Saw air tight and bogs

[Bedoug]

Hey All, I’m looking for some help. I have a Stihl 031AV that I have torn down to the case (without splitting it). I have checked each line, rebuilt the carb and cleaned all the ports and jets with torch tips, replaced the clutch side seal (now holding vacuum with no visible loss), have put in a new plug, gas filter and just about anything else for a tuneup. I replaced the piston and rings (aftermarket) and it fires up but bogs after 20 minutes or so. What would cause this on a saw that should, reasonably, work fine. Should I get OEM piston rings? Is it likely a carb tuning issue? Any help would be appreciated. Thanks, Bedoug

 

[Andyshine77]

Check compression.

 

[Bedoug]

Check compression.

Thanks for the response. If compression is low does that suggest I need OEM rings, or something else?

 

[Brent Nowell]

Is your metering diaphragm compatible with your need lever height? Some have a divot that screws with the lever arm which screws with fuel delivery.

Coils can cause running issues that are very similar to carb issues. Sometimes it will show spark but it’s a weak spark, a three point tester will show spark strength. Sometimes too coils are bad only when hot... they really can be frustrating to diagnose.

 

[Bedoug]

Is your metering diaphragm compatible with your need lever height? Some have a divot that screws with the lever arm which screws with fuel delivery.

Coils can cause running issues that are very similar to carb issues. Sometimes it will show spark but it’s a weak spark, a three point tester will show spark strength. Sometimes too coils are bad only when hot... they really can be frustrating to diagnose.

Brent, thanks. Everything seemed to be level and okay with the carb. I took it apart today to double check. The one thing I didn’t like was that float lever and needle seemed a bit stiff, not enough ease in the play up and down. I loosened the set screw a bit and it seemed okay after that. The coil does put out good, continuous spark. I have also put in a electronic chip. The coil is old though so it could be a cause. I’m wondering if I’m running a bit lean on the high side.

 

[Old2stroke]

Most obvious thing to check is the fuel tank vent. When it bogs, just loosen the tank cap to let some air in and see if it cures it.

 

[HarleyT]

Look closely at the fuel line dangling in the tank, after 40 years, it can become soft, gummy, and weak, and can collapse and pinch shut while running, sometimes intermittantly.

 

[Bedoug]

Most obvious thing to check is the fuel tank vent. When it bogs, just loosen the tank cap to let some air in and see if it cures it.

Old2stroke, thanks. Weirdly, that seems obvious, but I’ve forgotten to do this.

Look closely at the fuel line dangling in the tank, after 40 years, it can become soft, gummy, and weak, and can collapse and pinch shut while running, sometimes intermittantly.

Most obvious thing to check is the fuel tank vent. When it bogs, just loosen the tank cap to let some air in and see if it cures it.

Most obvious thing to check is the fuel tank vent. When it bogs, just loosen the tank cap to let some air in and see if it cures it.

Look closely at the fuel line dangling in the tank, after 40 years, it can become soft, gummy, and weak, and can collapse and pinch shut while running, sometimes intermittantly.

HarleyT, thanks for the tip. I was thinking about the fuel line. It is incredibly soft and flexible, so I haven’t really considered it a problem, as dry and brittle are things I look for. I’ll take a closer look at the line. Would you advise changing it regardless of tears based simply on its age?

 

[HarleyT]

The new line is fairly stout, so if your line feels limp and wimpy, I would change it. Imagine if it was a drinking straw, and you tried to take a sip, it would likely suck together and pinch off. If it feels a little gummy too, definitely replace it.

 

[HarleyT]

Looks like it is still available from Stihl.

 

[Andyshine77]

Thanks for the response. If compression is low does that suggest I need OEM rings, or something else?

All good suggestions above as well, I assumed if you went through the carb you would have replaced the fuel line. Tank vent is definitely something to check.

Now I would still check compression. If it's low you will need to take the muffler off and inspect the piston and cylinder through the exhaust port.[emoji111]

 

[Bedoug]

All good suggestions above as well, I assumed if you went through the carb you would have replaced the fuel line. Tank vent is definitely something to check.

