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Author Topic: Stihl 261s Scoring Pistons  (Read 2283 times)

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Offline JaredR

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Stihl 261s Scoring Pistons
« on: August 06, 2020, 09:15:08 PM »
Hi all,

I'm new to the forum and looking for some answers. We recently started building handcrafted log homes so I went to the local Stihl dealer and bought a 261 C-M for our notching and small saw work. After using it a few days (less than 10 hours on the saw) it wasn't starting and obviously had a problem. Took it back to the dealer and he takes the muffler off and the cylinder and piston are scored badly (see photo)  and he said the only time he had seen that happen was from unmixed fuel. However, we are running Stihl mixture at the suggested ratio and because we have built a good relationship with him he decided to look into it further instead of just saying we ran straight gas in it. He called his "higher-up" who asked what we were using it for and when he heard it was for log building he said immediately that the usage was too light and the saw had somehow self-adjusted and was over-revving, burning itself up. (We do a lot of brushing and light cutting, running it at half throttle). Apparently they had ran into this before with the smaller C-M models and log builders.

While he is replacing the piston and cylinder under warranty I went ahead and bought a regular carburated 261 which we have been using and it's been running great. However, this afternoon my brother sent me a photo of the piston on the new one and said it's scored and not starting. I have no idea what is causing this. Do Stihls seriously self-destruct if not ran under constant load? I'll be taking the saw back to dealer tomorrow as well as our 15gal tank of mixed gas that we have been using but would like to get some outside thoughts on this.

Thanks very much!



 

Offline Al_Smith

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Re: Stihl 261s Scoring Pistons
« Reply #1 on: August 06, 2020, 10:42:44 PM »
Although it might be unpopular  i might throw my 2 cents in .First I assume it's a fresh air part transfer and assume it's set up for 50 to one oil to gasoline ratio.
From my perspective firstly 50 to one is too lean .Secondly the saw had to be tuned too lean .Thirdly using fresh air with no oil laden fuel is asking for trouble .Although you might blow most of the fuel out on  fuel laden transfer you leave the oil film on the cylinder .With fresh air it's less oil .
Now I know why they did it because of EPA mandates but that doesn't mean it's a good idea .-2 cents nothing more 

Offline ZeroJunk

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Re: Stihl 261s Scoring Pistons
« Reply #2 on: August 06, 2020, 11:32:56 PM »
I have read the physics on why regular carburated two cycle engines don't like being run at half throttle. It was written before M Tronic etc. had been conceived , so not sure how that applies. But, the carburetors can be quite lean at half throttle. I don't know that it would ever be a problem limbing because the end destruction is heat, so even if it is lean you wouldn't be at a duty cycle high enough to melt the piston. But, if you are just steady cutting a log at half throttle you would.

Offline joe_indi

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Re: Stihl 261s Scoring Pistons
« Reply #3 on: August 06, 2020, 11:33:36 PM »
50:1 on these saws is begging for trouble. Here I recommend customers to go for ' other oils' ratio which is 25:1. Going midway on these two would be a 30:1 for any oil. We don't have Mtronics here but the carburettor versions of stratified saws and usually to be on the safe side I adjust the carb more generously. This prevents over heating. We are working in 34 C temperature and for you to get your piston scoured like this the temperature needs to be much higher or the carb leaner

Offline Ianab

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Re: Stihl 261s Scoring Pistons
« Reply #4 on: August 06, 2020, 11:46:40 PM »
I have heard of chainsaw carving guys having issues due to running the saws at part throttle for extended periods. The carbs on a chainsaw are relatively simple affairs, and are basically set to run at full throttle, or idle, as long as both those work, it's usually OK. They don't have the complicated fuel maps that a fuel infected car engine uses. The mixture might be wrong at 1/2 throttle, but saws don't usually spend much time running like that. 

A richer fuel/air mix and a bit more oil might help, but it's likely down to the operator. They aren't intentionally abusing the saw, but it's just not designed to be run like that. Let it idle, or give it the revs.  You might only be cutting through 2" sticks, but give it a blip of full noise to do it. That way it's not running lean, overheating AND starving for oil. 
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Offline barbender

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Re: Stihl 261s Scoring Pistons
« Reply #5 on: August 07, 2020, 12:06:16 AM »
Sorry about your saws! While I don't like a 50:1 ratio, I sure doubt that's causing your issue. When I was was brushing and such log building, I tried to just continually blip the throttle, instead of holding it at continuous medium rpms. 
Too many irons in the fire

Offline Spike60

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Re: Stihl 261s Scoring Pistons
« Reply #6 on: August 07, 2020, 05:51:27 AM »
50:1 isn't best for any saw, but it's at that ratio that modern saws will remain emissions compliant. That's why everything is supposed to be run at 50:1. I'm a 40:1 guy for my stuff; both newer AT saws and the old girls in the collection. But the deal here isn't the ratio, it's the constant 1/2 throttle running.

I think that just about every saw's owners manual will say that cutting should be done at full throttle. Probably more of a safety concern than anything. As far as leaning out at constant part throttle use I can see that happening. Every carb has a "lo" speed circuit and a "hi" speed circuit. There is no provision in the carb for low RPM running. At part throttle use, neither the lo or hi speed can operate as they are designed to. And low RPM's results in less cooling air from the flywheel.

