Troubleshooting: Stihl saw won't idle (dies)

Discussion in 'Mechanic and Repair' started by jdhupp, Dec 28, 2009.

  1. jdhupp

    jdhupp LawnSite Member
    Messages: 3

    Hi, all.

    I've been troubleshooting an idle problem with a Stihl saw (TS-400 cutoff saw) and I'm nearly out of ideas. Hoping someone here can help. I feel invested now and don't want to simply hand it over to the service department.

    By the way, I bought this saw used, so I don't know anything about its history.

    The saw runs fine at higher speeds, but if I try to let it throttle down it dies immediately below a certain rpm. It also hesitates when I throttle up (does not accelerate responsively -- may actually drop a little for a half-second).

    Making sure the saw is well warmed up does not help.

    It was interesting that earlier, I could adjust the saw so that it would idle for a little while (less than a minute). Trying to solve that, I disassembled the carburetor, gave it a dip treatment, and reassembled with a full carb rebuild kit. I also adjusted (bent down) an inlet control lever ever so slightly to sit flush with the carb chamber floor, just as it is supposed to. But afterwards the idle performance was actually worse, and no carb adjustment corrected that.

    I disassembled the carb and bent the inlet control lever the other way a bit, but that had little if any effect.

    The saw passes all 3 vacuum and pressure leakage tests for the carb and crankcase.

    The spark plug was replaced not long before I got serious about solving this.

    Also replaced:
    - a fuel hose
    - in-tank fuel filter
    - auxiliary air filter
    - spark arrest screen
    - exhaust gasket
    - decompression valve (lets the Stihl start easier with less kick-back)
  2. DT Lawn Care

    DT Lawn Care LawnSite Senior Member
    Messages: 850

    After all of this, I am guessing that at some point you checked the compression for bad rings/piston, but I am just checking. Still sounds carb related, you may have to purchase a new carburetor to fix your problem.
  3. rotti1968

    rotti1968 LawnSite Member
    Messages: 169

    Very common issue in the ts 400's , I work on these daily. replace the carburetor and the issue will go away. Its internal in the carburetor and more then likely you will not be able to correct it with cleaning and/or rebuild.
  4. tigerfan88

    tigerfan88 LawnSite Member
    from SC
    Messages: 82

    probably a few pieces of cope in carb after that "dip" treatment. LOL! i agree w/ rotti. replace carb. see this alot with all kind of carbs. you can try cleaning carb in ultrasonic cleaner, but odds are still likely that replacement will be your answer.
  5. rotti1968

    rotti1968 LawnSite Member
    Messages: 169

    Just so you know Stihl considers this a wear item and should every so often be replaced.
  6. jdhupp

    jdhupp LawnSite Member
    Messages: 3

    I just checked compression: 120 psi. How does that seem?

    I attach a couple photos of a light half-circle below the intake port, and a light scoured half-ring on the top of the cylinder on the intake side. I'd be happy to have a rating of how much piston wear that indicates.

    I know the manual says not to run wire through the channel, but if I'm looking at a new carb then it shouldn't hurt to try that to see if I've got debris in the low-speed channel. Agree?

    1 Light semi-circle below intake port (lo-res).jpg

    2 Scoured, light partial ring top of cylinder head above intake port (lo-res).jpg
  7. jdhupp

    jdhupp LawnSite Member
    Messages: 3

    Rotti1968, you nailed this on the first try.

    Nonetheless, because of other authoritative-sounding advice and also on account of the obvious wear on the piston skirt (worn semi-circular area on cylinder below intake port), I installed a new cylinder-and-piston assembly, but that did not solve the problem.

    So at this point, even though I had already done a carb dip, installed a carb rebuild kit, and double-checked the low-speed ports (squirted carb cleaner through with no apparent obstructions), I replaced the carb.

    Now it idles beautifully.

  8. rotti1968

    rotti1968 LawnSite Member
    Messages: 169

    No problem, sorry you put a new jug and piston on this. With 120 psi i would not have pulled the jug but may have pulled the exhaust and had a peak.That being said the wear you have on that jug is dirt ingestion, are you using stock filters or after market? Do you blow them out or just replace them when clogged? Do you get the filter kit and if so do you replace the rubber gasket in the filter housing? Dont blow them out, stay away from after market filters, make sure you tighten the filter cover completely, make sure you replace the rubber gasket, and when you change them look for debris / sand like material on the intake. If you see this you are not a: getting a good seal or b: have a bad filter. Most of the engine failures I see on these are due to dirt ingestion and lack of maintenance.Happy I was able to help you out.

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