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Stihl FS85 Weedeater

Discussion in 'Mechanic and Repair' started by zipposusa, Sep 15, 2007.

  1. zipposusa

    zipposusa LawnSite Member
    Posts: 6

    :confused: I have a Stihl FS85 Weedeater that I am having some serious problems with. I have been a mechanic for small engine equipment for 13 years now and never had a problem like this. Here is what it is doing. It will run at idle and at Max speed but will not go from idle to high speed on it's own. I have to manually choke it to get it there but once it is there it runs great until you drop it back to idle. It will then idle good but when you try to pick the speed up again it dies. The funny thing is that I have an Echo Blower that is doing the EXACT Same thing. Here is what I have done to them. I have rebuilt the carburetor, I have cooked the carburetor out 4 times in carb cleaner, the good stuff, I have tore the carb. down 4 times and the last time I took them all the way past the aluminum plugs that are in there and then cooked them out again. I have repalced the fuel filters, I have replaced the fuel lines. I have checked the cylinder for scoring, I have removed air filters and tried running as well with them in place. I have adjusted the carb. every which way possible. I have changed gas, Octanes, and oils. Used fresh mix. I have checked for bad seals in the crankcase. Nothing seems to fix this. The only thing that the 2 have in common is that they have Zama Carbs. The weedeater is a Stihl FS85 and the blower that is doing the same thing is an Echo. These machines have come from the same owner to be fixed with the same problems. I have racking my brain about this and I can't figure it out. The only thing I haven't done is replace the carb. with a new one. Anyone got any ideas.
  2. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,023

    I have ran across a few that just couldn't be cleaned, From the sound of it you should go ahead and replace to get these units out of your hair.
  3. Stillwater

    Stillwater LawnSite Platinum Member
    Posts: 4,834

    (OK first confirm you do not have the fuel line and prime bulb line reversed)
    (confirm that the needle valve is not sticking at the rubber tip)

    I had the same issue 3 weeks ago same exact problem,and after 2 days of doing exactly what you have done so far I made 1 last attempt at repair.

    this is what I did. It was a possible 3 issues but because I did 3 things at the same time I don't know what of the 3 things it was that fixed it.

    If I was to guess what fixed it I would say #3 a trapped pocket of moisture

    Put the carb cleaner away.........

    1)completely dismantle the carb again and take some super fine florist wire and gently ream all the ports.
    2) crank up your compressor and blow each port with the highest possible psi
    3) preheat your oven to 175-200 degrees, place the carb in the oven for about 20 or 30 minutes.

    My trimmer starts now on 1 or 2 pulls when cold and runs like new, carb cleaner and blowing air alone was not effective, I really think I had trapped moisture that the air and carb cleaner coulden't reach.

    let me know if this fixes yours 2
  4. zipposusa

    zipposusa LawnSite Member
    Posts: 6

    eshskis, I do not have them reversed. In fact I ran out of ideas so I swapped them just too see and nothing. Switched them back. I have cleaned the ports with a torch tip cleaner. But keep in mind it is doing the same thing that it did when I first got it, so the tip cleaner didn't do damage. Keep in mind this isn't the carb cleaner that you get in a can. This is cleaner you get in a 5gal bucket and is very potent. Come to think of it when I took the carburetors for the first time, BOTH OF THEM, there was a build up of some sort around the welch plugs and in the carb. but I have removed them and replaced them and the carb. cleaner took that away quick. I am using air to blow the crap out and I run every thing at 185psi. I haven't tried the oven though. The cleaner took the other stuff of so easy I don't any of it would be in there.

    I have read on another thread that the Zama Carbs. that start with C1Q S series 63 and another are having problems with the carb. in generally and sometimes fail to work and can't be fixed. I have the C1Q S63A carb.
  5. Stillwater

    Stillwater LawnSite Platinum Member
    Posts: 4,834

    frustrateing isin't
    mine has has a ton of hours on it and I did not think I could fix it. the oven was a last resort but it worked. if it did not work I was going to replace the whole trimmer.
  6. zipposusa

    zipposusa LawnSite Member
    Posts: 6

    Well I am thinking that it has to be the carburetor. Just because I am having 2 different machines by different manufacturers have the same problem. I sure it isn't spark or a spark plug. It is starving for fuel when it try to power up but will run at idle and fast speed once you baby it to the fast by choking it.

    And yes it is frustrating. That is what this guy wants to do is buy a new one but I am trying to discourage from it for now. What do you do after you bake it? Just blow it out with air? or ?
  7. Stillwater

    Stillwater LawnSite Platinum Member
    Posts: 4,834

    I just reassembled it and it worked I did not want to replace mine either those are not cheap trimmers
  8. topsites

    topsites LawnSite Fanatic
    Posts: 21,653

    Yeah but by the time I went through my echo I could've spent the $300 or so for the new FS-100rx I had to buy in the end anyhow, the hours I spent on labor...

    I like those new 4-mix'rs, mine's finishing out it's 2nd season.
  9. Roger

    Roger LawnSite Fanatic
    Posts: 5,922

    Is it possible the pumping diaphragm may be weak, and needs replacing? I know the suggestion sounds odd, since you have two pieces of equipment with the same problem, at the same time (If I understood you comments rightly).

    I had a problem with my FS85, and a rebuild kit solved the issue:


    Note, the Zama carb is a C1Q S45, not the same as your S63 model.

    I also have an FC85 (stick edger, but same powerhead). When I had problems with my FS85, I changed the trimmer head and handle, and used the FC for trimming. When I got the FS85 rebuilt, I kept it as my edger. Formerly, the FC85 was used far less. However, in the past week, it has been exhibiting the same problem, so I am looking for a new diaphragm and gasket kit for the FC85. The carb on both is the same model, C1Q S45.

    One other question regarding the suggestion of the oven treatment, ... wouldn't the heat damage the diaphragms in the carb? Or, do you think they get that hot when working at normal operating conditions, hence an "oven bake" will not damage?
  10. MowerMedic77

    MowerMedic77 LawnSite Bronze Member
    Posts: 1,164

    At this point with all the time and effort spent you are surely well past the point of a profitable repair for your shop, its now a matter of not getting beat by a couple pieces of 2 cycle and trying to take care of your customers. :cry: I know I have been there:cry:
    But that being said when a customer brings in multiple units with similar issues it tends to raise red flags for me.

    Let me ask you, how are you checking for cylinder scoring? Are you simply looking in on the intake and exhaust side or have you pulled the cylinders completely? I have seen units that did not appear to be scorn but more like a dull almost sandblasted finish on the piston/cylinder wall. Do you own the tools to pull a 2 cycle cylinder leak down test?

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