Not looking good for this MS391

[Doubledengines]

Having a tough time getting my friend's MS391 started... He ran 4 cycle fuel through it and said it just died on him...

Got spark.. and cant get it started even with starter fluid down its throat. Did a quick compression test and shes right at about 70 psi.

Quick looked down the spark plug hole and saw some vertical scoring on the cylinder.. Going to pull the muffler later on to get a better look.

I read online something about these 391s have about 160-170 psi compression brand new, is that right? I know it does have a de-compression valve at the top. I'm new to the decompression. I also checked Stihl's website and they said most saws should have about 110 psi. I couldn't find a chart for all their saws though.

 

[mkinslow]

If its scored,and compression is that low,its not gonna start. Most likely looking at a new piston and cylinder

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[RED-85-Z51]

160ish is healthy. If he straight gassed it...its toast

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[Doubledengines]

I kinda had a feeling.. when I pulled it, it felt alright... I first tested compression and was about 50.. threw some oil down the cylinder and jumped to what you see about 70. Probably looking at a whole rebuild on this one.

Just finished a rebuild on a MS250. That was kind of a pain. I’m hoping this “almost commercial” saw is a little bit easier.


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[pioneerguy600]

4 stroke fuel is straight unmixed gasoline, that causes severe scoring on the piston, it actually melts the aluminum of the piston and it gets deposited on the cylinder wall, pull it apart and show pics of the damage for furher accesment.

 

[Doubledengines]

4 stroke fuel is straight unmixed gasoline, that causes severe scoring on the piston, it actually melts the aluminum of the piston and it gets deposited on the cylinder wall, pull it apart and show pics of the damage for furher accesment.

Exactly. I was hoping it wasn’t too late for this one, but it might be. I’ll hopefully report back tomorrow with the muffler off.


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[pioneerguy600]

Exactly. I was hoping it wasn’t too late for this one, but it might be. I’ll hopefully report back tomorrow with the muffler off.


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Yea, with compression that low the rings are seized in their grooves/lands, likely plenty of aluminum stuck to the cylinder walls, this happens a lot , even to experienced users.

 

[anymanusa]

He ran 4 cycle through it? You mean it was straight gassed?

 

[Doubledengines]

He ran 4 cycle through it? You mean it was straight gassed?

That's correct, straight gas.

 

[Alphadelta]

Welp, that blows. I just got a meteor piston kit and I'm very impressed with the quality of both the cylinder and piston. They aren't badly priced either.

 

[Doubledengines]

How do they compare to OEM? Quality and pricing?


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[Alphadelta]

How do they compare to OEM? Quality and pricing?


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When I was looking around for my Jonsered 2171, over and over I got told to go with a meteor. Now that I have a meteor for my 440 I am over the moon with the quality. I quick google yielded $290 and OEM vs $150 for the meteor. I think for the quality and price, meteor is hard to beat.

 

[Arrowhead]

Meteor is the only aftermarket i would consider. Great quality. Most saws won't run on 110psi. Sure you read that right?

 

[Doubledengines]

When I was looking around for my Jonsered 2171, over and over I got told to go with a meteor. Now that I have a meteor for my 440 I am over the moon with the quality. I quick google yielded $290 and OEM vs $150 for the meteor. I think for the quality and price, meteor is hard to beat.

Good to know, weren’t you the same Person on another thread that had farm tech for the 440? And wasn’t too excited about it?


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[Alphadelta]

‘Twas me. That was my crankshaft. But I also picked up the meteor piston kit as insurance lol.


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[pioneerguy600]

Meteor is the only aftermarket i would consider. Great quality. Most saws won't run on 110psi. Sure you read that right?

He actually has 50 - 70 readings, I am sure that is too low even if the gauge is reading a little low.

 

[Doubledengines]

I thought so, ahh gotcha. Can never have too many.

And yes gauge read about 70 and saw did not run when it came in.


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[pioneerguy600]

Meteor pistons are good quality pieces, they are the only ones being made now that most of us builders will use and recommend as replacements for OEM.

 

[sb47]

Honda needs to make a chain saw. Those Honda 250's will run on plane dirt. You almost cant kill a Honda 250 2 stroke.

 

[Wood Doctor]

Last year I rebuilt this 310 with a 390 top end and a few other parts:

I am sure your saw is in better shape than this one was. It runs fine today. The owner calls me the chain saw whisperer.