Solution here would be a good electric saw run off a generator on the job site. Would also eliminate exhaust fumes as the house starts coming together and you are working in tighter areas. I don't think that you or the dealer would want to take a chance on burning up a 3rd saw.
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Offline Patrick NC

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Re: Stihl 261s Scoring Pistons
« Reply #7 on: August 07, 2020, 09:36:19 AM »
Ive never had any problems with 50:1. I used it in my ms260 pro for 10 years without a problem. Now I run a Husqvarna 372xtorq and a 550xp mk2 with the same mix. No problem yet. I only use premium synthetic oil ( husqvarna xp+ ) and never mix more than 2.5 gallons at a time. I agree with the theory of running full throttle when cutting. 

Offline Al_Smith

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Re: Stihl 261s Scoring Pistons
« Reply #8 on: August 07, 2020, 09:45:24 AM »
Oh goody the oil wars again . 8)

Offline Patrick NC

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Re: Stihl 261s Scoring Pistons
« Reply #9 on: August 07, 2020, 09:52:50 AM »
Oh goody the oil wars again . 8)
Sorry, I wasnt trying to say what was right. Just relaying my experience. If never argue for someone change what works best for them. 

Offline Patrick NC

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Re: Stihl 261s Scoring Pistons
« Reply #10 on: August 07, 2020, 09:54:27 AM »
Autocorrect. I meant to say that Id never argue to change what works for someone else. 

Offline Al_Smith

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Re: Stihl 261s Scoring Pistons
« Reply #11 on: August 07, 2020, 11:34:43 AM »
 :D This great debate of oil mix and brand is a long standing type of debate going back as far as the internet and online forums .It can become amusing if you don't take it too seriously .

Offline JaredR

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Re: Stihl 261s Scoring Pistons
« Reply #12 on: August 07, 2020, 02:11:33 PM »
Thanks for all of the replies, guys!

I took the carb one to the dealer this morning and he is going to tear it apart. He's curious if both saws came from the same batch.

As far as cutting with a full throttle, I don't think that applies to brushing. I've been told to run about half throttle and "blip" it as @barbender mentioned.

I'm going to wait to hear back from him, and am hoping that the saw has a defect. If it doesn't I'm not sure what to do as we've been following instructions from more experienced sawyers and builders. I've even asked Robert Chambers about it and we were using it as he recommends.

Offline mike_belben

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Re: Stihl 261s Scoring Pistons
« Reply #13 on: August 07, 2020, 02:22:37 PM »
Saw carbs dont have much in the way of midrange metering circuits like a sled or bike. Its just idle and main jet, the midrange is just drilled passages at best.  

Id richen the mix and increase the idle time for cooldown breaks.  I sawmill with my 395xp at about 3/4 throttle all the time on the 87octane with the cheapest oil i can find, typically 30:1 and tuned fat.  I get a lot of vibration at WOT.  Lot of idle breaks to reject heat. 
Revelation 3:20

Offline lxskllr

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Re: Stihl 261s Scoring Pistons
« Reply #14 on: August 07, 2020, 03:02:11 PM »
Is that common knowledge saws shouldn't be run at 1/2 throttle? First I've heard of it. That should probably be put in the manual, with the reason given, instead of a generic "Cuts should be made at full throttle". I've run my mill at half throttle with the intent of letting it cool down a bit before going WOT again. Sounds like that's a bad idea.

Offline gspren

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Re: Stihl 261s Scoring Pistons
« Reply #15 on: August 07, 2020, 03:21:57 PM »
I never heard or at least don't remember the why but my chainsaw mechanic said the throttle should be used like a light switch, all on or off.
Stihl 041, 044 & 261, JD 2355 4X4 w/fel, JD 620, Yamaha Kodiak 400 & trailer, Kubota 400 RTV,  P&M OWB, 75 acres to play.

Offline JaredR

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Re: Stihl 261s Scoring Pistons
« Reply #16 on: August 07, 2020, 03:35:20 PM »
Is that common knowledge saws shouldn't be run at 1/2 throttle? First I've heard of it. That should probably be put in the manual, with the reason given, instead of a generic "Cuts should be made at full throttle". I've run my mill at half throttle with the intent of letting it cool down a bit before going WOT again. Sounds like that's a bad idea.
Same here. I'm also hearing a lot of conflicting info from multiple sources. The manual only says that you shouldn't run WOT until you use about 5 tanks. I didn't see anything about damaging it if used at 1/2. So apparently don't run it WOT or at 1/2, and if you do run it make sure it's under full load at all times. Or chance it burning up.  smiley_furious3 I think I will be seriously looking into battery chainsaws.

Offline Al_Smith

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Re: Stihl 261s Scoring Pistons
« Reply #17 on: August 07, 2020, 03:49:39 PM »
 After I rebuild a saw I tune it fat,run a few tanks like that .Then lean it out and flog it like a rented mule .I don't baby them .

Offline Real1shepherd

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Re: Stihl 261s Scoring Pistons
« Reply #18 on: August 07, 2020, 05:01:10 PM »
On the Swedish saws....no reason to baby them new or after a rebuild. Use a good oil mix and run them like you're gonna use them, hard.

As been said above, an electric saw is less bulky and gets into smaller spaces. Running a chainsaw half throttle most of the time is not getting enough oil mix in the works.

Kevin

Offline JaredR

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Re: Stihl 261s Scoring Pistons
« Reply #19 on: August 07, 2020, 05:04:24 PM »
So are there any log builders here who have some input? How do you run your saws when notching and have you had this issue?


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