Stihl 031 fuel pump vacuum problem,..

[perry]

Ahoy, I'm pretty sure I have a problem with the vacuum that runs the fuel pump. Can anyone explain how this works inside the crankcase?
I bought the saw used and cut 2 chords of wood. It got hard to start and then it would only start if you poured gas down the carb, then it would idle but would load down and die if you tried to cut anything.
I took it for repair and they said it was the vacuum from the crankcase. They put in a piston and rings etc. and it ran like a champ, cutting, well about two chords of wood. Then I put it away for the winter and when I started it- same exact symptoms. I took it apart and all the lines are free- gas line to carb, line to crankcase. I can suck on the crankcase hose (yum) and just barely get it to pass air. I haven't checked the compression but it feels real tight if you pull the rope.
That's all I can think of for now. Thanks and happy cutting.

 

[taplinhill]

For a 2 stroke to work properly, the crank seals have to be tight. That is what allows for the vaccuum. It sounds like this is your problem. I would think they would have been replaced when they did the top end, but who knows. If they were replaced, they shouldn't have failed that quickly.

 

[Lakeside53]

Take off the carb, and put a small blob of grease on the impulse hose. Stick you thumb in the carb inlet hole (to block it) and have someone pull the saw over. The grease will suck in if you have crankcase vac.


It might be seals, but it also might be the usual suspects - fuel filters, fuel hoses, impulse hose, carb inlet screen etc. I could even be ignition...

 

[perry]

Take off the carb, and put a small blob of grease on the impulse hose. Stick your thumb in the carb inlet hole (to block it) and have someone pull the saw over. The grease will suck in if you have crankcase vac.

then perry says,..
Ok, I removed the black rubber boot behind the carb (intact- no leaks or cracks) and put some grease on the tiny orange/brown tube that goes into the case- I'm guessing that's the impuse tube- plugged the fuel mix inlet with my thumb and the grease sucked in so I'm kind of lost here. That means the seals are ok, right?
I checked my compression and it seems to be 175. I can hardly pull the rope without pulling the saw out from under my foot. It's mostly disassembled so it's kind of hard to hold on to.

It might be seals, but it also might be the usual suspects - fuel filters, fuel hoses, impulse hose, carb inlet screen etc. I could even be ignition...

and perry further remarks,..
I checked the foam filter inside the pickup in the fuel tank, it was intact but kind of discolored so I cleaned it in gas. The whole assembly from the tank to the carb sucks a little easier now but you still have to suck pretty hard and it doesn't seem to have a free air flow- I mean it's not easy.
The carb is a Walbro 21-80. Any idea what to do next?
Thanks for your help.
Great forum, hope I get the hang of posting soon.

 

[STIHL-KID]

Ah, the 031.......what a great saw! Well sounds like your in a little bit of a pickle. Basically it all has to do with negetive pressive (vacuum) for the fuel to deliver properly. I would do the following:

1. Pull off the impulse line and make sure it is absolutely clear, also carefully inspect the outer part of the line for cracks or damage.

2. If your in doubt about when your last fuel tank filter was changed, just get a new one and start fresh.

3. Inspect the fuel line from the tank to the carb for cracks or damage

Once you check these items and have varified they are good.....then you might want to dig deeper into it. I wish you good luck on your 031, they are cool saws!

 

[perry]

I checked all the lines in the fuel circuit and they seem restricted but they pass some air if you suck hard. No cracks or holes that I can find. What's weird is that if I put the gas cap on and suck on the feed to the carb I get nothing. If I remove the tiny set screw that's horizontally situated in the gas cap I can get some air flowing through but not much. If I take the cap off the tank I get lots of air.
Also, any exploded diagrams or info on the Walbro carb for an 031? Mine says 21-80 if I'm reading it right.
Thanks, guys.

 

[Lakeside53]

I wouldn't wory about the gas cap until you get it runnng. Most 031 leak like a sieve out of the cap... You might actually be working fine - it only has to make up a small amount of air.

The carb is WA2. You'll need a K10-WAT rebuild kit.

Here's an exploded view.

 

[perry]

Well, I rebuilt the carb using your suggested model kit. The only thing I could see wrong was the round screen which was black and pretty plugged.
Also, part number 27 in your exploded view has a vent hole in it. Does the orientation of the vent matter? Is it suppossed to be at the top?
I set the carb H/L for standard settings and pulled the chord but no joy. After putting a little gas down the spark plug hole it started and died a few times then ran, cut and idled ok even though it smoked more than would be ideal.
It seemded down on power when cutting so I started adjusting the L speed and improved it slightly but then it had trouble starting.
After the saw warmed up good, about 20 minutes or so of unspectacular cutting with a sharp chain in soft wood, it wouldn't idle properly then started to dog down in the cut, developed a low speed stumble and finally I couldn't start it for anything. This is about the same performance and sequence of events I've gotten since I bought this thing. If anyone has any ideas, I'd certainly be open to suggestion at this point. Thanks again.

 

[JohnO]

Have you tried a vacuum check? I have an 031 with similar symptoms, and found a huge leak in the crank seal. I couldn't even pull a vacuum. Anyway, it's an easy check for you to try out. Just put some gasket material under the muffler and carb, and pull a vacuum at the impulse hose fitting.

-John

 

[Bowtie]

Well, I rebuilt the carb using your suggested model kit. The only thing I could see wrong was the round screen which was black and pretty plugged.
Also, part number 27 in your exploded view has a vent hole in it. Does the orientation of the vent matter? Is it suppossed to be at the top?
I set the carb H/L for standard settings and pulled the chord but no joy. After putting a little gas down the spark plug hole it started and died a few times then ran, cut and idled ok even though it smoked more than would be ideal.
It seemded down on power when cutting so I started adjusting the L speed and improved it slightly but then it had trouble starting.
After the saw warmed up good, about 20 minutes or so of unspectacular cutting with a sharp chain in soft wood, it wouldn't idle properly then started to dog down in the cut, developed a low speed stumble and finally I couldn't start it for anything. This is about the same performance and sequence of events I've gotten since I bought this thing. If anyone has any ideas, I'd certainly be open to suggestion at this point. Thanks again.

My opinion only, you need to check it for an air leak. I have 2 saws now with those symptoms, and one I know for a fact has an air leak. Do a search on here for air pressure testers. You can make your own for little money. Good tool to have for chainsaws.