stihl 031 problems

Sounds like Idle/TAS/LA screw needs to be bumped up. Should be a lone screw on L side of saw - away from H-L screw pair. Turn it CW till chain barely doesn't spin. Make minor changes in steps - 1/8 turn, check 1/8 turn, check, etc.

I think Doug's on the right track with adjusting the idle speed. That's where I'd start, but the fact that it bogs and sometimes stalls on acceleration suggests the low-speed mixture screw might be set a bit lean as well. A typical carburetor has three adjustment screws. The one Doug is pointing you towards just limits how far the throttle butterfly can close when you let go of the trigger. Turning that screw clockwise will hold the throttle open wider when you're not squeezing the trigger, thus increasing the idle speed.

The other two screws, marked L and H, are needle valves that adjust the ratio of gas to air, i.e. the "mixture" screws. The one marked L, which will be the one closer to the engine cylinder, is the low-speed mixture adjustment, which affects idle and acceleration. The one marked H, which will be further from the engine, is the high-speed mixture screw, and affects how it runs when you're squeezing the trigger. Turning either of these counterclockwise will richen the mixture (i.e. make the fuel:air ratio higher), while turning them clockwise will reduce the amount of gas relative to the amount of air entering the engine.

I'd recommend you stay away from the H screw for now because if you adjust that one incorrectly then you can cause the saw to run "lean," meaning too much air and not enough gas, which can raise both engine speeds and combustion temperatures enough to do serious damage. It's fairly safe to mess with the low-speed mixture. The procedure I'd suggest would go like this:

1) Warm the saw up for a couple of minutes.

2) Turn the idle speed screw a turn or so clockwise, to raise the speed enough that the chain moves (slowly) even when you're not giving it any gas.

3) Slowly adjust the L screw while watching how fast the chain is moving. Try to get the chain moving as fast as possible only by adjusting that L screw. If it starts moving too fast to perceive differences, then back off the idle screw (from step 2) a bit. If the engine and carburetor are in good mechanical condition then this should put you pretty close to a correct low-speed adjustment.

4) Reduce the idle speed using the idle speed screw to the point where the chain just barely stops moving.

5) Check how smoothly the saw accelerates by squeezing the trigger rapidly, all at once. If it hesitates and bogs down, try richening the low-speed mixture by turning the L screw counterclockwise just a bit -- like 1/8 of a turn or so. Repeat, making small adjustments of the L screw until it accelerates well. Re-adjust the idle speed using the L-screw if necessary.

6) Set the saw on the ground and let it idle for 45 seconds or so, then pick it up and tip it nose-down. If it suddenly stalls, you've probably got the mixture a bit too rich. Turn that L screw clockwise in small increments, until the saw continues running smoothly when the nose is dropped after idling for a little while. Again, readjust the idle speed if necessary.


That's most of the process of tuning a saw. The last thing would be to adjust the high-speed screw, but like I said that part is risky, so I wouldn't recommend you try that until you're a little more practiced.

That doesn't sound good, unless you just ran out of gas. Hope you can restart it...

Excellent synopsis, Jon. I hope that last tidbit of low power isn't lean scoring.

dougand3 said:

Excellent synopsis, Jon. I hope that last tidbit of low power isn't lean scoring.

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Thanks. Me too.

iron said:

yikes. just read a bit about lean scoring. my saw was quite hot a few days ago. i've been noodling some pine rounds and it was running hot to the touch. it was also 90 degrees outside. i didn't think too much of it, but in hindsight, should've stopped when i noticed.

i let the saw cool down, cleaned out some gunk by the pull cord, and started just fine the next day.

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Ambient temperature shouldn't ever trigger lean scoring, so if something has gone really sideways then at least don't blame yourself.

Lots of good advice in good thread. So I won't ad any

I will say though if you get your 031 sorted out it's a great firewood saw. I've got one I believe model year 1976. It's slow cutting, low reving, but it doesn't bog even in bigger wood and gets the job done.

