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Stihl MS 391 Problem

effigy

effigy

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Recently i purchased a new Stihl MS 391 with the 25" bar, as well as the Stihl chain oil & premixed fuel from my local dealer.
On day 1 i flooded the saw on my first attempt to start it. Shortly there after I got it unflooded and spent the next hour cutting wood using about 75% throttle.
Once started and running it didn't give me any troubles and seemed to run well.
On day 2, (20 hours after day 1) I checked the bar oil (still mostly full) and fuel level (still 1/2 full) and when I went to start it pulling on the ripcord with the chain brake in the on position was impossible, it wouldn't budge, however, with the chain brake in the off position the ripcord would pull but the engine won't start...
when it comes to engines (in general) i understand how they function from a high-level perspective but im very inexperienced with them beyond that, having only owned & used a gasoline powered generator & gasoline powered pressure washer up until this point and my chainsaw needs were previously met by my battery powered DeWalt saw.

Ideas? suggestions? advice?

EDIT: I also hear what sounds like air pressure escaping when i attempted to start the saw on day 2.
 
effigy

effigy

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Thanks for all the insight, I sincerely appreciate it!

I do need to clarify something though, I read in the manual that for the first 3 tanks of fuel not to run the saw at 100% throttle when off load, the 75% was the throttle estimate for the second or 2 before i began each cut.
Once the saw started to eat at the wood i did squeeze the trigger tighter but then backed off again just before the saw came out the other side.
And since all my cuts were on wood that was 3-4 inches thick at most, and since the saw cuts like a hot knife through butter (in a matter of seconds, this thing's a beast) the average throttle was probably closer to 85-90% but definitely not 100%...

I did take the saw back to the dealer, hopefully the repairs will be covered under warranty and not cost me hundreds of dollars...

I'll also make sure to post when I get the saw back with whatever the dealer tells me the problem was.
 
cscltd

cscltd

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Usually if saw won’t pull over with brake on, most times the brake was left on position with fast idle on or higher throttle with chain not turning for more than 5 secs and melted sprocket bearing, stretchEd clutch springs and melted brake cover and sometimes oil gear.
and won’t start with brake off as chain is engaged because of above and is like trying to start a manual car in gear.
the same can also happen if you really load saw while cutting and not in ideal rpm range as clutch would be slipping and causing extra heat in an already hot area.
 
FrankHgh

FrankHgh

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Friend Gave me a 391 that Wouldn't start. Ends up the Decompression button was bad, I replaced the decomp button on my free saw and it works great.
 
effigy

effigy

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Got the saw back, was told it was a problem with the chain brake and the repair service was covered under warranty.
I've got another 40-50 feet of small wood (3-4" stuff) that needs to be cut down to sizes small enough to be picked up curbside by city waste disposal.
Hopefully enough small wood to burn through the 2.5 more tanks of gas to finish out the break-in time period.
Then i get to tackle the actual reason for the purchase of the saw, a handful of 20"+ diameter norfolk island pines that need to come down.

Thank you all again for the insight and advice, I truly appreciate it!
 
sled_22

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Got the saw back, was told it was a problem with the chain brake and the repair service was covered under warranty.
I've got another 40-50 feet of small wood (3-4" stuff) that needs to be cut down to sizes small enough to be picked up curbside by city waste disposal.
Hopefully enough small wood to burn through the 2.5 more tanks of gas to finish out the break-in time period.
Then i get to tackle the actual reason for the purchase of the saw, a handful of 20"+ diameter norfolk island pines that need to come down.

Thank you all again for the insight and advice, I truly appreciate it

I wish i knew more than just the "chain brake was the issue". Obviously the clutch was engaged still for the chain brake to hold the crank/starter back.
I suspect logically speaking, the chain brake wasn't correctly functioning and burned your clutch up as a result, thus they warrantied the clutch as well due to the chain brake failure. which doesn't totally make sense because the chain brake was working somewhat as intended when trying to start the second day.
I just put a brake band in my neighbors 311 and there isn't much room in there for the brake to be out of round or it's wearing on the clutch drum.
I'd also offer a word of advice and make sure your oiler is turned all the way up. That 311 has an 18" bar and oils just enough. I can imagine a 25" bar is to the limits of this style saw's oiler.
Have fun with your new toy/tool.
 
Wood Doctor

Wood Doctor

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Most owner's manuals say you should engage the chain brake when starting the engine. I personally have never done that. However, I have replaced burned out chain brakes for saw operators who engage the brake every time when starting. Go figure.
 
cscltd

cscltd

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Most owner's manuals say you should engage the chain brake when starting the engine. I personally have never done that. However, I have replaced burned out chain brakes for saw operators who engage the brake every time when starting. Go figure.
liability and lawyers- its always the manufactures fault that a person with subpar IQ or doesn't read manual or even stand back and think for a second what could happen with a tool with a rotational cutting tool on it on "what could/will happen?"
I think thats why Husqvarna manual use to state not to put hands on saw blade/bar when starting cause someone did and it was their fault for not stating that in manual.
 
