MS261C wont start

[Overkill338]

Afternoon all,
Might as well start off saying I'm not a Stihl tech and far from one, but I pick things up fairly quickly. I work for a fire department in California and we have A LOT of saws and a few of us are fairly saw savvy despite our lack of professional training. I have access to an SDS and a lot of tools and equipment to basically rival a small, small engine shop, we simply need to knowledge and experience. The InternWeb and YouTube has helped tremendously but i really need confirmation on my thoughts. Now to the challenge! I am trying to fix a MS 261 C-M Z that has less than 8 hours of run time. While it was being ran it began bogging down when the trigger was pulled until it eventually just stopped. Visually everything checks out, nothing obviously broken. When i try to start it on the 'choke' setting it'll pop once per pull for about 3-4 pulls then nothing. after pulling about 5-10 times fuel will start leaking out the exhaust and its plumb flooded. I ran it through the 'check engine sheet' or whatever its called and everything held up like it was supposed to. The station that dropped it off brought by a new black fuel solenoid and i replaced it and the saw fired right up! told then to take the saw and before using it to do a 'reset' of the carb with the 90 seconds idling while on 'choke'. Well...saw was brought back to me and it had been used without being reset and i am back to square one with a saw that wont start and is flooded after 5-10 pulls. I did the test to see if the solenoid was seated properly and it passed, did the leak down test where you pressurize it to .5 bar and count how many pulls to reach 0 and it passed with 7 pulls. I did used the SDS and turned off the solenoid and it held .5 bar will pulling that starter as well. I don't want to just tell the guys they ruined a solenoid and to just get a new one until I'm sure but i wouldn't think a solenoid would go back simply by not resetting that carb. Am I wrong? do they need ANOTHER solenoid and then reset the carb?

Mine did that once. The clutch had came loose and backed off tight against the washer and e-clip. Strangest thing I ever dealt with.

 

[Crispexx]

Mine did that once. The clutch had came loose and backed off tight against the washer and e-clip. Strangest thing I ever dealt with.

Yea after it wouldn't start again and he installed the bar and chain again he gave it a few pulls and thats where i saw the chain move. so i thought well maybe the clutch is messed somehow, maybe it did like your did and backed off the stud/chank shaft and pushed tight against the clutch cover. Alas it did not, clutch was on there snug. I still took the clutch off and cleaned it and everything in there just to make sure.

 

[holeycow]

If I was in emergency services I would not want overcomplicated gizmo equipment. Just sayin'.

Hope you get it solved.

PS, as a fire dept you should get special consideration from Stihl to warranty that basically new saw.

 

[Crispexx]

If I was in emergency services I would not want overcomplicated gizmo equipment. Just sayin'.

Hope you get it solved.

PS, as a fire dept you should get special consideration from Stihl to warranty that basically new saw.

This one particular saw has been bugging me for some time now, out of the 30+ saws were running this is really the only one that keeps coming back for more. The hardest part about this whole deal is who is going to end up operating any one of these saws. It could literally be anyone out there who decided to pick it up and run it that day, whether they come from a logging family and have been around saws their whole life or are from the heart of a big city and the first chainsaw they saw in real life was three days ago during their chainsaw class.

 

[Overkill338]

Afternoon all,
Might as well start off saying I'm not a Stihl tech and far from one, but I pick things up fairly quickly. I work for a fire department in California and we have A LOT of saws and a few of us are fairly saw savvy despite our lack of professional training. I have access to an SDS and a lot of tools and equipment to basically rival a small, small engine shop, we simply need to knowledge and experience. The InternWeb and YouTube has helped tremendously but i really need confirmation on my thoughts. Now to the challenge! I am trying to fix a MS 261 C-M Z that has less than 8 hours of run time. While it was being ran it began bogging down when the trigger was pulled until it eventually just stopped. Visually everything checks out, nothing obviously broken. When i try to start it on the 'choke' setting it'll pop once per pull for about 3-4 pulls then nothing. after pulling about 5-10 times fuel will start leaking out the exhaust and its plumb flooded. I ran it through the 'check engine sheet' or whatever its called and everything held up like it was supposed to. The station that dropped it off brought by a new black fuel solenoid and i replaced it and the saw fired right up! told then to take the saw and before using it to do a 'reset' of the carb with the 90 seconds idling while on 'choke'. Well...saw was brought back to me and it had been used without being reset and i am back to square one with a saw that wont start and is flooded after 5-10 pulls. I did the test to see if the solenoid was seated properly and it passed, did the leak down test where you pressurize it to .5 bar and count how many pulls to reach 0 and it passed with 7 pulls. I did used the SDS and turned off the solenoid and it held .5 bar will pulling that starter as well. I don't want to just tell the guys they ruined a solenoid and to just get a new one until I'm sure but i wouldn't think a solenoid would go back simply by not resetting that carb. Am I wrong? do they need ANOTHER solenoid and then reset the carb?

