Stihl 391 Stalling Issue

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djetelina

djetelina

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Nov 19, 2021
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Texas
Just bought a used Stihl 391 on Craigslist for $400 w/ 20" bar and new chain. Looked ok, started ok, seller told a pretty good story. Bucked thru 4 or 5 30" dia American Elm logs, ran strong. Then it stalls while cutting. Starts right up on 1st pull, runs about 15 seconds, then stalls (sounds like the fuel system is leaning out). Shot a video, posted to YouTube, https://youtu.be/SKfB7qj-LQI .
Stalls out like clockwork every 15 seconds, starts right up on 1st pull, runs for 15 seconds. It is full of high-dollar pre-mix gas, opened/closed fuel tank to verify it was not a tank venting issue.

Question for the experienced Stihl users, is there any particular issue that the 391 is known for that may be causing this strange stalling issue? ie weak fuel pump desigh, known carburator issues, or even something in the electronic ignition?
I will troubleshoot this saw and post the fix to this forum.
 
Vintage Engine Repairs
Joined
Aug 12, 2019
Messages
3,764
Location
Australia
Your neighbour in that video :laughing:

Check the carby hasn’t got any gunk in it
Check the function / sealing of the check valve / ball in the main nozzel.
Check passages are all clear.
Check fuel filter isn’t blocked / restricted
Check fuel line and impulse are good
Check spark screen
Pres and Vac test
View Piston through exhaust.
 
Woodslasher

Woodslasher

Make McCulloch Great Again!
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Commiefornia
Just bought a used Stihl 391 on Craigslist for $400 w/ 20" bar and new chain. Looked ok, started ok, seller told a pretty good story. Bucked thru 4 or 5 30" dia American Elm logs, ran strong. Then it stalls while cutting. Starts right up on 1st pull, runs about 15 seconds, then stalls (sounds like the fuel system is leaning out). Shot a video, posted to YouTube, https://youtu.be/SKfB7qj-LQI .
Stalls out like clockwork every 15 seconds, starts right up on 1st pull, runs for 15 seconds. It is full of high-dollar pre-mix gas, opened/closed fuel tank to verify it was not a tank venting issue.

Question for the experienced Stihl users, is there any particular issue that the 391 is known for that may be causing this strange stalling issue? ie weak fuel pump desigh, known carburator issues, or even something in the electronic ignition?
I will troubleshoot this saw and post the fix to this forum.
My 2 cents, running for a bit before dying typically means an air leak or an obstruction/impedance in the fuel system. Either way, choking it adds enough fuel for it to start, but once it burns off that initial boost it can't get enough fuel to keep running. That being said, I'd say Tom gave you a good list of initial stuff to check and I'd definitely start with that.
 
Vintage Engine Repairs
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My 2 cents, running for a bit before dying typically means an air leak or an obstruction/impedance in the fuel system. Either way, choking it adds enough fuel for it to start, but once it burns off that initial boost it can't get enough fuel to keep running. That being said, I'd say Tom gave you a good list of initial stuff to check and I'd definitely start with that.
Start with checking on his neighbour :laughing:
 
trains
Joined
Feb 23, 2015
Messages
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Location
Australia
I hear banjos.

Seriously tho, first thing, take off the muffler and check piston condition, its the first thing you should check on a new to you saw anyway.

In answer to your original question, no there are no common issues with the 391 that your saw is having.
They tend to be pretty good home owner range of saw.
But what your saw is doing has happened to many 2 strokes before, and will happen again, so now we just have to work thru a process to find what it is.

If your unsure of what the piston should look like, post up some pics here of it.
if its ok, then you can go forward from there.

part of removing the muffler is that the screen which is serviceable will have to come off, so you can check it then, make sure its not blocked with carbon.

Then make up a U shaped hook from some wire, ie coat hanger, and gently lift out the fuel filter, dont over stretch the fuel hose to do so.
Gently hold the fuel line with some pliers, and use a flat bladed screw driver (small) to gently push the edge of the hose back and over the barb of the fuel filter, sometimes they can be quite stuck to the filter barb, but just go easy, and gently work it evenly from side to side and it will then move over the barb of the filter without damaging it.
Blow thru the filter and see how blocked it is, after that its time to remove the covers, filter and some 8mm nuts to remove the carb.
with carb removed, you can post up what you have found, and we can go from there.

not sure how experienced you are with these type of toys, so will assume nothing for now, but the only dumb question is the one you dont ask.

