Stihl KM 55R bogs and dies

Discussion in 'Mechanic and Repair' started by Yuanding, Jul 9, 2013.

  1. Yuanding

    Yuanding LawnSite Member
    from England
    Messages: 82

    I'm having serious running problems on a KM 55R that's had only light use (only ever been a back-up). Basically it fires up ok, runs for 5 mins, then bogs under load and dies. After that it's hard to start and back in the van for the day.
    I've tried all the usual things: new fuel line, pick-up body, air filter, spark plug; I've taken the top of the carb off, unscrewed the little brass jet and checked that it's not blocked (cleared it out with acupuncture needle). The diaphragm is in great condition and everything looks in pretty good shape. I run Stihl Ultra. Is air getting in somewhere and I can't tell, or is there something else that can go wrong with these carbs that I'm not looking at?
    Would a new carb rebuild kit do the trick, or is a whole new carb the way to go?

    And while we're on it, can anyone tell me what carburetor (or rebuild kit) I should be getting for a Stihl KM 55R, 2008 model, and how much I should expect to be paying for it. I'm in the UK and I've been quoted nearly £100 (that's $150!) by two Stihl dealers for a new carb (excl. any labour), which is obviously silly, as you can get a whole new machine (KM 56) for £170.

    Are parts generally just much more expensive in the UK than the US? Another example that tells me this may be the case: I need a couple of retaining washers for my Honda walk-behind mowers and whilst in the US they're $0.10 each in the UK they're £1.15 ($1.70 each!). Does anyone know someone in the US who'll ship them cheap to the UK?
  2. BigFish

    BigFish LawnSite Platinum Member
    Messages: 4,190

    OK, for the umpteenth time...........did ya cleen the spark screen in the about the exh. port in the cyl.????

    Your carb ,# 4140 120 0619 B can be had on Ebay for like 25-35 dollars here in USA. Not sure about shipping to U.K. Don't you have an Ebay in the U.K ??
  3. Yuanding

    Yuanding LawnSite Member
    from England
    Messages: 82

    UK models don't have spark arrestor screens, so yes I did check that in a sense. Also I had the muffler off, and examined the exhaust port - only minimal carbon, which I scraped off anyway.
    These carbs are not available on Ebay UK at the moment, and US sellers won't ship to UK.
    Here is the link showing how much mainstream suppliers are asking in the UK.
    There is 20% VAT on top of the price shown. So all told nudging $200 US dollars for a freaking carb! If you can find me a US seller who'll ship to Uk for sensible money I'd appreciate the info.
  4. ricky86

    ricky86 LawnSite Silver Member
    Messages: 2,139

    There are two sides to the carb. One is the pump the other the metering. The pump side has a cover with one screw. There is a fine screen in it also. That screen, when partially clogged could give you the symptoms you describe. You make no mention of taking that side apart.
  5. Yuanding

    Yuanding LawnSite Member
    from England
    Messages: 82

    I'm not sure i quite follow. The primer pump (top side) is held down by four screws. The underside with "Stihl" etc written on it has one central screw. I hooked out the screens top and bottom; they did look a bit varnished, but nothing excessive.
    If I wanted to get just a rebuild kit what would the code be for that and how much would you pay in the US?
  6. Yuanding

    Yuanding LawnSite Member
    from England
    Messages: 82

    Quick update. I've just hooked out the screens top and bottom again and i can confirm that varnishing is almost non-existent. The bottom one, apart from an area the size of a pin-head, is completely clear. The carb parts really are in good order, and on the face of it there's no sign of anything untoward going on there. Is there something else I should be looking at?
  7. dboyd351

    dboyd351 LawnSite Bronze Member
    Messages: 1,124

    Could be a coil starting to go too. Try testing the spark when cold, then running until it gets hot and starts to cut out, then check for spark while it is still hot.
    Another possibility is a loose spark plug. If it's a little loose, it will lose compression as it heats up and won't make power.
  8. herler

    herler LawnSite Fanatic
    Messages: 5,139

    ignition coil
  9. MV Property Care

    MV Property Care LawnSite Member
    Messages: 169

  10. lawnboy dan

    lawnboy dan LawnSite Gold Member
    Messages: 3,716

    do yourself a favor-dont buy any more sthil

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