Stihl Ms 261C Help! Idles but dies at full throttle.

[prodigey]

I have the 1st generation Ms261c M tronic. starts and idles perfectly but dies instantly when throttle is fully pressed. Replaced solenoid all gas lines vent cap and reprogrammed it everytime but still no fix. Please help

 

[SawAddictedFarmer]

Has it sat more than a week or two? Sounds like you may need a new carb. I have also seen one where the coil failed and it did something similar.

 

[SawAddictedFarmer]

Some smarty-pants guy should be along shortly to say how little I know and tell you what's really wrong.

 

[two4spooky]

Is he building you a saw?

Some smarty-pants guy should be along shortly to say how little I know and tell you what's really wrong.

Yes, you should be quite with no opinion and learn here. Shush... Do not speak. You are over your head.... Let those with experience speak

Be quite here

 

[hacskaroly]

starts and idles perfectly but dies instantly when throttle is fully pressed.

Have you been into the carb yet? Double check that fines are not inside or possibly the diaphragms might be crunchy? I have a MS 170 that starts and idles with no issue, but when I give it throttle, it does just about everything but full throttle and even dies, but will restart without issue. It has a crunchy diaphragm that needs to be replaced.

I am guessing the guy that owns this saw, lets his personally mixed fuel sit in it - potentially ethanol fuel. He also brought me a hand held blower and two weed eaters with issues too...will see if this is a trend.

Edit: Welcome to the forum!!

 

[bogieboy]

I would start by checking the inlet screen of the carb (under the cover with the single screw) for debris, and if nothing there, chexk the other side of the carb for any gunk and possibly a stiff diaphragm. If its soft and supple, theres likely a failed check valve in the carb body itself, and while replaceable, requires a special tool and can be finicky, most would opt for a carb replacement at that time. Not sure if they are still available, but last year i was able to grab an Mtronic zama for my 261 off aliexpress for like $20... i couldnt even get one that cheap as a stihl tech at a stihl dealer... and it appears to be a genuine zama, not an obvious fake...

 

[SawAddictedFarmer]

Unless I'm wrong this should work

Pardon Our Interruption...

It's a ebay Link, I don't know why it shows up how it does.

 

[drf256]

The M-Tron just adds something else to the diagnostic algorithm.

That being said, it’s usually NOT the MT itself that’s the problem. Don’t get me wrong, it can be, and you did the smart thing. When it it the MT, it’s usually the solenoid.

My only advice is to forget it’s an MT. Pull the muffler. Clean the exhaust screen, it could be caked up with carbon. Check the piston condition. If the slug looks good and you feel good compression, your internals are generally ok.

Change the $2 spark plug. After spending countless hours and money on engines over the years, it’s become SOP on engines that aren’t right. Just because you see spark at atmospheric pressure, it doesn’t mean there’s spark in a hot cylinder under compression.

Vacuum test the engine. MT can cover up a lot of easily diagnosed problems and make them more difficult to detect. Pull the carb apart. See if it holds pressure. See if the popoff pressure is acceptable. See in the fuel screen is clogged w shít.

IMHO, I don’t think it’s the nozzle check valve. The function of the valve is to stop the saw from pulling air instead of fuel into the wet side of the carb during idle. It’s generally more of an idle than running issue. It’s possible that the MT is overcoming it, but doubtful.

If it all checks out to the old school testing, then dive into the MT itself. If it’s all good, the last step would be to swap out the coil. The ECM of the MT system is in the coil. Make sure the little wiring harness isn’t broken or shorting out.