Parts are hard to come across though. If you come across parts saws for cheap may be good idea to pick one or 2 up if you plan on running an 031 as a everyday work saw. They are getting up there in age.

The 031 is one of the best small Stihls ever made, because it can truly cut big wood in a way it was probably never designed to do. It's not as fast as a 026/260 in smaller wood or limbing but it also doesn't seem to work as hard in big hard wood. Where a 026 starts to feel like your putting it to the test with a bar buried in oak, the 031 just seems to behave like a diesel in that it just stats steady and the rpms just don't move. It stays steady.

iron said:

well, i took the saw into the local stihl shop (i was mistaken, it's an 021 not an 031). he pulled the cord once and said the engine was shot. now i'm questioning if i didn't add oil to the fuel mix this last time??? dumb.

he suggested either a 026 or 036 for my next saw. i really don't think i need something huge or powerful, just enough for occasional noodling or chunking downed logs into smaller logs. i'll take suggestions here, but i realize i probably have a lot of reading ahead of me.

boo for inexperience and making mistakes.

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Sorry to hear that. It's possible you straight-gassed it, but if that doesn't seem likely then consider that lean scoring is pretty similar, and that would be the fault of the guy you bought it from. Given that you had problems every time you've used it, that seems pretty likely. If you want to explore that possibility, then remove the muffler and look at the damage. If it's lean scoring then the damage will be concentrated on the exhaust side of the cylinder and piston, so you'll see lots of scoring on the face of the piston (through the exhaust port) but little or no marking on the intake side of the cylinder when you drop the piston down and look at the far side of the cylinder bore. On the other hand, if you straight-gassed it then the damage will be fairly evenly distributed all the way around the bore, so you'll see scoring on both the face of the piston and the far cylinder wall.

The bright side is that you'll find either an 026 or 036, or anything in that class, much more capable than the 021 was.

And if you want to learn saw repair - you have a good specimen. An aftermarket top end (= cyl and piston kit) costs $22. It's an 025 size of 42.5 mm. A whole engine costs $35 shipped. Check ebay for parts.

Regarding the piston and cylinder damage and if you straight gassed it by accident.......do you still have gas in the container ?? Did the mechanic pour out some gas from the saw to check ? If I were wondering what caused a saw to fail or seize before I even pulled the muffler to look at the piston I'd pour some gas out to see if it had oil in it.

If it's pre mixed with oil it will have some color to it. Now the color can vary base on the brand of oil, but it will have a tint to it that's obvious as soon as you pour it into a clear glass jar. Could be dark green, could be light green, could be sky blue, and could even be red.

Regular gas is fairly clear and see through. Pre mix is not.

http://www.ebay.com/itm/SHORT-BLOCK-ENGINE-FITS-STIHL-02
1-023-025-MS210-MS230-MS250-ASSEMBLED-USA-/380892964325?hash=item58aefd65e5:g:4f4AAOxyBotTW5FN

http://www.ebay.com/itm/ENGINE-MOTO...002207?hash=item5afdcb17df:g:Wt0AAOSwd4tT0NSJ

The USA sellers list 021, 023, 025 fitment. The China sellers only list 023, 025. It's an 1123 series. I'm guessing China parts list omitted 021. But that needs more investigation. Maybe someone more familiar with the 1123 will chime in.

On rechecking, here is one that does list 021 fitment and has 42.5mm MS250 bore.

http://www.ebay.com/itm/42-5MM-Cyli...534324?hash=item1c66e78974:g:rqYAAOSwiYFXFaPn

iron said:

do you have a link you could send me for the whole engine? i'm a newb with this stuff, so any direction is appreciated. i see this link, but don't think it's the full engine: https://www.amazon.com/dp/B00HX6QVCU/?tag=hearthamazon-20

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If you go for the rebuild, PLEASE find out how to do a pressure/vacuum test on your saw BEFORE you tear it down to rebuild it.

It would be a shame (yet also a learning experience) to install a new cylinder/piston only to have the new one trashed by the same air/vacuum leak that might have nuked your last engine.