New MS261

Magicman · May 27, 2024, 08:23:38 PM

I have had this saw for about 6 months and have used it around the sawmill but I had never had an opportunity to stick it in a 20" Post Oak log and call it by it's first name.

Wow, the most logical question that I can ask is "where have you been all of my life?" It has to be the fastest saw that I have ever used.

With the 18" bar, I let it roll around and cut the backside first and then had to be careful on the bottom to keep it out of the dirt. This end picked up after the butt log came off.

Anyway, this MS261 is bad to the bone!!

PoginyHill · May 28, 2024, 07:43:08 AM

Nothing bad about Stihl or the 261, but a lesson I learned this weekend. I've had the 261-C for probably more than 5 years. I use a 16" bar (only because I generally don't cut much bigger than that and have a larger saw/bar when I have the need - and don't need to extra weight or length to inhibit safe/easy handling). I think the bar I have came off an old 026 I had. I changed the saw from 0.325" to 3/8" to use some older stuff I had and to match the 391 and 462 I have. Anyhow this bar has occasionally thrown chains - mostly when I'm cutting small diameter brush to control slash pile height. It messed up the chain catcher enough I had to change it about a month ago.

Happened again Sunday. Was able to bend the chain catcher back in place, but the portion of the link that rides in the bar got damaged enough I had to grind them down with a Dremel tool. Chain would not ride in the bar otherwise. Put everything back together and threw the chain again Monday. This time it took off the E clip, washer and demolished the needle bearing.

Looking it over - and finally noticing that all of the original Stihl marking had worn off, I'm thinking the bar was badly worn and probably had a lot more use on it that I ever gave thought to. So I have new stuff on order. I'm hoping this is the cause and not something else. I don't think I had the same issue with any other saw - at least to the extent I've had with this one.

How does one determine the bar is worn beyond reasonable use?

Magicman · May 28, 2024, 08:16:58 AM

Can't answer your question, but for whatever reason I changed my old 028AV from .325 as you did.

After giving the MS261 a workout yesterday, I know that I will never change it. I figure that maybe Stihl knows something about the power ratio that I don't so I won't question it.

booman · May 28, 2024, 08:49:04 PM

I am near to 80 yrs old and have been running chainsaws all my adult life. In that time I have never seized a chainsaw until recently. One of my favorite chainsaws is my Stihl MS261c. It recently slowed down and then stopped. Couldn't turn it over until later but it then had no compression. My go to chainsaw mechanic said it was most likely the decompression valve getting gunked up and leaking air in and causing it to run lean and hot, therefore seizing. I really dont ever use this valve to start the saw. Got it back today and it is running fine with a new piston and rings and a plug where the decomp valve used to be. Mechanic said the decomp valve needs to be replaced about every 2 years. Anyone else had this problem?

Old Greenhorn · May 28, 2024, 09:04:15 PM

I have never heard this before. Can you comment about whether this is related to the 261, the Stihl brand, or is it related to the valve itself which would imply all makes and models? I have never had an issue with a decomp valve, so I am just trying to learn a bit more. I've got saws that have had those valves in there a lot more than 2 years.

booman · May 28, 2024, 09:50:29 PM

First I have heard of this and this saw is a number of years old. I have other saws that have the valve and have not had a problem. Like I said, never had a saw seize on me before. I run non ethanol gas with Stihl synthetic oil.

Hilltop366 · May 28, 2024, 11:14:48 PM

A issue (weak point) that I have heard of with the 261 is the clutch side crank bearing can go bad and cause the seal to leak which would cause it to run lean.

I do know a guy that made a bearing puller so he could remove the bearing after taking the seal out without tearing down the saw. Was told that one of the advantages of the Stihl M-tronic is it will compensate for the lean if the seal starts to leak a bit.

Over tightening the chain / tightening the chain when it is hot are things to avoid with any saw but the 261 seems a little more sensitive to this.

Booman, make sure your mechanic has checked the crank bearing and seal.

doc henderson · May 29, 2024, 08:17:21 AM

I have decomp valves on my pro saws, and never had one go bad. When you talk to a guy that fixes stuff all day, doc or mechanic, all they see are the problems. Good to be aware of but doubt I would start replacing them as a consumable.

booman · May 29, 2024, 04:07:20 PM

Tech said he vacuum and pressure tested the seals and they were not leaking. Rotated crank, applied pressure, no leaks. He said if the crank moves enough to cause the seal to leak, that crank bearing is toast.

ladylake · May 30, 2024, 04:09:01 AM

Quote from: booman on May 28, 2024, 08:49:04 PMI am near to 80 yrs old and have been running chainsaws all my adult life. In that time I have never seized a chainsaw until recently. One of my favorite chainsaws is my Stihl MS261c. It recently slowed down and then stopped. Couldn't turn it over until later but it then had no compression. My go to chainsaw mechanic said it was most likely the decompression valve getting gunked up and leaking air in and causing it to run lean and hot, therefore seizing. I really dont ever use this valve to start the saw. Got it back today and it is running fine with a new piston and rings and a plug where the decomp valve used to be. Mechanic said the decomp valve needs to be replaced about every 2 years. Anyone else had this problem?

There is no vacuum in the cylinder on a 2 stroke, pressure on the power stroke and then pressure with the intake charge. How does it suck air though a bad decompression valve. Steve

booman · May 30, 2024, 07:32:14 AM

My understanding is you seal it off and pull a vacuum on the chamber to see if there are leaks. Same with pressure. Is this correct? Also, when the engines is running and the piston goes down, what comes in to fill the vacuum being created? I don't work on 2 strokes but are they that much different than a 4 stroke? Help me out.

booman · May 30, 2024, 07:42:19 AM

Didn't mean to hijack your thread Magic.

PoginyHill · May 30, 2024, 07:51:41 AM

Every time the piston comes down, it is forced down by the ignition of the fuel. There is no point at which the combustion chamber is under vacuum. Only the crankcase goes under vacuum - when the piston is going up to draw in fuel/air from the carburetor.

booman · May 30, 2024, 08:29:37 AM

So there is a vacuum involved. My point is that during this vacuum could extra air be drawn in through a partially open decomp valve?

PoginyHill · May 30, 2024, 08:47:21 AM

Not through the decomp valve (it's in the combustion chamber), but anything related to the crankcase is possible - gaskets, seals, cracks, etc...

booman · May 30, 2024, 09:55:37 AM

I agree with what you are saying, just trying to find a way that air gets drawn in through the decomp valve. Trying to figure out why my 261c fried. Same fuel I use in everything else I run. No other leaks.

Magicman · May 30, 2024, 10:24:18 AM

When the piston goes down under compression, that creates crankcase pressure. That pressure which also contains the fuel mix from the carburetor is then dumped into the cylinder above the piston. The piston is now again in it's compression stroke. There is valving arrangements between the crankcase and the carburetor to keep this crankcase pressure from going anywhere except to the cylinder.

The cylinder has pressure from the compression stroke and pressure from the crankcase pressure. There is never a vacuum in the cylinder.

Note to booman, there is never a hijack here on the FF. We are all students and learning.

doc henderson · May 30, 2024, 10:28:15 AM

maybe the scoring of the piston rings resulted in the brief period of not running well, and then it finished off.

booman · May 30, 2024, 12:22:36 PM

I read this online, even tho it doesn't affect the decomp valve: As the air/fuel mixture in the piston is compressed, a vacuum is created in the crankcase. This vacuum opens the reed valve and sucks air/fuel/oil in from the carburetor. Once the piston makes it to the end of the compression stroke, the spark plug fires again to repeat the cycle.