Stihl MS261c M-Tronic Won’t Idle

[singinwoodwackr]

Those are My Thoughts as well. It doesn’t miss a Beat at Wide Open Throttle in or out of wood.
I Bit the Bullet & Ordered a New Carb - will be in Mid-Week.
I Spoke to My local Stihl Tech earlier in the Week and He had Me Repeat the Model Number 3 Times and I had to Explain what M-Tronic is. They don’t have Diagnostic Tools and Apparently have Never worked on one. They Offered to Check it out “Mechanically”. I Politely Declined.

If he doesn't have the m-tronic diagnostic gear he's not a certified stihl shop.

 

[MMSC]

I Apologize for the Time that Has Passed - I’ve been Dealing with some Ongoing Health Issues and Haven’t Been in the Shop Much in the Evenings.
The New Carburetor came in - $110 + Tax - OUCH. It has the White Fuel Solenoid. Installed it and a New Orange Stihl Fuel Filter Expecting it to Fire Right Up, Idle and Go Through the 90 Second M-Tronic Reset - None of Which Happened. Still Flooding to the Point of Fuel Pouring Out and Under the Muffler.
Took a Few Steps Back and Started Over. Pulled Wiring Harness from Coil to Carb and Wiggled and Bent Every Which Way - No Loss of Continuity. Good Spark.
Compression is the Same 140-145.
Pressure & Vacuum Tested the Cylinder - Held.
Pressure Tested Tank, Fuel Lines & New Carb - Fuel was Still Pouring Out Muffler. Was Beginning to Think Stihl Possibly Put the Diaphragm against the Cover instead of the Carb Body. Almost Overlooked the SIMPLE Issue - The Tank Vent. I Replaced that and it is No Longer Flooding. However, STILL Will Not Start without Throttle Pegged & Will Not Idle.
Runs Great From Just above Idle to Wide Open Throttle. I Pulled the Muffler again and Checked the Spark Arrestor Screen. I Sprayed the Muffler with Brake Cleaner, Burned it Out, Heated Again with a Torch & Blew Out with Compressed Air.
Covered Everything I can Think of.
Interesting to me is the Fact that the Original Operator told Me this Morning that it Idled the 1st Week He had it and never had Since. I don’t know if the M-Tronic can get so far Out of Whack that it won’t be able to be Reset or what is going on.
I have Read Every Forum & Watched Every YouTube Video I can Find on M-Tronic in General as Well as MS261c-m Specific & I’m Still Scratching My Head.
I May Cut with it a while this Weekend & See if anything changes.

 

[huskihl]

This has been my dilemma a few times. Sometimes it’s the carb and sometimes it’s the coil. And nearly impossible to tell which

 

[user 122190]

I Apologize for the Time that Has Passed - I’ve been Dealing with some Ongoing Health Issues and Haven’t Been in the Shop Much in the Evenings.
The New Carburetor came in - $110 + Tax - OUCH. It has the White Fuel Solenoid. Installed it and a New Orange Stihl Fuel Filter Expecting it to Fire Right Up, Idle and Go Through the 90 Second M-Tronic Reset - None of Which Happened. Still Flooding to the Point of Fuel Pouring Out and Under the Muffler.
Took a Few Steps Back and Started Over. Pulled Wiring Harness from Coil to Carb and Wiggled and Bent Every Which Way - No Loss of Continuity. Good Spark.
Compression is the Same 140-145.
Pressure & Vacuum Tested the Cylinder - Held.
Pressure Tested Tank, Fuel Lines & New Carb - Fuel was Still Pouring Out Muffler. Was Beginning to Think Stihl Possibly Put the Diaphragm against the Cover instead of the Carb Body. Almost Overlooked the SIMPLE Issue - The Tank Vent. I Replaced that and it is No Longer Flooding. However, STILL Will Not Start without Throttle Pegged & Will Not Idle.
Runs Great From Just above Idle to Wide Open Throttle. I Pulled the Muffler again and Checked the Spark Arrestor Screen. I Sprayed the Muffler with Brake Cleaner, Burned it Out, Heated Again with a Torch & Blew Out with Compressed Air.
Covered Everything I can Think of.
Interesting to me is the Fact that the Original Operator told Me this Morning that it Idled the 1st Week He had it and never had Since. I don’t know if the M-Tronic can get so far Out of Whack that it won’t be able to be Reset or what is going on.
I have Read Every Forum & Watched Every YouTube Video I can Find on M-Tronic in General as Well as MS261c-m Specific & I’m Still Scratching My Head.
I May Cut with it a while this Weekend & See if anything changes.

