Stihl MS 391 looses power when warm

[REJ2]

Blow up pic

 

[lone wolf]

Blow up pic
View attachment 1065928

Yup!

 

[Wood Doctor]

OP gone?

Perhaps but the thread survives. I have to agree with Lone Wolf and 105 psi compression is too low. I have a friend with a Husky 262 that is having a similar problem -- loses power in the heavy cuts after warming up. I will be likely be giving his a compression test later this week.

If it does start and run, you can shut it down after a minute or so and warm test it to help confirm the cold test. Just a suggestion.

 

[Farmer_Nate]

What in the world is that yellow stuff in the gouge?

And none of my saws have ever had all the black coke inside. Very weird. That looks like quite a cool exhaust for all that junk to accumulate.

But now, who cares? OP left.

 

[lsaami]

Wow! You guys are animals around here. Had to take care of some things last night and didn’t get back on here.

How about a partially clogged gas tank vent?

That is a good thought. I’ll put her back together and run it until it acts up. If I can open the fuel tank and it comes back to life, that would tell.

No good! Keep going.

Pulled the sucker 15+ times with the same result.

 

[lsaami]

What in the world is that yellow stuff in the gouge?

And none of my saws have ever had all the black coke inside. Very weird. That looks like quite a cool exhaust for all that junk to accumulate.

But now, who cares? OP left.

No idea what the yellow stuff is. Seems uniform on all the edges which made me thing it was a carting mark or something.

 

[lsaami]

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.

The old air filter was pretty dirty. It has a fresh one on now.


Sooty/rich could also point to compression or ignition issues, correct?

 

[lone wolf]

Perhaps but the thread survives. I have to agree with Lone Wolf and 105 psi compression is too low. I have a friend with a Husky 262 that is having a similar problem -- loses power in the heavy cuts after warming up. I will be likely be giving his a compression test later this week.

If it does start and run, you can shut it down after a minute or so and warm test it to help confirm the cold test. Just a suggestion.

He only pulled it over 5 times, I am waiting to see the true compression reading is. Either the compression or the Coil is suspect . If i knew the compression numbers I

Wow! You guys are animals around here. Had to take care of some things last night and didn’t get back on here.


That is a good thought. I’ll put her back together and run it until it acts up. If I can open the fuel tank and it comes back to life, that would tell.

Pulled the sucker 15+ times with the same result.

Well now we don't know if the gauge is accurate. You have any new saws to cross-check it?

 

[lsaami]

He only pulled it over 5 times, I am waiting to see the true compression reading is. Either the compression or the Coil is suspect . If i knew the compression numbers I

Well now we don't know if the gauge is accurate. You have any new saws to cross-check it?

Nothing new, but that’s what I was saying earlier. I cross-checked it with my reasonably healthy MS180C and that only tests at 90 PSI after 15 pulls. No way that thing even runs at 90PSI.

 

[SteveSr]

Nothing new, but that’s what I was saying earlier. I cross-checked it with my reasonably healthy MS180C and that only tests at 90 PSI after 15 pulls. No way that thing even runs at 90PSI.

Based on all the fine sawdust collected on the engine it looks like you need to up your chain sharpening skills.

Also, on these saws the transfer covers are GLUED on and enough heat will soften the glue and cause an air leak leading to dirt ingestion and a blown engine. That is one way that these engines die young.

 

[lone wolf]

Nothing new, but that’s what I was saying earlier. I cross-checked it with my reasonably healthy MS180C and that only tests at 90 PSI after 15 pulls. No way that thing even runs at 90PSI.

So get a gauge that works in small engines. Other options are to pull the cylinder and look for obvious problems, or change the coil. If you could prove the compression is good, which you have not at this point, then you could assume it's a bad coil. Does the saw start in under 5 pulls cold, and will it hold idle.?

 

[lsaami]

Based on all the fine sawdust collected on the engine it looks like you need to up your chain sharpening skills.

Also, on these saws the transfer covers are GLUED on and enough heat will soften the glue and cause an air leak leading to dirt ingestion and a blown engine. That is one way that these engines die young.

I can confirm, I nuked the chain this summer cutting deadfall. Went full chisel on crap wood sitting in sandy soil. Stripped the back, but still only so much you can do. Rookie mistake. I have a fresh bar and chain for it now. Old one is now on permanent deadfall duty.

