Stihl MS391 suddenly lost power.

[Paul Tomo]

Today was supposed to be a day for cutting logs but after 10 mins into using my not so old MS 391 which is in excellent condition just suddenly lost power and then cut out. I tried to re start the saw but on pulling on the starter handle there was no tension in the starter cable at all.

 

The chainsaw will not start it and it won’t even try to start, it’s like there is no compression at all. What could be the problem with my saw.

 

thanks Paul Tomo

[Stubby]

1 hour ago, Paul Tomo said:

Today was supposed to be a day for cutting logs but after 10 mins into using my not so old MS 391 which is in excellent condition just suddenly lost power and then cut out. I tried to re start the saw but on pulling on the starter handle there was no tension in the starter cable at all.

 

The chainsaw will not start it and it won’t even try to start, it’s like there is no compression at all. What could be the problem with my saw.

 

thanks Paul Tomo

I think you have answered it already ...no compression . Were you using old fuel ?  You could take the muffler off and have a gander at the piston . You could have had an air leak from any number of places . Did it rev up higher than you remember as normal from last time you used it ?  Stale fuel can cause it to nip up as will an air leak as it will be running lean and faster therefore hotter with less lube . Look at the piston .

Edited December 10, 2020 by Stubby

[Exmoor Chris]

Yes, sounds like a melted top end. Was the fuel correct? If so you must have had an air leak. Bad luck

[Moose McAlpine]

Maybe it's sheared the flywheel woodruff key and the flywheel is spinning on the crank?

 

You can remove the starter housing to check this easily, or remove the clutch cover and see if the clutch turns when you pull it over.

[Paddy1000111]

Check the flywheel, check the spark plug (they can fail and lose compression) otherwise you've toasted a cylinder and you need to pull the jug

[spudulike]

It sounds like the spark plug or decomp valve may have come out as even a seized engine will give slight compression enough to tension the starter. Check these two things first, if the plug has come out, check the thread of the hole and make sure it is still OK as it often fails once a plug has come out under combustion pressure.

If it isn't this and your "no tension" is more like not much tension then you have probably left fuel in the saw for a while then cut hard with it and seized it. 

[bmp01]

15 hours ago, Paul Tomo said:

Today was supposed to be a day for cutting logs but after 10 mins into using my not so old MS 391 which is in excellent condition just suddenly lost power and then cut out. I tried to re start the saw but on pulling on the starter handle there was no tension in the starter cable at all.

 

The chainsaw will not start it and it won’t even try to start, it’s like there is no compression at all. What could be the problem with my saw.

 

thanks Paul Tomo

Damn, frustrating day, dissapointing experience to say the least. Nothing to add to the above, i expect by now you already know what's happened. Worth seeking more advice on here regarding how to fix it. 

 

 

[Paul Tomo]

19 hours ago, Exmoor Chris said:

Yes, sounds like a melted top end. Was the fuel correct? If so you must have had an air leak. Bad luck

I’ve took the saw down to a local Stihl dealers and said there’s still compression and the problem maybe the gasket that’s at the top of the piston ? He said the card that’s at the top might be to flexible and gone. Stripping it down will give me a better idea, looks like I’ll be doing this at the weekend. 
 

 

 

[Moose McAlpine]

36 minutes ago, Paul Tomo said:

I’ve took the saw down to a local Stihl dealers and said there’s still compression and the problem maybe the gasket that’s at the top of the piston ? He said the card that’s at the top might be to flexible and gone. Stripping it down will give me a better idea, looks like I’ll be doing this at the weekend. 
 

 

 

 

 

There's no gasket at the top of the piston. 😕

[Paul Tomo]

25 minutes ago, Moose McAlpine said:

 

 

There's no gasket at the top of the piston. 😕

What would he be talking about then?