Stihl MS 391 help, please

[red2002]

I melted my year-and-a-half old Stihl MS391 chainsaw a month or so ago, holing the piston.

A 50-1 mix of Stihl synthetic oil in 89-octain had been used and I have since run a couple of other chainsaws and a weed-eater on the same can of gas, so it's doubtful that that was the culprit.

While the piston was toast, the cylinder had only light scuffing, so I installed a new piston and small-end bearing. Permatex Moto Seal had been recommended by a couple of web sites, so it was used to seal the cylinder and case.

A compression test shows 120 psi (seems Stihl wants a minimum of 110, so that was good) and I have a bright, blue spark.

But, no fuel to the cylinder, so no start. A shot of gas through the carb lets the thing run for a few seconds.
The fuel filter is clean and flows fuel.

There is no impulse line on this model, but it appears the the carb pulls fuel with the piston-pulse via the upper of the three holes on the manifold. The manifold and carb are tightly sealed. The carb's diaphragms and screens are clean and intact.

I did not, however, do a vacuum/pressure test, but have a new pair of crank seals on hand.

I've certainly missed something and thought you folks with more experience can assist.

Any thoughts?

Thanks very much.

Mike

 

[Andyshine77]

When you rebuild these saws a vac/pressure test is a must. 120psi of compression is very low, pull the exhaust and have a look at the piston and cylinder through the cylinder port. Check all the lines for pinholes, may need to go through the carb. I assume you know how to properly tune the carburetor?

If you have an engine failure of an unknown cause always throw out the old fuel mix, equipment can run a long time on bad mix, but that doesn't mean damage hasn't or isn't being done.

 

[red2002]

Andyshine77,

I’ll double-check the fuel lines and you’re right about discarding suspect fuel.

Appreciate your advice.

Mike

 

[huskihl]

Impulse comes through the bottom small hole, between and under the lower bolt holes.



If any of the diaphragms are stiff at all they won't work properly.

 

[red2002]

Thanks, Kevin.

Didn’t know that, and I’ll check it.

I’ve used chainsaws for 50 years, am a rookie in their innards.

Mike

 

[Andyshine77]

Andyshine77,

I’ll double-check the fuel lines and you’re right about discarding suspect fuel.

Appreciate your advice.

Mike

Good deal. However if the saw really does only have 120psi of compression it's not going to run well. How many times did you pull the engine over to get that reading? You want to keep pulling in until the pressure stops climbing.

 

[red2002]

Andyshine77,

I got the thing running and it seems to be good!

Will know for sure when I put it all back together, and run it gently for a while.

Initially, I had tightened the compression tester adapter finger-tight for the 120 psi reading; snugged it up and it went to 135 psi. Not sure what the minimum/good number is, but my "finely calibrated" bicep says it is as hard to pull as it was new.

Replaced the fuel line, per your suggestion; something worked and that may have been the culprit.

So we'll see.

Huskihl,

Thanks for the pic of the impulse hole. I ran a small plastic rod through the cylinder hole and it was clear, as was the orifice in the carb. Dunno, but appreciate the info.

Mike

 

[huskihl]

Fuel lines, filters, and carb diaphragms can make saws do weird things

 

[Andyshine77]

135 is just a tad lower than normal for that saw, but you may just be getting a slightly low reading from the gauge.

Anyway sounds like you got her going.[emoji106]

 

[Charles P]

I melted my year-and-a-half old Stihl MS391 chainsaw a month or so ago, holing the piston.

A 50-1 mix of Stihl synthetic oil in 89-octain had been used and I have since run a couple of other chainsaws and a weed-eater on the same can of gas, so it's doubtful that that was the culprit.

While the piston was toast, the cylinder had only light scuffing, so I installed a new piston and small-end bearing. Permatex Moto Seal had been recommended by a couple of web sites, so it was used to seal the cylinder and case.

A compression test shows 120 psi (seems Stihl wants a minimum of 110, so that was good) and I have a bright, blue spark.

But, no fuel to the cylinder, so no start. A shot of gas through the carb lets the thing run for a few seconds.
The fuel filter is clean and flows fuel.

There is no impulse line on this model, but it appears the the carb pulls fuel with the piston-pulse via the upper of the three holes on the manifold. The manifold and carb are tightly sealed. The carb's diaphragms and screens are clean and intact.

I did not, however, do a vacuum/pressure test, but have a new pair of crank seals on hand.

I've certainly missed something and thought you folks with more experience can assist.

Any thoughts?

Thanks very much.

Mike

Hi Mike, the problem may have been not enough fuel WHEN you burned the piston. or, did the decompress not kick off? Both will burn it up.

 

[red2002]

Hi Mike, the problem may have been not enough fuel WHEN you burned the piston. or, did the decompress not kick off? Both will burn it up.

Good point, Charles, that will sure lean it out. Thanks for that idea!

I’d been running the saw for most of the day and it had been running fine when it abruptly quit. Rarely used the decompression valve anyway, but anything’s possible.

Mike

 

[Justin Taylor]

Did you check the rubber intake from the carb to the cylinder for cracks