# Stihl 031 AV Chainsaw won't stay running



## lawnburner (Feb 15, 2006)

Hi Guys, I have a Stihl 031 AV chain saw that suddenly won't stay running. I put a new tank of freshly mixed gas (40:1), a new plug and cleaned the air filter, which really didn't need it. I thought it was the carb so I pulled it off and cleaned it and blew it out with compressed air. It looked perfect. Same thing.... starts and runs for a few seconds and shuts off. I took the carb apart again... no improvement. I pulled the muffler and checked for blockages... none. The piston and rings look great and show no scoring or buildup of carbon. I also put in a new fuel line and filter.

Thoroughly frustrated, I did something I rarely do, I took it to 2 repair shops in mid-CT and was told by both to take it home and bury it because parts were no longer available. I know it is 40+ years old but it still looks almost new and ran like a race car until it quit. I ordered a new Walbro carb to replace the Tillitson carb... same thing, starts and dies.

This thing ran like a bear and doesn't even look its age. Any suggestions before I give up on it would be greatly welcomed. Thanks, Lawnburner


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## lawnburner (Feb 15, 2006)

Uh oh, No replies. I guess it's time to buy a new suit for the funeral. Thanks anyways guys, Ron


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## geogrubb (Jul 28, 2006)

While the engine is running spray/dribble some carb/brake parts cleaner behind the carb and along all mating surfaces if the engine tempo changes you have an air leak and can take the appropriate action. Have a good one. Geo


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## 72chevynova (Dec 19, 2018)

Check the coil..air gap..ohm the coil..ive seen it all...The switch wire could be grounding out...Plug wire breaking down


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## 72chevynova (Dec 19, 2018)

What year is it??


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## crazy mike (Aug 26, 1999)

At 40 years old check any parts between the carb and tank for damage/swelling. MTBE and ethanol are death to plastic/rubber parts not designed for oxygenated modern gas.


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## finaprint (Jan 29, 2006)

X2, the popping off to run for a second says the ignition is working. 

People just do not grasp how rough modern fuel is on those dinky little carbs now. 

9 times out of 10 it is the flapper valves on the diaphragm, they get too stiff from exposure to fuel and then they don't meter right and the engine then dies. Why it runs at first, the flappers lose fuel ramp control over the next second and then engine dies when fuel ratio goes to crap. The actual constantly changing fuel metering method these cheap devices use is easily knocked out of whack and then they don't run. 

You got a main metering valve that meters enough fuel to just barely cover the rpm/load needs and the spring to it can be easily damaged by air pressure blowing parts off, then spring rate altered screws up the fuel map. The flappers on diaphragm must positively close and open or they lose fuel control as well. If the saw simply sits with fuel in the pumping chamber that commonly kills the flappers, they get too stiff and then don't work right.


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## lawnburner (Feb 15, 2006)

These are all great ideas to try guys, Thanks I just had my right knee replaced and can't even navigate the basement stairs to get the original paperwork or sales slip for the year of the saw but I haven't thrown the saw out yet either. I will gladly try everyone's ideas when I am mobile once again. You guys rock. Ron


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## niagara transom (Jan 28, 2019)

Hi. Don't throw out the saw. It's something stupid causing your issue. The dealers don't want to talk to you because they only SELL you saws. Parts are avail all over, I'm doing a 1963 Contra Lightning 1106 saw (090) and the parts are being produced. You did the carb, you checked the fuel hoses etc. Did you check tank venting? Loosen cap. I'm not familiar with your saw so I may be wrong. It would be a simple solution.
Next if you ohm out the coil and verify spark would it be possible to have a vacuum or pressure leak either carb base gaskets or crank seals? There is a tool to check this but you can make a plate and tap it with a vacuum t and bolt it where carb was. A bit of work but I think the tool is $150 on ebay. I actually need to make one myself probably for my post on a 510. You can pressurize and use soap as well.
If crank seals are shot make sure you do the bearings or you will be back at square 1.
Possibly stuck piston ring(s)???? Starts under low comp and then dies???
Good luck!


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## lawnburner (Feb 15, 2006)

Hi niagra transom, I never thought to check the vented cap. That's a great idea. I'm still recovering from a total knee replacement but I'll definitely check that out next, and thank you for your wisdom. Ron


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## lawnburner (Feb 15, 2006)

I tried to run it without the gas cap, and I think it stayed running 2 or 3 seconds longer than before but it still slows down and dies. A spritz of staring fluid and she fires immediately and goes wide open sounding very strong, but 5 seconds later it dies again.

Earlier I said I replaced the carb, fuel line, fuel filter, gas and spark plug. Today I re-installed the original carb and it made no difference. It starts up runs for 5 seconds and dies. It seems to start easier if the throttle is locked wide open and the choke is on full but still dies if you take it off choke or release throttle.