This is the solenoid I took out of my 261 when I rebuilt it. It ran fine, but I had a new one already, so I used it. If you want to try this one, drop me your address.


I also have this if you need one

 

[drf255]

IDK, but isn’t the new solenoid white and not black? I’ve heard the white is the universal upgrade and the black won’t always run right with the newer coils. Sounds like your saw is on the newer side.

The “brain“ of the MT is in the coil. I was told that the coil was lifetime guaranteed by Stihl to the original owner. I could be wrong there, but just going by what I have been told.

The MT carbs have only one jet and the solenoid controls the tune and rpm for choke/idle/WOT. It’s simplistic, but don’t I ever wanna fix them with a BFH when they act up and a screwdriver is useless.

Try different mix. I had one act up on me and that’s all it was. The mix was 32:1 Motul800 which is quite viscous. The time of year caused warm saw running conditions, but the fuel sat overnight and was chilled, adding to the viscosity. A swap to a 40:1 mix of a different oil and fuel solves my issue. This issue seems to be more prominent in the smaller saws like the 261. I’m assuming that the smaller tighter jet can have issues with flow secondary to viscosity.

 

[super3]

Anyways, when he shut it off he pushed from Choke straight to off. correct me if I'm wrong but with those you let them run their 90 seconds then tap the trigger to get it of choke then turn it off correct?

No straight to off.

 

[DND 9000]

I’ve heard the white is the universal upgrade and the black won’t always run right

The white one is not a universal usable part. It depends on the control unit if it can be used or not. Ask your Dealer what you can use.

 

[CR888]

Just with the symptoms you describe, I would pull out the spark arrestor screen in the muffler and make sure it's not plugged up with carbon. Hold it with lookers and hit it with the propane torch which will burn off all the carbon. This is just a small thing that MUST be checked before ordering $$$ parts.

 

[SteveSr]

The white one is not a universal usable part. It depends on the control unit if it can be used or not. Ask your Dealer what you can use.

So the solenoid defaults to open (rich) to allow starting and then the coil module closes it periodically to allow the saw to run? Am I correct?

So if the saw is flooding then it would appear that the solenoid isn't getting the signal to close from the coil... or there is an issue with the wiring between the coil and solenoid? I recently read a thread claiming very delicate wiring on M-tronics that is easy to break if mis-handled.

So you may have an intermittent wiring connection to the solenoid. I would get out an ohmmeter and start measuring resistance/continuity while gently stressing/wiggling all of the wiring and connections. You may be surprised to find a wiring/continuity issue. This could have happened at the factory due to improper crimping and may be exacerbated by damp/wet storage conditions or exposure to a corrosive environment like salt air (if near the coast).

 

[Crispexx]

Just with the symptoms you describe, I would pull out the spark arrestor screen in the muffler and make sure it's not plugged up with carbon. Hold it with lookers and hit it with the propane torch which will burn off all the carbon. This is just a small thing that MUST be checked before ordering $$$ parts.

I checked the spark arrestor and it looked clean, but i pulled it and set it aside for now until we can get it to run


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[Crispexx]

So the solenoid defaults to open (rich) to allow starting and then the coil module closes it periodically to allow the saw to run? Am I correct?

So if the saw is flooding then it would appear that the solenoid isn't getting the signal to close from the coil... or there is an issue with the wiring between the coil and solenoid? I recently read a thread claiming very delicate wiring on M-tronics that is easy to break if mis-handled.

So you may have an intermittent wiring connection to the solenoid. I would get out an ohmmeter and start measuring resistance/continuity while gently stressing/wiggling all of the wiring and connections. You may be surprised to find a wiring/continuity issue. This could have happened at the factory due to improper crimping and may be exacerbated by damp/wet storage conditions or exposure to a corrosive environment like salt air (if near the coast).

So after reading this I pulled the cover off the flywheel and got super excited cause i thought i saw frayed wires coming out of the coil! But once i turned the lights on I saw it was just debris stuck to the wires... so that was a bummer. I’ll have to check the coil as best I can, I’m not super electric savvy so I’m sure I’ll fumble for a while.


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[SteveSr]

So after reading this I pulled the cover off the flywheel and got super excited cause i thought i saw frayed wires coming out of the coil! But once i turned the lights on I saw it was just debris stuck to the wires... so that was a bummer. I’ll have to check the coil as best I can, I’m not super electric savvy so I’m sure I’ll fumble for a while.

Check your PM. Looks like you have a bad/intermittent wiring harness from the factory or highly unlikely a bad coil. You likely won't be able to see the defect in the wiring or connectors. The tell-tale is watching the solenoid resistance change as you wiggle / pull on the wires.

When the connection makes the saw runs. When it doesn't the saw won't start and floods. One other thing to check is the coil/magnet gap.