VintageEngineRepairs has given you some good things to look at, just tried to give you some good order of doing them.
 
MagraAdam

MagraAdam

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Tasmania
I had one of these saws, and cut a lot of wood with it. That's a pretty good price, based on what they go for here.

The exhaust being blocked (usually the spark arrestor) is a common problem for all saws. Especially if run rich and oily. I'd give that and the fuel filter a look at first. I assume the idle is set fast enough and low screw rich enough? If not that could be the problem, but if you're new to tuning touching it could make things worse.

Once you get it going well get a full chisel chain on it, and go get a set of inner and outer dogs (you'll need to drill some 5mm holes in the cover under the nylon inserts) and you'll have a great saw. I'm a fan of slightly bigger dogs personally. The air filter is the style that can be cleaned. Turn the oiler to max too. (And flush the oil tank with petrol to give it a good clean as they often get dust and gunk in them).

Best of luck.
 
67L36Driver
Joined
Nov 1, 2010
Messages
14,481
Location
St. Joseph, MO
I have heard of a couple folks having issues with loose side covers, so you might peek at those as well.View attachment 946715

I have one out in the garage like that. MS311 transfer port cover on right side sucks air. [emoji90]

It’s no coincidence that the aftermarket cylinders are one piece. [emoji19]

Per my favorite Stihl tech, they have covered a bunch of them under warranty.
 
Captain Bruce

Captain Bruce

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Apr 4, 2015
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Michigan
I had one of these saws, and cut a lot of wood with it. That's a pretty good price, based on what they go for here.

The exhaust being blocked (usually the spark arrestor) is a common problem for all saws. Especially if run rich and oily. I'd give that and the fuel filter a look at first. I assume the idle is set fast enough and low screw rich enough? If not that could be the problem, but if you're new to tuning touching it could make things worse.

Once you get it going well get a full chisel chain on it, and go get a set of inner and outer dogs (you'll need to drill some 5mm holes in the cover under the nylon inserts) and you'll have a great saw. I'm a fan of slightly bigger dogs personally. The air filter is the style that can be cleaned. Turn the oiler to max too. (And flush the oil tank with petrol to give it a good clean as they often get dust and gunk in them).

Best of luck.

Defintely a simple fuel/air matter. The large set of spikes and a 28" set-up, with FULL-chisel chain.......You'll be buried in ankle-deep chips
 
hkstwn7

hkstwn7

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Joined
Jun 22, 2021
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hookstown,pa.
Just bought a used Stihl 391 on Craigslist for $400 w/ 20" bar and new chain. Looked ok, started ok, seller told a pretty good story. Bucked thru 4 or 5 30" dia American Elm logs, ran strong. Then it stalls while cutting. Starts right up on 1st pull, runs about 15 seconds, then stalls (sounds like the fuel system is leaning out). Shot a video, posted to YouTube, https://youtu.be/SKfB7qj-LQI .
Stalls out like clockwork every 15 seconds, starts right up on 1st pull, runs for 15 seconds. It is full of high-dollar pre-mix gas, opened/closed fuel tank to verify it was not a tank venting issue.

Question for the experienced Stihl users, is there any particular issue that the 391 is known for that may be causing this strange stalling issue? ie weak fuel pump desigh, known carburator issues, or even something in the electronic ignition?
I will troubleshoot this saw and post the fix to this forum.
Check fuel tank vent.
 
trains
Joined
Feb 23, 2015
Messages
1,322
Location
Australia
Check fuel tank vent.
Op said he had already lifted the caps to isolate that.
I think HarlyT is on the money, as long as it hasnt damaged the piston.

Just have to find the reason why it was sold, sometimes, its because they dont need it, other times its because it dosent work right.
 
Dean-O

Dean-O

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Dec 6, 2019
Messages
181
Location
Jefferson
$400 for a used 391???? You can buy a brand new one with a warranty and being test ran/adjusted before purchase for $589.
Check that piston for scoring first and foremost
 
djetelina

djetelina

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Joined
Nov 19, 2021
Messages
6
Location
Texas
Many thanks for the most excellent suggestions from everyone. Yesterday was nasty, will make an effort to troubleshoot my MS-391 today, reporting back w/ pics as applicable. Gotta get this thing going, another huge American Elm awaits.
 

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