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  1. #11
    New Member
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    Oct 2011
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    13
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    Default Re: Stihl 031AV will not rev up, idle is fine

    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!

  2. #12
    Platinum Member
    Join Date
    Oct 2010
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    588
    Tractor
    New Holland TC 29D

    Default Re: Stihl 031AV will not rev up, idle is fine

    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
    TC-29DA
    BX-1500

  3. #13
    Gold Member
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    Mar 2010
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    344
    Location
    South PA/Western MD

    Default Re: Stihl 031AV will not rev up, idle is fine

    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.
    Last edited by 955Lincoln; 10-25-2011 at 03:46 PM.

  4. #14
    Elite Member
    Join Date
    Dec 2002
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    2,806
    Location
    Foster, RI
    Tractor
    Mahindra 3016

    Default Re: Stihl 031AV will not rev up, idle is fine

    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
    Last edited by arrow; 10-28-2011 at 10:18 AM.

  5. #15
    New Member
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    Dec 2010
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    7
    Location
    Dime Box, TX
    Tractor
    JD 6410, JD 332D w/mulcher, MF 261

    Default Re: Stihl 031AV will not rev up, idle is fine

    Did you check the fuel tank vent?

  6. #16
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    Oct 2011
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    Default Re: Stihl 031AV will not rev up, idle is fine

    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.
    Last edited by JimLev; 11-20-2011 at 08:18 AM. Reason: Added more info

  7. #17
    Veteran Member
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    Mar 2007
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    1,306
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    Midwest
    Tractor
    Iseki TA 247. Husqvarna 54", Cub Zero Turn.Polaris 850 ATV's Stihl 018 026 Pro, 029, 034 Super AV, 039, 044 Mag. Stihl Pole Saw.

    Default Re: Stihl 031AV will not rev up, idle is fine

    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 ?

  8. #18
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    Oct 2011
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    Default Re: Stihl 031AV will not rev up, idle is fine

    Quote Originally Posted by yomax4 View Post
    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.
    Last edited by JimLev; 11-21-2011 at 11:59 PM. Reason: correct spelling

  9. #19
    Elite Member
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    Dec 2002
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    2,806
    Location
    Foster, RI
    Tractor
    Mahindra 3016

    Default Re: Stihl 031AV will not rev up, idle is fine

    Quote Originally Posted by JimLev View Post
    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.

  10. #20
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    Default Re: Stihl 031AV will not rev up, idle is fine

    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.

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