Stihl MS261 mtronic

[BlueMax]

So the latest. I tore down the carb,  removed the gauze filter and cleaned, removed the fuel  pin and tiny spring and cleaned, flooded the main body with carb cleaner in every hole I could find. Rebuilt the carb with a new membrane kit (both sides) as the old one looked a bit worn. So I have a new air filter, plug is clean and gap correct, new white solenoid, set gap on control unit to business card width. New fuel filter. Clean fuel.

it starts easy on choke now and run smooth…I tried the reset approach, I have the early mtronic so run for 90 seconds on choke and switch off. But as soon as I knock the choke off it dies, it simply will not run on idle at all. I can run it WOT and it screams but as I ease off to starts to chug/bog at half throttle and then dies below that. I am now stumped. Any ideas welcome.

[Trailoftears]

Sounds VERY trying ☹️

Bit of a wildcard but nothing to lose,try the later recalibration-

STIHL M-Tronic technology is designed to make sure that your tool delivers an optimum performance, all day, every day, by making automatic adjustments to regulate the fuel and air mix to the machine.

However, there are a couple of instances where you may need to reset your M-Tronic machine. Firstly, if you carry out any work on the fuel supply system, such as changing a solenoid valve, replacing or cleaning a carburettor or even just replacing an old fuel filter, you will need to do an M-Tronic reset.

For newer M-Tronic machines (such as the MS462 chainsaw) the process is different so check your Owners’ Manual to make sure that you are following the right procedure. The procedure for newer models is listed below:

Remove the scabbard

Ensure the chain brake is engaged (on)

Put the machine in the ▲ Cold start position

Start the machine and run it in the ▲ Cold start position for at least 30 seconds but no more than 60 seconds (the period marked as A in the diagram below)

Do not touch the throttle or accelerate the saw during this time

Disengage/release the chain brake

Open/squeeze the throttle fully for at least 30 seconds (B in the diagram)

Engine speed varies noticeably during calibration

As soon as the engine speed drops noticeably (C), release the throttle back to the idle position

When the engine is back to idle speed, engage the chain brake and switch off

Your saw is calibrated and ready for operation

 

[bmp01]

As per adw's question a few posts above how does the impulse signal connect to the carb on the MS261 ?

You need an impulse pressure signal to make the pump side of the carb work. The pressure pulses causes the membrane to move in and out ---> fuel is pumped through the one way flapper valves.

 

Worth noting:

-Engine will run on choke without the impulse pressure signal because the closed choke causes sufficient vacuum to suck fuel directly from tank.

-Engine will often run at wide open throttle without the impulse signal because the high engine speed often causes  sufficuent vacuum in the carb ventui to suck fuel directly from tank.

-Most other scenarios,  no impulse pressure signal usually means engine wont run.

 

[BlueMax]

So, the latest. I tore down the carb again, made sure it was squeaky clean, checked the jets where the solonoid site by blowing air form a compressor through. I checked the impulse path all the way through the carb mount and it’s clear. I have bought a new wiring loom form solenoid, and also a brand new genuine ignition module. None of this has made any difference!!! I have tried both reset procedures, I don’t think it’s the new method as if yiu let it idle for 30+ seconds then WOT is stays WOT without any change in revs, so I tried the first method. I am really stumped. It will run on choke and WOT but nothing in between. It will not idle or start without choke even when warm. I am now completely out of ideas as to what to try next. It’s now had a new ignition, solenoid, loom, carb diaphragm set, carb gauze filter clean, fuel and air filter and plug cleaned and set. I have spent a lot to date and achieved nothing, What do I try next?

[Dan Maynard]

What's the history of the saw? Did you buy it new or secondhand? The other recent 261 thread was resolved because the wrong flywheel had been fitted by a previous owner.

 

If it's not fuel system then it's got to be something else, pressure vac and compression tests like @pleasant said earlier. Piston scored? Seals? By the sound of it you don't know it's making power at WOT just that it runs.

[BlueMax]

I will ask, but pretty sure bought from new. My son is a trainee for a sole trader Tree surgeon, and this is one of his saws, he only ever uses Stihl and buys genuine spares. I think it was bought new, used a fair bit then started to be unreliable when climbing. He is not mechanical at all, and possibly did not maintain it well, and so just left it.

I volunteered to help ( free labour, I am a bit of an engineer and I spent many years fiddling with model 2 stroke engines in my youth) to try to resurrect it as they need another saw and can’t afford to buy a new one as he just had to have his Kubota chipper engine rebuilt.

As you can see I started with the service items, cleaned lots of wood dust out, then did the filters and plug, no joy, then solenoid, no joy, then carb diaphragms, no joy, etc. I havw checked all the bolts are tight for the carb and throat.

I can’t tell if it’s making power as I have removed bar for safety while I fiddle. I don’t have the tools to check compression and vacuum so am I at the end of the road for home fixing?

[Dan Maynard]

Have you looked at the piston?

[lurch_917]

well i don't know the saw but i have had issues with others where the decomp button was leaking when i put a new button in mine ran sweet again just a thought . you could test this by the old washing up liquid trick .

[Dan Maynard]

That's a good idea. Thing I'm thinking is if doing all that work to the carb made no difference then the problem is elsewhere, seems next most likely to me either scored piston so no compression or it's fighting an air leak.

[Muddy42]

Where are you? Someone nearby might lend you a mityvac. If you still can't tell where the leak is coming from with the mityvac, no leak can hide from a dunking in a bucket of water.

 

I've often wondered if you could make a basic pressure tester with a bicycle pump.