Stihl MS 391 looses power when warm

[lsaami]

Hey AS!

Love this community and all the helpful individuals here. I'm up against the ropes with this one. I acquired a few used Stihl saws from fellow who was offloading them, and they are WELL used (price was right). the topic of this post is a old-model MS391. It has served me very well for the last couple of years, but recently started having some issues. It start and runs great for the first few minutes, but after about 10-15 minutes of cutting firewood, it starts cutting out and has a lack of power, almost like it's running out of gas, only, it has plenty. Once it exhibits these symptoms, I cannot tune them out with the high jet. It makes no difference.

• Spark plug is a nice medium brown
• once it fully cools, it fires right back up again.

I was thinking it could be a plugged jet, but that shouldn't be affected by engine temp, should it? Maybe ignition issues?

I did a cold compression compression test this afternoon and it read about 105 PSI, so that was like punch to the stomach. Seems like that's really low? It's a brand new Chinese gauge, so I don't know how accurate that actually is.

Any Stihl wizards have some advice? Is this thing ready for the scrapyard?

 

[RileyT]

Check your fuel lines and filter.

Also not all gauges are created equal, it needs to have a air stem valve at the plug threads.

Pop the muffler off and check for scoring of the piston, may be where your compression is going.

 

[PV Hiker]

Also check the muffler screen to see if it is plugged.
Look under the starter cover and see if the coil is packed with sawdust that it maybe over heating.

 

[lsaami]

Pulled the muffler. Piston looks pretty dang good to my eyes. I left the tester on to see if it will leak down.

 

[lsaami]

Also check the muffler screen to see if it is plugged.
Look under the starter cover and see if the coil is packed with sawdust that it maybe over heating.

Not sure if this one came with a screen, or if PO removed it, but there doesn’t seem to be one now.

There’s definitely some sawdust, but not enough for it to be overheating that quick when it’s so cold outside. At least in my mind.

 

[PV Hiker]

Did not find a screen in this location... see image. If it is there and plugged use a torch and burn it clean.

 

[lsaami]

Did not find a screen in this location... see image. If it is there and plugged use a torch and burn it clean.

View attachment 1065701

Looks like it’s long gone.

 

[lone wolf]

Simple cross-check that gauge on something in great condition that will tell you if your problem is low compression or not which I suspect .

 

[lsaami]

Simple cross-check that gauge on something in great condition that will tell you if your problem is low compression or not which I suspect .

Good thinking. I just checked compression in my MS180C which is in serviceable condition and it tested cold at only 90 PSI. I doubt it would even run with compression that low. Am I right?

 

[lone wolf]

Good thinking. I just checked compression in my MS180C which is in serviceable condition and it tested cold at only 90 PSI. I doubt it would even run with compression that low. Am I right?

Right, but you have to pull till it stops rising about 15 times, don't just stop at a number. Did you do this?

 

[lsaami]

Right, but you have to pull till it stops rising about 15 times, don't just stop at a number. Did you do this?

yes. It wasn't 15 times, more like 5-6.

 

[stihltech]

Piston is not pristine. I would be looking at the intake side. Should still see machine marks around the piston
Pressure and vacuum test are a must. Definitely check complete fuel system from fuel to carburetor including tank vent.
Get it to quit and check spark at that time with a spark tester ( plug with BIG gap).
Cleaning will help with diagnosis.
Throw the compression tester away. Does it hold when picked up by the starter handle?

 

[Kenskip1]

How about a partially clogged gas tank vent?

 

[lone wolf]

yes. It wasn't 15 times, more like 5-6.

No good! Keep going.

 

[Farmer_Nate]

Looks like a significant gouge/scratch on that piston to me.

Wonder what that gold looking material is in that one scratch? Is that an attempt at repair with braze perhaps?

There is an overabundance of black soot in that saw. Those ports should be clean on a saw in good shape. Looks like it was run extremely rich.

I am no saw builder/expert.

as #lone wolf says, lots of sawdust caked all over that thing. Clean it and it will at least cool better.

Also, air filter is likely clogged by the looks of all of those pictures, which I think will reduce air flow and making it run richer, which may also help explain all the soot it seems to me.

If it was run on oil other-than that designed for a 2-stroke air-cooled engine, would that also maybe cause soot?

Again, I am no saw builder/expert.

 

[lone wolf]

Looks like a significant gouge/scratch on that piston to me. I am no saw builder/expert.

It certainly does look like you said. I'm still waiting to see his correct compression numbers. If I was working on that, I certainly would pull it all apart. Just looking at all that dirt and fine sawdust, I would say its sucked lots of dirt and wore things out. That fact that is loses power hot could be low compression to start with and when hotter it drops more, therefore noticeably less power hot.

 

[Farmer_Nate]

My previous reply updated...fyi

 

[rupedoggy]

Clean that saw and put in a known good coil. Mike

 

[Farmer_Nate]

Good point by @rupedoggy.

Classic sign of coil failure is that it runs O.K. when cold, then stops working, sometimes intermittently, when hot.

 

[Farmer_Nate]

OP gone?