I SWEAR it's a fuel delivery problem but I'll be darned if I know what to look at next. I've cleaned the old carb, installed a new carb, replaced the fuel, fuel line, fuel filter, checked for leaks.........?????


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## geogrubb (Jul 28, 2006)

Is it possible the carb gasket is blocking the pulse port and not letting the diaphragm pump. Have a good one. Geo


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## finaprint (Jan 29, 2006)

'A spritz of staring fluid and she fires immediately and goes wide open sounding very strong, but 5 seconds later it dies again.'

Give a longer burst of fluid and it will run longer. 

Fuel delivery problem. 

Careful with starting fluid, it has no lube in it. 

A NEW carb may still be years old from sitting on shelf...............meaning the valve flaps in it may be hard too. Those flaps MUST be very flexible and then after that you have to get daring to adjust the small needle valve in there to pump more fuel. That needle provides in effect the same fuel volume (float level) you find inside a car carb, you can't feed fuel through jets if you don't have fuel there to feed. The flappers feed a certain amount of fuel based on rpm and the needle has to cover that but no more, but it can not cover it too, carb works fine but cannot get enough fuel to stay running because the needle does not open enough. Why it only runs for a second or two. 

They give you gauges that supposedly set that needle but the setting is only a gross one and sometimes off far enough to make the engine either flood or do what is happening here. I've seen .003" mess that up and no way are those gauges that close in accuracy.

All that of course assumes things like saw in good condition with no leaks in carb. I go after the flaps first as not closing 100% is what allows the pumping of fuel to die off. Then once that is good I go for needle next and good luck. I have taken many pieces of equipment that did that and they run perfectly when done, but getting that needle right can be a pain.


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## finaprint (Jan 29, 2006)

BTW, with a problem like this make sure the hi and low needles are always cracked open some to at least run, if off of course that is an issue too. I start at 1 1/2 turns each.


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## DougM (Feb 2, 2021)

lawnburner said:


> Uh oh, No replies. I guess it's time to buy a new suit for the funeral. Thanks anyways guys, Ron


I have an 031 that I got used 20 years ago. It has always been a great saw, and this year it did the same thing. While I was using it, it bogged down and quit. If I turn the choke on it will start, but when I push the choke off, it will quit. Have not taken it into a shop yet. I found that any stihl dealer will not work on the old saw because they can't get stihl parts any more. I took it to a handy man shop before and he did not have a problem finding and off brand part to fix it with.
The one that really pissed me off was when I took my big 076 stihl to a stihl dealer to get fixed, he gave it back to me in a box full of pieces. Said he could not get parts for it. I thought it odd that he couldn't figure that out before he tore it apart.


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## paulr44 (Oct 14, 2008)

It is sad, as I liked the 031 but yeah, most parts are NLA. More than likely the carb. is gunked up, but I never forego checking fuel (for water/debris), filter and supply hose first. I've seen water laden filters, split supply hoses, and plenty of water and sh*t in the fuel. Also, an inspection of the impulse line and manifold is often a good idea - you said it died while using it. I've seen saws get pinched and users get rough with the handles and the impulse line get pulled off. That line is what makes the fuel pump in the carb. work.
Sometimes, albeit rare, the inlet screen in the carb. gets clogged up with sawdust.
The carb. should be a Walbro (also NLA), but a kit would still be available for it. The killer I saw a few times on that model was the oil tank vent is unique, and NLA.


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## Tuffelhunden (Jun 9, 2021)

lawnburner said:


> Hi Guys, I have a Stihl 031 AV chain saw that suddenly won't stay running. I put a new tank of freshly mixed gas (40:1), a new plug and cleaned the air filter, which really didn't need it. I thought it was the carb so I pulled it off and cleaned it and blew it out with compressed air. It looked perfect. Same thing.... starts and runs for a few seconds and shuts off. I took the carb apart again... no improvement. I pulled the muffler and checked for blockages... none. The piston and rings look great and show no scoring or buildup of carbon. I also put in a new fuel line and filter.
> 
> Thoroughly frustrated, I did something I rarely do, I took it to 2 repair shops in mid-CT and was told by both to take it home and bury it because parts were no longer available. I know it is 40+ years old but it still looks almost new and ran like a race car until it quit. I ordered a new Walbro carb to replace the Tillitson carb... same thing, starts and dies.
> 
> This thing ran like a bear and doesn't even look its age. Any suggestions before I give up on it would be greatly welcomed. Thanks, Lawnburner


I just stumbled on this post. I’ll add my two cents. I just finished a complete rebuild on an 031 and 041. My 031 did the exact same thing. After tearing into it, in a rather anger driven and swearing feat. I found the fuel line got pinched. I’m not sure how, but pulled it around with a long skinny pick, shortened it, and it’s up and ripping again. Hope this helps! The older the better when it comes to most mechanical stuff!


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