Progress is good. I agree, cut with it, and use it hard, and it might get itself corrected.

 

[MMSC]

This has been my dilemma a few times. Sometimes it’s the carb and sometimes it’s the coil. And nearly impossible to tell which

I’ve Never Personally Ran into a Defective Coil.
I’ve Heard of Them “Breaking Up” at Higher RPMs and the Saw Sort of Cutting Out or Burbling most often when They Fail. I am Testing Spark with an In-line Tester and it is Showing Bright Orange Spark. I Figured, maybe Incorrectly, that between that and it Running Perfect at Wide Open Throttle that the Coil is OK.

 

[SteveSr]

I’ve Never Personally Ran into a Defective Coil.
I’ve Heard of Them “Breaking Up” at Higher RPMs and the Saw Sort of Cutting Out or Burbling most often when They Fail. I am Testing Spark with an In-line Tester and it is Showing Bright Orange Spark. I Figured, maybe Incorrectly, that between that and it Running Perfect at Wide Open Throttle that the Coil is OK.

The coil in the M-tronic saws has the computer in it to control the solenoid in the carb which determines the mixture. If it is so rich at idle that it won't run I would think that the EPA (and Stihl) would like to know about it. I would kick this can up to your regional Stihl tech rep or Virginia Beach. It looks like they laid a turd ignition module that won't reset. I don't know if they still do but at one time Stihl had a lifetime warranty on their coils.

 

[huskihl]

I’ve Never Personally Ran into a Defective Coil.
I’ve Heard of Them “Breaking Up” at Higher RPMs and the Saw Sort of Cutting Out or Burbling most often when They Fail. I am Testing Spark with an In-line Tester and it is Showing Bright Orange Spark. I Figured, maybe Incorrectly, that between that and it Running Perfect at Wide Open Throttle that the Coil is OK.

It’s not the spark portion of the coil, it’s the mtron control unit portion

 

[MMSC]

It’s not the spark portion of the coil, it’s the mtron control unit portion

Gotcha.
I ran it through a Tank of Fuel This Evening & it Ran Great from Just above Idle all the way through. Still having to Start with Throttle Pegged & won’t Idle.
Not the End of the World - I can live with it if it doesn’t clear up. I’ll Keep My Eyes Open for a Coil - Would Probably Track Down Used Carb, Flywheel, Coil and 3 Position Multimaster Switch from Non M-Tronic Version if I hadn’t Bought a New M-Tronic Carb. If Nothing Else I’ve Learned a Few Things on this one… I Just HATE Getting Stumped!

 

[singinwoodwackr]

It’s not the spark portion of the coil, it’s the mtron control unit portion

Gonna find out if this is the issue on an older 661 I‘m working on.
same flooding issue…

 

[huskihl]

Gonna find out if this is the issue on an older 661 I‘m working on.
same flooding issue…

It’s usually the solenoid. About 10 times. 3 times it was the coil. 3 times it was the carb

 

[singinwoodwackr]

It’s usually the solenoid. About 10 times. 3 times it was the coil. 3 times it was the carb

New solinoid…fired right up and runs great.

now…I have a 261mt that I can’t even get to start. It will fire once as if it were a regular carb and the nothing. It isn’t pouring gas out the muffler like the 661, however…just won’t start.
I have swapped carbs and coil. Don’t have another harness but it seems fine.

this was a from scratch rebuild…bearings on up.

