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Thread: Old Stihl FS80 Trimmer-I'm Exasperated!!!

  1. #1
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    Default Old Stihl FS80 Trimmer-I'm Exasperated!!!

    I have an old style Stihl FS 80 Brushcutter/Trimmer that is about 30 years old.

    I really, really like this machine.
    However, I have had a lot of trouble getting and keeping it running. Parts are hard to come by and the dealers only want to sell you a new one.

    Anyhow, I have a lot of new parts and probably put way more into it than I should.
    Also, I am less than mechanically clever.

    I have a brand new carb, intake, filter and lots of other stuff.
    I have gotten it further than all the dealers I have taken it to.
    I just can't get it to run right.
    It starts easily and will idle all day.
    However, I can't get it to run full throttle. I have messed with the carb screws over and over

    Am I looking in the wrong place?
    In battle, Free Men will almost always defeat slaves
    Herodotus after the Battle of Marathon

  2. #2

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    I suppose you have already de-carboned the exhaust port/muffler/spark arrestor?

    Hows the fuel line from the tank?

    Isa 8:16,20 Bind up the witness, seal the Torah among my taught ones...To the Torah and to the testimony! If they do not speak according to this Word, it is because they have no light in them!

  3. #3

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    Do you get more RPM or performance with it choked? If yes, its likely starving for fuel. If it has a brass screen or pickup filter they can get a buildup that will let enough fuel through for idle but not high speed.

  4. #4
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    I asked my son who owns a landscaping company. He has a lot of Stihl equipment. This is his answer, don't know that it will help since it sounds like you already covered it.

    I'd say check the fuel filter; usually in the tank. The carb could use cleaning I'm sure. The spark plug could be the problem. Another thing to check is the exhaust screen, when they clog with carbon they tend to run bad. Hope this helps..

  5. #5
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    Thanks everybody.
    Muffler is completely cleaned. Not sure what the spark arrestor is
    New fuel lines from tank to primer and primer to carb.
    But, I can see an occasional air bubble in the line.
    Maybe sucking a little air?

    It runs best with the choke open.

    Fuel filter is "brand new". I put that in quotes because I replaced it last fall/winter and I don't have an hour's running time through it

    I say that because I ran the original fuel filter for at least 10 years without replacing it.

    All gas is now ethanol, though.
    I don't have a problem with any of my other older equipment.
    New gas with a new mixture.

    I may put a little Sea Foam in the tank.
    Can't hurt.
    In battle, Free Men will almost always defeat slaves
    Herodotus after the Battle of Marathon

  6. #6

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    The spark arrestor is the fine mesh screen within the exhaust. They can really clog.

    Also speculating the gasket on the carb mount...you may even have a gasket/manifold combination. There is a vacuum hole that needs to be lined up and not impaired. May wanna check that too and be sure it's all mated correctly.

    Another thought. On your throttle (top of the carb that unscrews), you have a slide that goes into the carburetor that has a needle valve usually 'U" clipped onto it. I've seen those come off an exhibit the problem you describe.

    Isa 8:16,20 Bind up the witness, seal the Torah among my taught ones...To the Torah and to the testimony! If they do not speak according to this Word, it is because they have no light in them!

  7. #7
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    Thanks, Al.
    I think the screen is clean.

    When I put the new manifold and gasket on, I checked to be sure that was lined up (or at least I knew to do it)

    What do you think about an air leak somewhere?

    I do see a few air bubbles in the carb side of the primer while it's running.
    Maybe the new hoses need heated so they shrink better around the fittings?
    My 22 yo son is doing some of the work and he sometimes may want to cut a corner or two
    In battle, Free Men will almost always defeat slaves
    Herodotus after the Battle of Marathon

  8. #8

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    Quote Originally Posted by Davy Crockett View Post
    Thanks, Al.
    I think the screen is clean.

    When I put the new manifold and gasket on, I checked to be sure that was lined up (or at least I knew to do it)

    What do you think about an air leak somewhere?

    I do see a few air bubbles in the carb side of the primer while it's running.
    Maybe the new hoses need heated so they shrink better around the fittings?
    My 22 yo son is doing some of the work and he sometimes may want to cut a corner or two
    Fine wire makes good clamps for small fuel lines. Have you looked at the reed valves? They can seat at idle and fail to seat at higher rpm.

  9. #9

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    New hoses shouldn't need shrinking of any sort. Wouldn't hurt to recheck for cracks and tight fittings though. These rascals can be finicky.

    Just wondering if you unscrewed the top of the carb and checked to see if that needle valve (inside the slide) was still secure. You can check it when you get the top off by just pushing up lightly with your finger on the pointy end. It shouldn't move up or down.

    Isa 8:16,20 Bind up the witness, seal the Torah among my taught ones...To the Torah and to the testimony! If they do not speak according to this Word, it is because they have no light in them!

  10. #10
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    I did not yet check for that needle valve.
    This is a brand new (not rebuild) carb.

    I'm out of daylight for today.

    Which side is the top?

    I get nervous around carbs.
    When I was a kid, I tore enough carburetors apart and never successfully reassembled one
    In battle, Free Men will almost always defeat slaves
    Herodotus after the Battle of Marathon

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