Stihl MS261 backfiring, running backwards and generally misbehaving

[spudulike]

Keep an eye on ebay, you may get a second hand one for £40 if lucky. 

[AWG81]

Just another update - I checked the flywheel with L&S and the part code should be 1141 400 1200 so it looks like I have the wrong flywheel fitted. I'm wondering if I change the flywheel it might work with the coil I have. Out of stock on L&S at the moment though...

[ChrisNewport]

Dont know if it helps but this is the coil / flywheel set up on a knackered 261 i have, same part number as the flywheel you have.

[spudulike]

That helps enormously....and no sarcasm this time..... interesting, some muppet has fitted a Mtronic flywheel on a 2012 machine....no wonder it isn't liking it! On Chris's machine you can see the Mtronic connector on the coil.

The flywheel must be wrong as long as Chris's machine hasn't been messed with.

So, you need an old flywheel and hopefully you still have the old coil although the aftermarket one may work with the correct flywheel.

Good thing I said check the flywheel is correct!

[spudulike]

Nothing on eBay, perhaps someone on here has a scrapper Pre Mtronic MS261 - must be a fair few out there judging by the clutch bearing issues with this model. 

[AWG81]

Thanks for posting this Chris. Now it all makes sense! Hopefully I can get hold of an older 261 flywheel from somewhere.

[AWG81]

Hi all, just an update on this MS 261.

 

I bit the bullet and bought a new flywheel from L&S with the correct number. It now will fire up and run (hooray), but I'm still getting a bit of backfire and my decomp valve is popping out every time I pull it over. When it backfires I get a puff of smoke out the exhaust and decomp valve.

I seem to be having trouble with the master control lever too and am wondering if it's related - if I have it set on the choke setting (all the way down) it pops back up on its own every time I pull it over without the engine coughing. It will only start when the lever is on the run position.

 

Once it goes it seems to idle and rev up fine (although a bit smoky) but the starting issue is puzzling me. I'm about ready to throw this thing in the bin but I've spent too much time on it now to give up!

 

Thanks for the help so far - it's running better than it was thanks to info provided here!

[pleasant]

I haven't read the whole of the comments on here, but reading your last comment (above) is it flooding the combustion chamber? Popping the decomp valve out without starting plus all the other symptoms you have listed above suggest it is flooding somewhere...or is very very rich. The flywheel rotating backwards on the compression stroke could suggest the same......return pressure trying to compress a liquid?

Edited May 11, 2023 by pleasant

[spudulike]

Is the machine running with the aftermarket coil? It is possible that the manufacturer got the initial start up ignition timing too retarded causing the backfire.

The decomp flipping out may be the backfire causing it or just another aftermarket part which is a bit weak.

The choke lever may have a bit of wear or a weak flat spring. Without the machine in font of us it will always be difficult to analyse what part is wrong with it.

[pleasant]

12 minutes ago, spudulike said:

Is the machine running with the aftermarket coil? It is possible that the manufacturer got the initial start up ignition timing too retarded causing the backfire.

The decomp flipping out may be the backfire causing it or just another aftermarket part which is a bit weak.

The choke lever may have a bit of wear or a weak flat spring. Without the machine in font of us it will always be difficult to analyse what part is wrong with it.

True.....i regularly get phone calls along these lines, and tell them to bring it in and show me because after a minute or so of a customer self diagnosing I tend to glaze over. Once you have it in front of you, even with the customer rabbiting in your ear you can quietly visually analyse it and come to a conclusion before the customer has finished talking. Personally i dont like taking machinery on that a weekend warrior has had a go at, as you have to check what they have done...put that right, before you can find the original issue.

 

'It's not starting'

 

'Ok, have you done anything to it? Whats the history? Well, it was working and now it doesn't and I've played around with those screws on the side near the carburettor i think'

 

'Jeez.....ruddy marvellous'

 

🙄

Edited May 11, 2023 by pleasant