 

[Crispexx]

Check your PM. Looks like you have a bad/intermittent wiring harness from the factory or highly unlikely a bad coil. You likely won't be able to see the defect in the wiring or connectors. The tell-tale is watching the solenoid resistance change as you wiggle / pull on the wires.

When the connection makes the saw runs. When it doesn't the saw won't start and floods. One other thing to check is the coil/magnet gap.

Awesome, I just replied. I did remove, clean and regap the coil just to make sure. I’m going to pull out the meter and see what I can do


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[DND 9000]

The white one is not a universal usable part. It depends on the control unit if it can be used or not. Ask your Dealer what you can use.

Correction, made a mistake:
The white valve can be used as a special assessory on the carburetors of the MS 261 and 362 which had originally the 0000 120 5110 valve.

This white valve can be used there on these models, but not in generall on the other m-tronic models.

 

[SteveSr]

Awesome, I just replied. I did remove, clean and regap the coil just to make sure. I’m going to pull out the meter and see what I can do

It may help if you could take some photos of the wiring harness and all of the connections between the coil and solenoid so we can see how it is laid out. I have actually never worked on an M-tronic saw but have spent a career in embedded electronics. I also don't have the repair manual for this version of the saw.

@backhoelover if he is still around might be able to help out. I am sure that he has messed with more M-tronics.

One other thing that happens with M-tronic is that if the computer (in the ignition module) doesn't detect the solenoid coil it will disable the spark after a few seconds.

A good shop who knows and understands M-tronic (don't assume that they all do) should be able to hook up the computer to the saw and read out the error codes and failed start attempts which should provide a good indication of what is going on with the saw.

 

[Crispexx]

It may help if you could take some photos of the wiring harness and all of the connections between the coil and solenoid so we can see how it is laid out. I have actually never worked on an M-tronic saw but have spent a career in embedded electronics. I also don't have the repair manual for this version of the saw.

@backhoelover if he is still around might be able to help out. I am sure that he has messed with more M-tronics.

One other thing that happens with M-tronic is that if the computer (in the ignition module) doesn't detect the solenoid coil it will disable the spark after a few seconds.

A good shop who knows and understands M-tronic (don't assume that they all do) should be able to hook up the computer to the saw and read out the error codes and failed start attempts which should provide a good indication of what is going on with the saw.

This is how it’s laid out. I’m going to start checking resistance and continuity between all the wires and connections here in a min. Hopefully I’ll fine something!


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[SteveSr]

This is how it’s laid out. I’m going to start checking resistance and continuity between all the wires and connections here in a min. Hopefully I’ll fine something!


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Lots of opportunity for connection issues. I would start by measuring the resistance of your old solenoid so you know the range that you are looking for. Probably in the range of hundreds of ohms.

Next I would remove the screw and pull the connector from the coil. Look closely for any corrosion on the terminals. This will take the coil out of the picture and provide access to the rest of the circuit. It may take two people one to hold the meter leads on the connector and another to start wiggling and pulling on wires.

BTW, where do the wires go after the solenoid? down to the kill switch?

 

[Crispexx]

Lots of opportunity for connection issues. I would start by measuring the resistance of your old solenoid so you know the range that you are looking for. Probably in the range of hundreds of ohms.

Next I would remove the screw and pull the connector from the coil. Look closely for any corrosion on the terminals. This will take the coil out of the picture and provide access to the rest of the circuit. It may take two people one to hold the meter leads on the connector and another to start wiggling and pulling on wires.

BTW, where do the wires go after the solenoid? down to the kill switch?

So the yellow wires are the grounds, the yellow with green goes to the off switch and the black wire coming from the coil goes to the switch as well. So far I’ve checked everything I can see for continuity and it all checks out, even the ground that goes to the MDG port which was squished pretty good. But again I’m not an electrician so I’m not 100% sure what I’m looking for when it Cole to resistance. I reached out to the station that this saw belongs to and they tossed out the old solenoid.

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[SteveSr]

So the yellow wires are the grounds, the yellow with green goes to the off switch and the black wire coming from the coil goes to the switch as well. So far I’ve checked everything I can see for continuity and it all checks out, even the ground that goes to the MDG port which was squished pretty good. But again I’m not an electrician so I’m not 100% sure what I’m looking for when it Cole to resistance. I reached out to the station that this saw belongs to and they tossed out the old solenoid.

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See that smushed yellow wire going from the coil ground ring terminal to the solenoid connector. That is NOT right and shouldn't be smushed. The saw and/or wiring has been incorrectly (re)assembled. This may or may not be your problem but deserves closer inspection.

Also, what is going on with the tape/sleeve on the red/black coming from the coil. Is that factory or an improvised repair? Looks like the wire color changes from red to orange?

Pull the remaining screw out of the coil and remove that connector. You should be able to measure the solenoid between the yellow ground ring terminal and the red/orange wire.