Now I would still check compression. If it's low you will need to take the muffler off and inspect the piston and cylinder through the exhaust port.[emoji111]

Thanks, I’ll give the lines a look, but the piston and rings are brand new.

 

[Bedoug]

The new line is fairly stout, so if your line feels limp and wimpy, I would change it. Imagine if it was a drinking straw, and you tried to take a sip, it would likely suck together and pinch off. If it feels a little gummy too, definitely replace it.

HarleyT, thanks. The fuel line inside the tank feels like like a well-stretched balloon-extremely pliable, so maybe, based on your suggestion, I’ll start there.

 

[Bedoug]

All good suggestions above as well, I assumed if you went through the carb you would have replaced the fuel line. Tank vent is definitely something to check.

Now I would still check compression. If it's low you will need to take the muffler off and inspect the piston and cylinder through the exhaust port.[emoji111]

Andyshine77, when you say “tank vent” are you referring to the vent built into the gas cap, or the connector at the top of the tank that leads down to the tank and out to the carb? Thanks

 

[Andyshine77]

The cap. Empty the fuel tank, remove the fuel filter. Remove the fuel line from the carb and drain the fuel from the line. Put the fuel cap on and hook your vacuum pump up the the fuel line at the carb end, if it doesn't bleed down immediately the vent isn't working properly.

 

[Bedoug]

The cap. Empty the fuel tank, remove the fuel filter. Remove the fuel line from the carb and drain the fuel from the line. Bit the fuel cap on and hook your vacuum pump up the the fuel line at the carb end, if it doesn't bleed down immediately the vent isn't working properly.

Thanks, I’ll give that a go.

 

[Bedoug]

Thanks, I’ll give that a go.

It didn’t even hold vacuum for a little bit, should it have?

Also, when I was playing with the trigger, gasoline poured through the carb like it was the Jamestown flood. I suppose that’s pointing to a metering issue. I took the carb apart and saw the needle spring was folded over. I fixed and reassembled. Should the carb hold vacuum from both sides. It only was holding from the input line side, not the fuel line side. Thanks.

 

[lone wolf]

The new line is fairly stout, so if your line feels limp and wimpy, I would change it. Imagine if it was a drinking straw, and you tried to take a sip, it would likely suck together and pinch off. If it feels a little gummy too, definitely replace it.

Still talking about saws?

 

[lone wolf]

It didn’t even hold vacuum for a little bit, should it have?

Also, when I was playing with the trigger, gasoline poured through the carb like it was the Jamestown flood. I suppose that’s pointing to a metering issue. I took the carb apart and saw the needle spring was folded over. I fixed and reassembled. Should the carb hold vacuum from both sides. It only was holding from the input line side, not the fuel line side. Thanks.

If it keeps pissing you off just buy a new carb.

 

[Brent Nowell]

It didn’t even hold vacuum for a little bit, should it have?

Also, when I was playing with the trigger, gasoline poured through the carb like it was the Jamestown flood. I suppose that’s pointing to a metering issue. I took the carb apart and saw the needle spring was folded over. I fixed and reassembled. Should the carb hold vacuum from both sides. It only was holding from the input line side, not the fuel line side. Thanks.

The vent is working properly, it is supposed to allow air in to displace fuel used in order to prevent a vacuum from forming when the engine is running using fuel.
There are great 2 stroke carb tests on YouTube by walbro that show proper testing procedures.

Basically the pressure test with a mighty vac will indicate that the needle is seating correctly. It should also hold a vacuum indicating that the check valves for the pump area are sealing correctly.
To test the main check on the portion that goes from the metering area to the Venturi you will need to use a hose and your mouth videos are online to show this.

Make sure that the carb is wet with fuel when you do this, a dried out carb will fail every time. The seats and valves function properly only when wet with fuel.

Again these tests only indicate the above is working properly, they will not tell you if the metering arm is set correctly, or if the metering diaphragm is too stiff to work fast enough for fuel delivery.

*edit*
Btw don’t go past 10psi . Unless a service manual actually says otherwise for pop off tests etc
I’m not an expert on this at all, I just learned about these things 2 weeks ago, so I’m not sure how it damages the carb exceeding pressures.. I can imagine that you could blow out a gasket maybe?