Stihl MS 391 -- sticky chain brake

[jerrycastle]

Hello All,
I'm new to the Forum.  First post here.....

I have a 2-year old Stihl MS 391.  I bought it new and use it for felling and firewood on my own property. The chain brake has become very difficult to engage.  I have taken the side cover and clutch cover off and used both compressed air and Brakleen to clean the chain brake.  ...and I mean clean.  There appears to be nothing obviously mechanically wrong with it.  It's just really difficult to engage.  Here's the kicker.... when the saw is cold, it engages pretty well/easily.  After running the saw for 3-5 mins, I need to use the butt/palm of my hand to "slam" it... not safe or practical. 

My little MS 210 is 10 years old and I have NEVER had an issue with the chain brake.

Any Stihl guys out there have this issue? 

Thanks.

[dougand3]

Welcome. Never worked on this model, so guessing here. Gotta think heat off cylinder or exhaust is changing plastic or metal. Big spring heats, gets longer, harder to push flag?
Here is IPL:

https://forestryforum.com/gallery/albums/userpics/36829/Stihl_MS_391_chainbrake.jpg

[joe_indi]

If you open the brake cover, you will see these two parts.
Check whether that spring is in place on both its ends.
One end on the lever shown her, the other on a pin on the body.
Also check whether any other components are loose on their pins. If any are loose, the resultant play will cause extra travel in the brake handle.
Joe

[joe_indi]

accidentally re-posted

[jerrycastle]

Joe and Doug--
Thanks for your replies as well as your insight.  All of the mechanical linkages appear to be connected and "throwing" when I pull vs. push the chain brake handle.  I agree that the issue is very likely heat related, but I still can't figure out why after two years and/or how to remedy it. 

I might have to swallow my pride and bring it to the dealer.  Otherwise, I'm just throwing parts at the thing without an educated conclusion.

Until next time....

[motohed]

I'm just wondering if the pivit pin the lever arm rides over is tight , or is it tweaking a little when the saw heats up . You may what to try some heat shielding under the muffler . It may be the arm is slightly worn and binding as the saw gets warm , also just an FYI brake clean can melt some plastics so be careful , trust me on this and don't ask .

[jerrycastle]

Motohed,
Thanks for the reply. 

I asked at the saw shop a few months ago and they assured me that Brakleen won't harm/melt the plastic on a Stihl chainsaw.  They said the worst that could happen is that the plastic might discolor.  We shall see.

[motohed]

Motohed,
Thanks for the reply. 

I asked at the saw shop a few months ago and they assured me that Brakleen won't harm/melt the plastic on a Stihl chainsaw.  They said the worst that could happen is that the plastic might discolor.  We shall see.

I was just sitting here thinking about your problem . It may be a stretch  , but if you use the chain brake a lot for starting purposes the spring tension may have changed , or is being changed , somehow by the heat of the saw . Maybe a new spring is in order . Somethings , are just unexplainable gremlins that get over looked . I would atleast put it next too a new one to see , if the length is a little different , the forces that apply will be different , also check the arm for any little bit of wear . The tolerences of these chain brakes are pretty close and a little wear maybe making it out of spec , when the saw warms up . Like I said it's a stretch . You are the one using the saw , it makes it hard to diagnose for the rest of us . we are just using our previous saw repair experiance and overall use of saws in general . I'm thinking if it been used enough to weaken the  spring , IE , making it a little longer and then adding a little heat to make it weaker when warm , then add in a little wear to the arm that actuates the brake band , maybe allowing the arm to bind a little , adding to that a spring that is a little long and then is weakend a little more by the engine heat . This would allow for the resulting problem . This is one of the reasons , I liked the old husky saws that actually had an adjustor , that would reset the spring pull length and tension . They could be dialed in spot on .

[jerrycastle]

Well, I broke down and brought the saw to the shop yesterday.  The guys said they have never seen/heard anything like my issue and said they can get it on the bench at some point this week. 

Thank you all for chiming in.

[Ada Shaker]

Yeh, sounds kinda thermal, anyone's guess really. Do let us know what caused it if you can, cheers.