MS261C-M v.II Help Needed

[Benjo]

Running out of ideas on this one, any help is welcomed!

This is only the second 261 v2 I've worked on, and also the second Stihl mtronic, so forgive any basics I've missed! I liked the first 261 I worked on, and ended up buying this one for myself to see if it would unseat my 550 as most-used in my collection.

Saw ran when I got it but didn't 4 stroke at WOT and when hot had an erratic idle. Eventually found the flywheel side bearing had spun on the crank and had a bit of play, so I split her and put in new:

• Crank bearings
• Seal kit (crank seals, case gasket, base gasket, aluminum case bolts)
• Crank (old one was pretty loose on the new bearing)
• Fuel filter
• plugged the decomp
• P/C look good, piston has some common wear but makes 155lbs compression and when put in the bore without rings it provides plenty of resistance pushing it in and out

Figured that would do it, pressure/vac tested fine, but same exact symptoms remained. Pressure/vac tested again, still great, pressure/vac tested fuel line, also perfect. Figured it must be the solenoid or dirty carb (tank was pretty dirty when I got it). So I replaced:

• tank vent
• solenoid
• when that didn't work, an entire new carb (getting desperate...)

Finally got that put back together last weekend, no change. Still lean, still no 4-stroking at WOT, and now she often dies after making a short cut and releasing the throttle (but starts back up immediately).

Did another pressure/vac test (sealed between carb and boot if that's important). It's very tight, held 8psi for 2 hours with no drop including rotating crank by hand, left it with 15in/hg and it held unchanged for 2 hours, then after rotating the crank both by hand and with a drill(!) trying to get it to leak somewhere, then leaving it for 24 hours, it dropped to about 14. Even dunked the whole thing in water thinking there might be a super small leak somewhere that opens when warm...nada.

Opening the tank does nothing (and the vent is new), fuel filter is new and fuel flows easily...spraying carb cleaner at various points while running offered nothing notable.

My instinct is to target the coil/harness next, but that's fully into guessing/throwing part$ at it.

Anyone experienced anything similar? Any new ideas that are better than my idiotic "replace every part until it works" campaign?

Thanks again for any help!

 

[huskyboy]

@drf256

 

[jed1124]

I know this sounds elementary but did you do a reset on the M-tronic? I'm not sure if the procedure is the same on the mark 2, but to reset the older m-tronic saws the procedure is to run them on choke for 90 seconds then immediately shut the saw down.

Someone will by shorty who know much more than me on this topic.

 

[Gary Courtney]

Which solenoid? White or black? My dealer told me Tuesday he just got back from a Stihl seminar and they have come out with another solenoid to trump the white one!

 

[Benjo]

I know this sounds elementary but did you do a reset on the M-tronic? I'm not sure if the procedure is the same on the mark 2, but to reset the older m-tronic saws the procedure is to run them on choke for 90 seconds then immediately shut the saw down.

Someone will by shorty who know much more than me on this topic.

I did do the reset with every attempted fix, forgot to mention that!

 

[Benjo]

Which solenoid? White or black? My dealer told me Tuesday he just got back from a Stihl seminar and they have come out with another solenoid to trump the white one!

It came with a black solenoid, I replaced it with a new black solenoid, then the totally new carb from Stihl also has a black solenoid.

 

[Gary Courtney]

upgrade on 661 from 2.0 to 3.0 is 1144 007 1801 stihl rep says this is all you need. it has a coil.fuel filter,and solenoid

 

[Gary Courtney]

should have one for a 261

 

[Dustin4185]

http://opeforum.com/threads/261-362-462-661-mtronic-info.18639/page-2#post-998944

Read through this thread. My MMWS 261 started acting wonky. I bought the update kit for it and did the reset. Fixed my issue. Glad I didn’t F up Randy’s work!

 

[Benjo]

Thanks so much for the responses!

Is the 1140 007 1802 part # still correct for the 261 "kit" (that's just white solenoid and orange filter)?

Again, thank you for all the help!

 

[DND 9000]

Yes, that is correct.

