Stihl MS 391 looses power when warm

[stihltech]

Tear it apart. Find the issue. Or junk it.

 

[lsaami]

Seems like it’s running okay for now. No sense tearing it apart while it’s operational.

I’ll report back if anything changes.

 

[Husky77]

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

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Blow up pic
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Does that score go through the ring or only marked the skirt, piston still has machine Mark's which is good. Cheap gauge you get cheap results I know because I had one and one saw was showing 50psi but ran great, tested it with a friends gauge and well over 100psi so dont rely on a gauge that's a bargain! Your saw needs a good clean, tank vent is blocked for sure you've indicated that by unscrewing the cap and it working. So if the outside is that bad the inside probably has more crap in the tank than you can see. First thing I would do is clean it ALL, new filter carb kit and fuel line. Either spend the time and do a good job and bin it. I come across so many saws that are never looked at until something goes wrong then expect a cheap repair. Do the maintenance look after it and they last a very long time.

 

[lsaami]

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Does that score go through the ring or only marked the skirt, piston still has machine Mark's which is good. Cheap gauge you get cheap results I know because I had one and one saw was showing 50psi but ran great, tested it with a friends gauge and well over 100psi so dont rely on a gauge that's a bargain! Your saw needs a good clean, tank vent is blocked for sure you've indicated that by unscrewing the cap and it working. So if the outside is that bad the inside probably has more crap in the tank than you can see. First thing I would do is clean it ALL, new filter carb kit and fuel line. Either spend the time and do a good job and bin it. I come across so many saws that are never looked at until something goes wrong then expect a cheap repair. Do the maintenance look after it and they last a very long time.

Thanks for the advice. I have a carb kit, new lines, etc. on order.

 

[bavermont]

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?

Good morning - new to the group. Happy to report that reading some of the threads on chainsaw repair I was able to fix my saw. It was the spark arrestor screen that was fouled and causing loss of power. Simple fix - thanks.

 

[lone wolf]

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

Did the needle stop rising?

 

[lone wolf]

Did the needle stop rising?

A pic of the screw in the end of that tester might help .

 

[Husky77]

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?

I checked a saw that was showing 50psi, it would run fine so wanted to check the gauge. My gauge was off ebay and cheap, the one I checked it with was showing 90psi so almost double. So I ended up buying a more accurate gauge and that shows 130psi. The saw runs great but your not saving anything with a cheap gauge, its frustrating and very time wasting.