Stihl99

Stihl99

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Like it has been previously stated turn the oiler wide open, 1 or 1 assures you are oiling at optimum result.
I would have liked to have seen a photo of your clutch drum, brake band, and clutch, but hey it is fixed under warranty other than an additional trip to the dealer so that is most important.
 
BradSt

BradSt

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If the brake being on prevented you from pulling the cord, the clutch shoes were most likely stuck to the drum. I'd have to see the clutch to say for sure, but I'm betting something got hot there. Plastic cased clam shell saws do NOT like overheated clutches. It gets ugly quickly. Don't repeatedly cut at 75% throttle. The manual doesn't say don't run full throttle, it says no full throttle "off load" for 3 tanks, meaning no piss revving. Engine RPM is what engages the clutch. The higher the RPM, the more pressure on the clutch shoes, and the less likely they are to slip. Your saw doesn't need an elaborate break in. Cut whatever you want at this point. And do NOT rev the engine above low idle with the brake engaged.
 
effigy

effigy

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I wish i knew more than just the "chain brake was the issue". Obviously the clutch was engaged still for the chain brake to hold the crank/starter back.
I suspect logically speaking, the chain brake wasn't correctly functioning and burned your clutch up as a result, thus they warrantied the clutch as well due to the chain brake failure. which doesn't totally make sense because the chain brake was working somewhat as intended when trying to start the second day.
I just put a brake band in my neighbors 311 and there isn't much room in there for the brake to be out of round or it's wearing on the clutch drum.
I'd also offer a word of advice and make sure your oiler is turned all the way up. That 311 has an 18" bar and oils just enough. I can imagine a 25" bar is to the limits of this style saw's oiler.
Have fun with your new toy/tool.
I stopped back by the dealer a couple days ago, saw the mechanic as he was busy walking by and asked him again what was the problem was with the saw, he looked confused, i said 391 chain brake, he smiled and started talking as he continued walking away into the back of the shop but i distinctly heard him describe excessive vibration and a spring.
hope that little more bit of info helps.

If the brake being on prevented you from pulling the cord, the clutch shoes were most likely stuck to the drum. I'd have to see the clutch to say for sure, but I'm betting something got hot there. Plastic cased clam shell saws do NOT like overheated clutches. It gets ugly quickly. Don't repeatedly cut at 75% throttle. The manual doesn't say don't run full throttle, it says no full throttle "off load" for 3 tanks, meaning no piss revving. Engine RPM is what engages the clutch. The higher the RPM, the more pressure on the clutch shoes, and the less likely they are to slip. Your saw doesn't need an elaborate break in. Cut whatever you want at this point. And do NOT rev the engine above low idle with the brake engaged.
as I said, 75% off load, full speed on load, easing off as the bar comes out the other side, but since all the wood i was cutting was only 3-4" diameter (small but still too big to break by hand) each cut only took a couple seconds.
but that was last week and 2 tanks of fuel ago...



this week i've been cutting the large pines and the saw runs nice.
the only quirk i noticed is that immediately after starting it and turning off the brake the chain crawls while idle at first. however after i give the throttle a squeeze for a second or 2 then let it idle again the chain stops and stays stopped.
I also added the 3-Point Westcoast Saw Felling Dogs (bumper spikes) + Chain Catcher and immediately felt the benefits when making face cuts.
 
David Mayers

David Mayers

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Sep 5, 2015
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56
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Surrey
Recently i purchased a new Stihl MS 391 with the 25" bar, as well as the Stihl chain oil & premixed fuel from my local dealer.
On day 1 i flooded the saw on my first attempt to start it. Shortly there after I got it unflooded and spent the next hour cutting wood using about 75% throttle.
Once started and running it didn't give me any troubles and seemed to run well.
On day 2, (20 hours after day 1) I checked the bar oil (still mostly full) and fuel level (still 1/2 full) and when I went to start it pulling on the ripcord with the chain brake in the on position was impossible, it wouldn't budge, however, with the chain brake in the off position the ripcord would pull but the engine won't start...
when it comes to engines (in general) i understand how they function from a high-level perspective but im very inexperienced with them beyond that, having only owned & used a gasoline powered generator & gasoline powered pressure washer up until this point and my chainsaw needs were previously met by my battery powered DeWalt saw.

Ideas? suggestions? advice?

EDIT: I also hear what sounds like air pressure escaping when i attempted to start the saw on day 2.
Good day David here, I think that you have a possible compression leak from the crankcase, it could also be the fuel tank, if I may suggest try spraying some lube spray with the sprocket cover and starter covers removed, remove the spark plug and use a air line from a compressor with a cloth and a little air just to pressurise the cylinder/crankcase, I would suggest try the same with the petrol tank, look/listen for leaks, hope this helps regards David.
 
HarleyT

HarleyT

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If a mechanic is busy, they are likely to say almost anything to get away from a customer quickly.
P.M. me the serial number, so we can see what they filed under warranty, if they did at all.
 
RED-85-Z51

RED-85-Z51

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If a mechanic is busy, they are likely to say almost anything to get away from a customer quickly.
P.M. me the serial number, so we can see what they filed under warranty, if they did at all.
I can relate to that statement

Sent from my SM-G996U using Tapatalk
 
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