Stihl 031AV will not rev up, idle is fine

[JimLev]

I've got an old Stihl 031AV that I've had for years, never had a problem with it until a few weeks ago. It was running like a champ one day, the next day it started fine but won't speed up, sounds like it's bogging down.
I replaced the plug, cleaned the air filter and (Walbro) carb, didn't help.
I pulled the carb off and got a rebuild kit for it. Didn't help, still won't speed up.
I've set the low and high speed idle screws to the manual.
All of the passages in the carb are clean and not plugged.
The local saw shop didn't know what was wrong with it, but they wanted to sell me a new carb for $90 even though I don't think the carb is the problem.
It's got a good spark and compression, almost no carbon on the piston or top of the head.
Anybody got an idea of what the problem could be? I'm stumped.
Thanks in advance.

 

[steveky]

Check your muffler, had a blower that did the same thing...hope this helps.

 

[Furu]

I agree, look at the muffler but specifically the spark arresters in the muffler output. They will/can coke up and give the symptoms you describe. There is a real easy fix if that is it. Let me know and I will describe it later when I am on the computer instead of the smartphone.

 

[North West Farmer]

Jim Lev,

Any saw, especially an older saw is very subject to problems with the fuel intake line and/or the fuel filter.

An older saw has had some time to accumulate foreign material in the fuel filter, thus reducing fuel flow.

In this case I would be very suspect of the fuel line, especially if you have been using gas with alcohol content and not draining the fuel tank when being stored or running the fuel out of the fuel line(s). If the fuel line has been subjected to the modern doctored fuels with the additives and all it is not at all unusual for the fuel line to get soft or spongy so when you throttle up the line collapses and blocks fuel flow. If the filter is even partially plugged it makes the fuel line collapse even more likely.

On an older saw also check for pin holes in the fuel line. I have had saws give me fits when the fuel line somehow developed a pin hole and until I happened to see some air bubbles through the clear fuel line material I didn't have a clue. From that time on, that is one of the early items I check for.

You describe good things on the ignition side and the spark arrester being plugged can definitely prevent normal fuel air flow and present with the symptoms you describe. After checking the spark arrester, as suggested, give the fuel line(s) and fuel filter a real close examination.

Good luck. Older saws commonly suffer fuel supply maladies.

Nick, North West Farmer

 

[20 20]

This is why I open up the exhaust, and I agree check the intake fuel line

 

[JimLev]

Thanks for all the quick replys.
I should have mentioned that I had pulled the muffler and it is clear, it doesn't have a spark arrestor on it. I did blow thru it and it was clear.
Just for the heck of it, I'll pull the muffler off and see if it makes a difference.
I did inspect the fuel lines and filter, they also look fine.

Any other suggestions are appreciated, winter is coming and I have wood that needs to be cut.
Thanks

 

[RobertN]

It may be possible that the carb is still clogged; the orfices are really tiny, and hard to clean very well.

Also, is the vent clear? Possible it is getting a vacuum in the tank and not getting enough gas? Or a soft gas line that is collapsing?

The diaphragm on the carb is good? The vent hole on the bottom of the carb clean and clear? That would prevent the diaphragm from pumping enough gas...

You might post this question on aboristsite too...

 

[aczlan]

Our farmboss had similar symptoms last month. It had a bad fuel line and a dirty fuel filter.

Aaron Z

 

[Dennis Mickle]

Our farmboss had similar symptoms last month. It had a bad fuel line and a dirty fuel filter.

Aaron Z

I have an 031 it was doing the same thing it acted like you turned the choke on you tried to throttle up. I checked everything turned out it was the air filter was clogged even though I had cleaned it with a brush and a little air pressure. I put a new one on and the saw ran fine.

 

[aczlan]

I have an 031 it was doing the same thing it acted like you turned the choke on you tried to throttle up. I checked everything turned out it was the air filter was clogged even though I had cleaned it with a brush and a little air pressure. I put a new one on and the saw ran fine.

I thought of that and tried running ours without an air filter to check, but no dice.

Aaron Z

 

[JimLev]

Yes, I did try it without the air filter on, didn't make a difference, but I did see a little mist of gas being pushed out the air intake end of the carb when I tried to speed the engine up. It did the same thing before I rebuilt the carb.
The vent hole is open and I ran a fine wire thru all of the internal carb passages, blew carb cleaner thru them too so I know they are not plugged.
I tried three different needle valve levels just to make sure that wasn't the problem.
I pulled the head cylinder off to take a look at the rings and piston, they look fine. After putting it back together it started on the first pull but won't speed up. Took the muffler off, same thing.
I'm still stumped why this won't run!

 

[V1Rotate]

Two stroke engines require a certain amout of "crankcase compression" which actually provides a small amount of pressure and vacuum to the carburetor. The carb should see mostly a vacuum in order to draw the air-fuel mix into the engine. If the seals at the ends of the crankshaft are leaking, you will have less crankcase compression, so you might want to check that.
Also, the diaphragm fuel pumps on many engines require pressure pulses from the crankcase, if those pulses are weak, the pump may not work properly

 

[955Lincoln]

When you say bogging down, is it sort of a missing type condition where the saw sounds like it wants to take off but just falls flat on its face slightly above idle speed?

