linkage and butterflies are all in good working order, i have ran it without the air cleaner and it still did the same. When I adjust the high on the carb I can get it to go rich but when I try to lean it out it doesn’t budge in regards to the RPMs.


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[Crispexx]

The oem pistons for 261 are sub-$50 at dealers. I’d clean up that jug as well, it sure looks usable.

I use a hybrid method of acid on a Q-tip and sanding as well, as mentioned above.

I have some OEM rings on order, I’ve been looking for acid and I’ll be damned it’s a pain to try and find locally!


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[BradSt]

I have some OEM rings on order, I’ve been looking for acid and I’ll be damned it’s a pain to try and find locally!

Home Depot/Lowes/Hardware stores should all have it

 

[Crispexx]

Home Depot/Lowes/Hardware stores should all have it

Yup that’s where I’m headed tonight


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[pioneerguy600]

I have some OEM rings on order, I’ve been looking for acid and I’ll be damned it’s a pain to try and find locally!


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Brick,mortar and stone outlets handle it here, used to wash concrete and mortar droppings off stone and brick. Swimming pool accessory stores also sell it for shocking water. Should add that all floor tile outlets also sell acid.

 

[Justsaws]

If all else is in good shape I would suspect that the carb adjustment screws are limited in rotation based on what Stihl has determined to be needed for an OEM P/C and cannot be turned in to lean out enough for the less effective AM cylinder. The aftermarket P/C is probably not up to Stihls assumed potential and cannot move mix effectively through combustion. I would remove the screw limiters and go from there. Having said that, I do not know if that carb has limiters, as well if you put the OEM cylinder back on you will need to readjust to the original position. If the coil is not limited then it is pretty straight forward, limited coil makes it a bit trickier.

With the AM P/C any Stihl tech info is worthless, it would be best to simply try the saw and see how it runs during use. The rpms are not as important if it is tuned decent during use.