Stihl MS261c M-Tronic Won’t Idle

[MMSC]

I know this saw has been discussed extensively on here & abroad. I’ve gone over everything I can find on it and it’s STILL winning the Argument.

Stihl MS261c M-Tronic 1st Gen.

Coughs on Start & Run Position. Will Start with Throttle Trigger Squeezed. Will Not Idle.

Has 140 PSI Compression (Piston & Cylinder Look Great), Spark & Fuel. New Plug - Plug is wet every time I Pull it and Fuel Seems to be Leaking out BELOW Muffler - Possibly through outlet and running down. Definitely seems to be Flooding. Spark Arrestor Screen is Clean.

Have Gone over Carb , Welch Plug Removed - All Passages Clear. Pressure Tested to 7 PSI - OK.

Swapped Black Fuel Solenoid Valve for Green. Interchanged several times with No Difference.

Repaired Bare Wires in M-Tronic wiring harness between Carb & Coil.

Pressure & Vacuum Test on Cylinder / Crank / Intake Boot - OK.

NEED to get it to Idle to Reset / Calibrate M-Tronic.

Ran it through 5 Cuts - Runs Great from Just above Idle to Wide Open Throttle.

Can’t Figure Out why it won’t Start without Pegging Throttle - or Idle. I Plan on Picking up a New Orange Fuel Filter this afternoon - about the Only thing I haven’t Ruled Out.

What am I Missing?

 

[Vintage Engine Repairs]

I’m pretty sure even though they’re m-tronic there is a way to raise the idle manually, a little screw somewhere. Someone will chime in

Edit: found a video on how to:

 

[MMSC]

Thank You, I appreciate it! I will definitely look into that this evening.

 

[MMSC]

You’re Right, There is a Set Screw that Controls how far the Butterfly Valve is Open at Idle. I Turned it Clockwise to Open it in 1/4 Turn Increments up to 2 Turns in and then back out 1 Turn with No Difference. Tried with both the Original Black Fuel Solenoid Valve and the New Green One. No Luck.

 

[Joedangelo04]

Idle passage is blocked? Soak in berrymans or similiar.

 

[SteveSr]

Welch Plug Removed - All Passages Clear. Pressure Tested to 7 PSI - OK.

Welch plug removal is generally a BAD idea as they are almost impossible to get them to re-seal. Even the carb manufacturer can't do it without proprietary sealant. This will cause a direct fuel path leak from the metering chamber into the engine.

The other leak path is through the needle and seat and possibly the solenoid. fuel inlet should hold 10+ psi indefinitely. Any leak down is fuel going into the engine.

 

[MMSC]

The Idle Path was Blocked, Removing the Welch Plug was a Last Ditch Effort because I Couldn’t get it Cleared. Still Holding Pressure.

 

[SteveSr]

The Idle Path was Blocked, Removing the Welch Plug was a Last Ditch Effort because I Couldn’t get it Cleared. Still Holding Pressure.

Still holding pressure has nothing to do with welch plug sealing. Totally different mechanisms.

The fact that you have gas running out the muffler says that it is way too rich, likely at idle which is why it won't. The only way to check welch plug sealing is to pressurize the venturi of the carb and look for soap bubbles around the welch plug. You can make a welch plug leak tester from a small Irwin wood clamp with rubber "feet" to seal the carb and a small section of fuel hose through one of the end caps to add the pressure.

 

[singinwoodwackr]

Did you do a reset? Maybe a stupid Q but...
I've never had an issue with a 261 after a full reset.

 

[MMSC]

No Sir. I can’t get it to Idle at all, let alone 90 Seconds for the Reset.

 

[singinwoodwackr]

No Sir. I can’t get it to Idle at all, let alone 90 Seconds for the Reset.

May need to replace the carb.
Had to do that with 2 201ts.
Couldn't reset, rebuilt x2, no joy.

Have you tried a sonic tank (in carb cleaner bath ,( i use Berryman's) on the carb? Disassemble and tank the sucker. This has worked countless times for me. Sometimes it is a tiny piece of crud in the carb that just a blow-out won't get.
Just remove the electronic stuff...

 

[stihltech]

Sometimes you just need to replace the carburetor. It is called more cost effective. AND get the orange fuel filter.

 

[Dennisthemenace]

Sounds like it's flooding? And 1st gen? Why not replace the solenoid if original or well used? If that does no good replace the carb.
And what's this about repairing bare wires in the harness?
That doesn't sound good.

 

[MMSC]

I’m leaning towards replacing the Carburetor. I have a New Green Solenoid and have Swapped them frequently as I have been told the Green were compatible with Gen 1 and others have said They aren’t.
The Harness from the Coil Up had some insulation off the Wires. I watched the West Coast Muscle saw video on those harnesses and put Heat Shrink on those areas to make sure nothing was Shorting or Grounding out as Spark was Erratic. I Plan on Ordering another Harness. It Has Much Better Spark Now.
Failed to Mention, this was a Bonepile Saw at Work. It didn’t have Too Many Hours as it was Used a couple hours a day to Cut LVL Beams on the Yard, However - Company Saw- wasn’t taken care of like mine are. Got Scrapped instead of Serviced and some TLC. The Operator said it hadn’t been Idling for Some Time and Always had to be Started with the Throttle Pegged.

 

[huskihl]

If the M Tronic wires were bare, to me it’s more likely that the coil got fried in the process than the carburetor

 

[MMSC]

That may be the case. Nothing Looked Bubbled or Like the Wires got Hot, More Dry Rotted than anything. I know Wires have a Tendency to Break there from Vibration.

 

[SteveSr]

That may be the case. Nothing Looked Bubbled or Like the Wires got Hot, More Dry Rotted than anything. I know Wires have a Tendency to Break there from Vibration.

If t he wiring harness was wonky it wouldn't run at all. The control unit checks for harness integrity and will interrupt spark if it doesn't like what it finds.

 

[MMSC]

If t he wiring harness was wonky it wouldn't run at all. The control unit checks for harness integrity and will interrupt spark if it doesn't like what it finds.

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.

 

[user 122190]

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.

Labors rates are crazy. You could rake up shop time to exceed the price of the saw, in educating them on working on an M-Tronic.

Have you thought of sticking something down the carbs throat to hold open the throttle while you probe for problems?

 

[Dennisthemenace]

You sure that he is a real Stihl tech/mechanic? He says he's never seen a 462, doesn't get the service bulletins, doesn't have the mandatory service tool that all Stihl dealers must have.
Never heard of mtronic?
My missus knows more about Stihl than he does.
I'd be getting a 2nd opinion