If you have a point ignition my guess is a bad condenser. Don't bother testing the condenser because it will test ok, just replace it. And the points also while you are in there, if you can find these items.

 

[arrow]

You received this answer before. . There are microscopic cracks in the fuel lines that you cannot see. Change them out anyway even if you think they are ok..Also, do not forget to replace the rubber line connecting the fuel components to the crank case. My next step would be to change points and condenser or better yet, do a module conversion. If that fails, you may have a sloppy union between bottom ring and piston, replace the rings. You may need a new carb but that would not be my first move.. Do the line changes first as they are the easiest. Make sure all lines are not crimped

 

[bakjr]

Did you check the fuel tank vent?

 

[JimLev]

Sorry for the long delay, my computer crashed last month. Finally back up and working.
After taking everything apart again and checking everything all of you have told me, it still will not rev up.
I can find nothing wrong with the carb but still think it is a carb problem.
I do have pulses coming from the crankcase to the carb.
One interesting thing I noticed when I had the the muffler off (with the engine idling) is there seems to be a lot of raw gas spitting out of the exhaust port on the head. I also see fuel spitting out of the carb air intake (air filter off) when I try to rev it up. Seems like it is getting too much fuel.
I've tried lowering the adjustment for the needle valve, didn't help.
I pulled the head off, the piston and rings look fine, as do the cylinder walls.
I'm at the point where I maybe should just spend the ~$85 for a new carb.
I looked at new saws a few weeks ago, but with Christmas coming I can't justify spending $500+ for a good saw after I just had a new roof put on our house.

995Lincoln, it has electronic ignition, I haven't pulled the flywheel off yet. Wonder if I have an ignition problem and the spark is coming late?
Looking at the spark with the plug out, the spark looks strong, the engine starts and idles fine.
Once in a great while it will rev up the max speed but doesn't stay there for long.

 

[yomax4]

Usually, ( on snowmobile engines ) if it's the crank seals, It will run wide open but not idle. Has to be diaphram/needle and seat rocker or hoses ?

 

[JimLev]

Usually, ( on snowmobile engines ) if it's the crank seals, It will run wide open but not idle. Has to be diaphram/needle and seat rocker or hoses ?

I'll pull it apart one more time and look at everything again.
What's a "seat rocker"?
I did buy a rebuild kit for the carb, I replaced all of the gaskets, the diaphram, the needle valve, the pin, and the lever that operates the needle.

 

[arrow]

Sorry for the long delay, my computer crashed last month. Finally back up and working.
After taking everything apart again and checking everything all of you have told me, it still will not rev up.
I can find nothing wrong with the carb but still think it is a carb problem.
I do have pulses coming from the crankcase to the carb.
One interesting thing I noticed when I had the the muffler off (with the engine idling) is there seems to be a lot of raw gas spitting out of the exhaust port on the head. I also see fuel spitting out of the carb air intake (air filter off) when I try to rev it up. Seems like it is getting too much fuel.
I've tried lowering the adjustment for the needle valve, didn't help.
I pulled the head off, the piston and rings look fine, as do the cylinder walls.
I'm at the point where I maybe should just spend the ~$85 for a new carb.
I looked at new saws a few weeks ago, but with Christmas coming I can't justify spending $500+ for a good saw after I just had a new roof put on our house.

995Lincoln, it has electronic ignition, I haven't pulled the flywheel off yet. Wonder if I have an ignition problem and the spark is coming late?
Looking at the spark with the plug out, the spark looks strong, the engine starts and idles fine.
Once in a great while it will rev up the max speed but doesn't stay there for long.

With all due respect, it seems to me you are doing a lot of looking. What is the compression of your saw? 120-130? Then I'd say your rings are fine. A reading of 70 can cause your problems. If you saw no score lines within the piston wall, that is one less worry. Theses saws actually spit some gas out of the carb anyway when they get long of tooth. Coming out the exhaust is another matter. Your strong spark should be blue-white and quite evident, if so, it is a fuel delivery problem. Did you change out the choke spring so the choke does not stay shut? You rebuilt the carb so I will take for granted all settings like needle seat are proper. Many carbs I rebuilt on small engines were always hit or miss for me. I always buy carbs now but before you do, check the compression reading. Chainsaw timing does not get whacked out as much as say a lawnmower that hits a rock. It happens but is a rare occurrence in my experience.

 

[JimLev]

Yup, have been doing a lot of looking after I've had the carb apart 4 times.
Don't know what the compression is but I can check it after I get home from Thanksgiving travels.
There were no score marks on the walls or piston.
The wierd thing is it worked fine one day and then a week later it didn't.
The spark was blue/white.
There is no spring on the choke, it has a lever that just blocks off the intake to the carb.
The low and high speed adjustments were set to the book.
If the compression checks out I'll order a new carb.
Thanks.