 

[drf256]

Benjo,

You mention the flywheel bearing went. Was there any damage to the coil or flywheel? Did they hit or rub? If they hit at all, your flywheel magnets may have been damaged. MT saws have 2 sets of magnets.

What mix and ratio of fuel are you using? I had a problem in one of mine, but it was ported. The mix was simply too viscous to flow enough.

My bet is that the white solenoid fixes your issues. If all of the above doesn’t, I’d pull the carb.

 

[Benjo]

Benjo,

You mention the flywheel bearing went. Was there any damage to the coil or flywheel? Did they hit or rub? If they hit at all, your flywheel magnets may have been damaged. MT saws have 2 sets of magnets.

What mix and ratio of fuel are you using? I had a problem in one of mine, but it was ported. The mix was simply too viscous to flow enough.

My bet is that the white solenoid fixes your issues. If all of the above doesn’t, I’d pull the carb.

Well there’s something I should’ve thought of! Thanks for the reply!

Just gave it a look, seems ok to me, thoughts?:

 

[Mastermind]

I swap parts off of a new saw to pinpoint an issue like you're having. There really ain't much it could be though. Coil, wiring, solenoid.

 

[Mastermind]

The Bubba just rebuilt two 261s. Adjustable versions though. One needed a coil.

 

[Benjo]

Benjo,

You mention the flywheel bearing went. Was there any damage to the coil or flywheel? Did they hit or rub? If they hit at all, your flywheel magnets may have been damaged. MT saws have 2 sets of magnets.

What mix and ratio of fuel are you using? I had a problem in one of mine, but it was ported. The mix was simply too viscous to flow enough.

My bet is that the white solenoid fixes your issues. If all of the above doesn’t, I’d pull the carb.

And also, for the record, I've tried my usual XP+ oil at 40:1 as well as some borrowed red armor at 45:1 and a can of 40:1 TruFuel to try to rule out fuel issues.

I'd have thought the entire new carb I put in would've eliminated any carb-related issues (even if it has the black solenoid), but after coil/harness that's where I'll be headed!

 

[Benjo]

I swap parts off of a new saw to pinpoint an issue like you're having. There really ain't much it could be though. Coil, wiring, solenoid.

Yeah, you're right, not many parts left to eliminate!

I'm in no hurry because it's my own saw, so I think I'll see if I get another one through that I can use to swap out coils/wiring/solenoids etc. until I get it right. Or the last one I did was for an old coworker, I'll see if I can borrow it the next time we talk.

Thanks again for all the help, it's very much appreciated!

 

[drf256]

That flywheel rubbed. Not saying it is the issue, just saying it could be the issue. The only time I’ve seen one fail is when it rubbed the coil because the bearing went. The hit caused the magnet polarity to change and the flywheel made no spark. I never thought it was possible, but it happened to me.

I may have a flywheel on my builder case you could try. I’d just need yours back in exchange. You might not want to wait all that time though.

I’d swap out the flywheel (and coil if possible) like Randolpho said. The MT system gets a lot of info from the flywheel, so any damage there is a possibility.

 

[Benjo]

That flywheel rubbed. Not saying it is the issue, just saying it could be the issue. The only time I’ve seen one fail is when it rubbed the coil because the bearing went. The hit caused the magnet polarity to change and the flywheel made no spark. I never thought it was possible, but it happened to me.

I may have a flywheel on my builder case you could try. I’d just need yours back in exchange. You might not want to wait all that time though.

I’d swap out the flywheel (and coil if possible) like Randolpho said. The MT system gets a lot of info from the flywheel, so any damage there is a possibility.

Flywheel is definitely on my list of parts to swap from a runner to find the issue along with coil, solenoid, etc.

I'm 90% I can get a flywheel from a junked saw a friend has (chip truck incident), but it has no carb or coil etc., so for the other parts I'm prepared to wait a bit for. I also know someone who may buy a new 261 this spring, I'll offer to open the muffler and advance the timing if he lets me swap around parts at the same time.

Yet again, thanks for the help, it's great to have a source of expert help when this sort of thing crops up.