Stihl MS261 backfiring, running backwards and generally misbehaving

[AWG81]

Hi all,

 

This is my first time posting but I have got a fair bit of help by lurking on here over the years so thanks for that.

 

I have a problem MS261 (from 2012) that has got me stumped. I got this for cheap and was told it was running but it actually wasn’t (Now I know why it was cheap I guess).The first symptom was that it was extremely hard to pull over - the decompression valve would pop out after the first pull and rip the starter cord from my hand. It would occasionally fire up and run at full revs but wouldn’t idle. The cylinder seemed to have fuel leaking out around the muffler and the plug was black. It would also occasionally pop or misfire.

 

Then I seemed to be able to get it running ok, but when I was using it kind of burped and started running backwards!

 

I checked the flywheel which was fine (it has a cast flywheel key) so I replaced the ignition coil (aftermarket). It now seems to run in the right direction but I’m still having major trouble getting it started. It’s still backfiring and seems to flood straight away. Again, it will occasionally fire and run but only on the half choke setting and then it either stalls OR runs too fast so that the chain goes around at idle.

 

When it backfires I get a little puff of vapour or smoke that comes out of the carb.

 

I don’t have the means to test the compression, but seeing as it starts occasionally I’m assuming the compression is ok?

 

I’m not sure where to go from here and would appreciate any ideas...

 

Cheers!

[Stubby]

11 hours ago, AWG81 said:

Hi all,

 

This is my first time posting but I have got a fair bit of help by lurking on here over the years so thanks for that.

 

I have a problem MS261 (from 2012) that has got me stumped. I got this for cheap and was told it was running but it actually wasn’t (Now I know why it was cheap I guess).The first symptom was that it was extremely hard to pull over - the decompression valve would pop out after the first pull and rip the starter cord from my hand. It would occasionally fire up and run at full revs but wouldn’t idle. The cylinder seemed to have fuel leaking out around the muffler and the plug was black. It would also occasionally pop or misfire.

 

Then I seemed to be able to get it running ok, but when I was using it kind of burped and started running backwards!

 

I checked the flywheel which was fine (it has a cast flywheel key) so I replaced the ignition coil (aftermarket). It now seems to run in the right direction but I’m still having major trouble getting it started. It’s still backfiring and seems to flood straight away. Again, it will occasionally fire and run but only on the half choke setting and then it either stalls OR runs too fast so that the chain goes around at idle.

 

When it backfires I get a little puff of vapour or smoke that comes out of the carb.

 

I don’t have the means to test the compression, but seeing as it starts occasionally I’m assuming the compression is ok?

 

I’m not sure where to go from here and would appreciate any ideas...

 

Cheers!

Could be any number , or a combination of things . Metering arm not set correctly , an air leak coming from one or more of several potential places . I think you have to methodically  go through everything . Spud will start you off @spudulike

[Dan Maynard]

Sounds like you have to take nothing for granted. If it's been sat for a while before you had it quite likely gummed up or bunged up in carb so I think that's a logical thing to strip and replace rubbers.

 

If the behaviour definitely changed with coil change then that sounds like it was a problem, maybe you need to think about how much to spend though, could be aftermarket coil is junk and it needs a new carb so with decent OEM parts not cheap.

 

Are you sure the cylinder and piston are good? Also, it's from the period 261 were notorious for crank wear so you should check that.

[AWG81]

Thanks for this Dan and Stubby. One thing I should have mentioned, I replaced the carb for a cheapo one just to see if it made any difference and it didn't. As for the piston and cylinder, I checked just now and I'm not 100% convinced either way - it looks like it has very light scoring, but it's really very hard to tell. Would scoring this light be enough to cause such drastic problems? I'll try and get a photo tomorrow and post here.

 

I have it on the bench in bits at the moment - I'm happy replacing seals and the piston etc but I've not had anything to do with coils / timing before. Is there a definitive way of ruling this out?

 

Thanks again.

[spudulike]

The backfiring is almost definitely down to incorrect ignition timing. This would usually be down to a sheared flywheel key and the flywheel moving on the crankshaft so the ignition timing happens at the wrong time.

You say you checked the key, was this a real good check as it would be the most likely cause.

If it isn't this, it is more than likely that you either have the wrong coil or flywheel on the machine. Seeing as the coil has been replaced, it is most probable that the coil isn't the right one. 

There have been five types of flywheel for this machine and probably a fair few coils.

The reverse running of a two stroke is something of legend - not seen one myself, ever.

[peatff]

26 minutes ago, spudulike said:

The reverse running of a two stroke is something of legend - not seen one myself, ever.

 

I've never seen it but a friend of mine claimed it happened to his old MZ bike a few times and he did have a witness to confirm it.

[spudulike]

Yup, it appears to be talked about more on motorcycle and scooter sites so very possibly...MZ, that takes me back. 

[AWG81]

Thanks Spud. I think I checked the flywheel pretty good, but I'll check again tomorrow and post a photo here. The key is part of the flywheel itself and it looked intact but I'll have a closer look.

 

How would I go about checking I have the correct flywheel and coil? The serial number of the saw is: 175665202 if that helps.

 

I couldn't believe it when it started running backwards. I had to check against my mate's saw to make sure I wasn't seeing things. I wish I'd taken a video...

[Dan Maynard]

Other questions would be if piston or cylinder has been changed already?

[Dan Maynard]

1 hour ago, AWG81 said:

- it looks like it has very light scoring, but it's really very hard to tell.

If very light scoring was the only issue the saw would run fine, maybe slightly down on power and slightly more smoky.

 

If scoring is hard to tell then doesn't sound like that can really be the issue.