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