So get a gauge that works in small engines. Other options are to pull the cylinder and look for obvious problems, or change the coil. If you could prove the compression is good, which you have not at this point, then you could assume it's a bad coil. Does the saw start in under 5 pulls cold, and will it hold idle.?

It start in 2-3 pulls every time. One pull on cold start, move to warm start and it usually fires right up, no matter the temperature.

I’ll see what I can do about getting a better compression gauge. Any recommendations?

 

[lone wolf]

I can confirm, I nuked the chain this summer cutting deadfall. Went full chisel on crap wood sitting in sandy soil. Stripped the back, but still only so much you can do. Rookie mistake. I have a fresh bar and chain for it now. Old one is now on permanent deadfall duty.

It start in 2-3 pulls every time. One pull on cold start, move to warm start and it usually fires right up, no matter the temperature.

I’ll see what I can do about getting a better compression gauge. Any recommendations?

Well, I know you don't want one with an adapter on the screw in end. I have a homemade one and a Snap On, they both are very good.

 

[Jeff Lary]

Looks like that piston has a gouge in it big enough to hide a toothpick in it.

 

[Wood Doctor]

I can confirm, I nuked the chain this summer cutting deadfall. Went full chisel on crap wood sitting in sandy soil. Stripped the back, but still only so much you can do. Rookie mistake. I have a fresh bar and chain for it now. Old one is now on permanent deadfall duty.

It start in 2-3 pulls every time. One pull on cold start, move to warm start and it usually fires right up, no matter the temperature.

I’ll see what I can do about getting a better compression gauge. Any recommendations?

Use the dirt cheap method. Hold the saw by the pull-cord handle waist high and release it. If the saw drops towards the floor on its own weight after a slight tug, you are short on compression. Not an acid test, but it is probably as accurate as your meter is now showing.

 

[lsaami]

Use the dirt cheap method. Hold the saw by the pull-cord handle waist high and release it. If the saw drops towards the floor on its own weight after a slight tug, you are short on compression. Not an acid test, but it is probably as accurate as your meter is now showing.

If I do that, it takes about 3 seconds for it to turn over. About 15 seconds to hit the floor.


I put the saw all back together and ran it for a while. It started loosing power, so I stopped and opened the fuel tank. That *seemed* to get it going again.

I’m thinking this is likely a case of this thing being very tired, and one minor thing causes it to not run.


That begs the question - is it worthwhile to rebuild a saw of this caliber? Or do I look to upgrade to a 362 or something of that nature?

 

[SteveSr]

If I do that, it takes about 3 seconds for it to turn over. About 15 seconds to hit the floor.


I put the saw all back together and ran it for a while. It started loosing power, so I stopped and opened the fuel tank. That *seemed* to get it going again.

I’m thinking this is likely a case of this thing being very tired, and one minor thing causes it to not run.

I think that you overheated and scored the P&C by forcing it to cut with a dull chain. As it gets warm the piston expands more than the cylinder and starts getting into the transfer that is already there causing the lack of power.

Seized 2-strokes will sometimes run again after they cool down but not run well. The only way to confirm this diagnosis is to tear it apart and do an inspection to confirm what the photos show. You may also find a loose transfer cover as well.

That begs the question - is it worthwhile to rebuild a saw of this caliber? Or do I look to upgrade to a 362 or something of that nature?

Unless you can find a 391 parts saw cheap with a known good engine it is not worth rebuilding.

A 362 is a good pro level saw and is more powerful and should be harder to kill.

 

[stihltech]

Yellow mark = plastic bearing cage failure IMO.

 

[Farmer_Nate]

I think that you overheated and scored the P&C by forcing it to cut with a dull chain. As it gets warm the piston expands more than the cylinder and starts getting into the transfer that is already there causing the lack of power.

Seized 2-strokes will sometimes run again after they cool down but not run well. The only way to confirm this diagnosis is to tear it apart and do an inspection to confirm what the photos show. You may also find a loose transfer cover as well.


Unless you can find a 391 parts saw cheap with a known good engine it is not worth rebuilding.

A 362 is a good pro level saw and is more powerful and should be harder to kill.

I am certainly sold on the merits of saws designed as "pro" for daily hard use.

 

[pioneerguy600]

The piston ring is likely stuck in its land/groove causing the engine to loose compression, that deep score mark tells me the ring was likely impacted, move the piston down til the rings show through the exhaust port, try moving the rings to see if the gouge has pinned the rings too tight in the lands.