I started with 3 basket case saws…make one from 3. I figured to go with the Mt as I’ve never had issues with several other 261s. Mistake…
at this point I’m thinking I’ll just swap out all the Mt parts and built it as one of the regular carb saws. This way I can get a saw back in the customer’s hand and figure out the Mt later. I’ve scrounged enough parts for two complete builds. Those solinoids are pricy…$43, ouch…

 

[huskihl]

New solinoid…fired right up and runs great.

now…I have a 261mt that I can’t even get to start. It will fire once as if it were a regular carb and the nothing. It isn’t pouring gas out the muffler like the 661, however…just won’t start.
I have swapped carbs and coil. Don’t have another harness but it seems fine.

this was a from scratch rebuild…bearings on up.

I started with 3 basket case saws…make one from 3. I figured to go with the Mt as I’ve never had issues with several other 261s. Mistake…
at this point I’m thinking I’ll just swap out all the Mt parts and built it as one of the regular carb saws. This way I can get a saw back in the customer’s hand and figure out the Mt later. I’ve scrounged enough parts for two complete builds. Those solinoids are pricy…$43, ouch…

Flywheel key maybe? Spark plug?

 

[singinwoodwackr]

Flywheel key maybe? Spark plug?

Brand new and gapped plug. Flywheel is good…I have 4 of them

 

[SteveSr]

Brand new and gapped plug. Flywheel is good…I have 4 of them

Likely a harness/solenoid/switch issue. Failed the solenoid check so it kills the spark.

 

[huskihl]

The coils and carburetors may need to be matched to each other (if that’s a thing with Stihl). Perhaps they’re from different generations of Mtronic?

 

[singinwoodwackr]

I have 2 MT coils and 2 carbs. One pair came from the trashed MT saw. I didnt keep parts separated as i didnt figure they needed to be. I've swapped those 4 units, trying all combinations...no joy.
I'd have a hard time believing the coil and carb are a pkg as I can order either as a replacement and there is no literature stating they are paired.

 

[SteveSr]

I have 2 MT coils and 2 carbs. One pair came from the trashed MT saw. I didnt keep parts separated as i didnt figure they needed to be. I've swapped those 4 units, trying all combinations...no joy.
I'd have a hard time believing the coil and carb are a pkg as I can order either as a replacement and there is no literature stating they are paired.

I am not an M-tronic expert but with the Stihl system (as opposed to Husky) all of the "brains" are in the ignition module and not split with the carb. The only possible match would be for the solenoid to ignition module.

If you have a multi-meter I would verify continuity of the switch and solenoid at the ignition connector. Someone here may have the expected readings based on switch position. With the switch in "run" I would expect to see the solenoid resistance at the ignition harness terminals (ignition module disconnected).

 

[singinwoodwackr]

Well, I think I’ve got it figured out. A year ago I built 3 other 261s from throw away carcasses. After ordering ebay carb kits and rebuilding carbs I couldn’t get any to run. Turned out the kits were wrong…and still sold, by the way. Correct kit, fired right up.
I tried 2 different regular carbs after swapping all the MT stuff out…no joy. My guess is that I used what was in my misc kit pile when I checked carbs I had and none of what I had was correct. Looked close, but…
have reordered the right kits so will see what happens.

 

[atpchas]

After ordering ebay carb kits and rebuilding carbs I couldn’t get any to run. Turned out the kits were wrong…

Trying to educate myself here... Were you able to pinpoint what was wrong in the kits? Wrong pump diaphragm material? Miscut diaphragm or gasket? Wrong tit length on metering diaphragm? Spring too weak or strong?

 

[singinwoodwackr]

Trying to educate myself here... Were you able to pinpoint what was wrong in the kits? Wrong pump diaphragm material? Miscut diaphragm or gasket? Wrong tit length on metering diaphragm? Spring too weak or strong?

I'll try and find the old thread...has pics
https://www.arboristsite.com/threads/carb-not-getting-fuel.359538/page-5
May or may not be the same issue. Definitely not thee exact issue as I didn't use the wrong parts shown in that thread.
But, I did not use a complete new kit here, just some pts of kits I had on hand. It doesn't take much to screw these things up could be any of them. I'll rebuild with full kit and see what happens.