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Chainsaw issue

Discussion in 'Tools and Materials' started by RickH, 12 Jan 2019.

  1. RickH

    RickH

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    I have a McCulloch 360T
    Starts fine .. either hot or cold ... which in itself is a good indicator that many things are Ok with the saw.

    In use ... be cutting fine then revs will die, then get to a point with chain out of the wood .... tickover is fine, but when I try to rev it picks up, but after about 20-30% of power then dies off ... often to a stall. Can't get it up to speed.
    It can often take 10 or more attempts before it will get to point that revs will open up .. then it cuts fine ... but a few minutes later it will start again.

    When it is revving it cuts fine with no loss of power.

    I have cleaned air filter, checked plug gap, used new (correct mix) fuel, general clean to get rid of any oil or wood chip build up.

    I just took plug out again ..after today's running - nice biscuit colour so all seems well there.
    Used compressed air to blow through carburettor breather pipe back to tank - all clear.

    Carburettor is a Zama W26 .... not sure if it has a diaphragm or not, if so I suppose that could be suspect.

    Any suggestions ?
     
    Last edited: 12 Jan 2019
  2. Burnerman

    Burnerman

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    For sure, you are on the right track!
    Your saw is suffering with fuel starvation, and the W26 is a C1M carburettor which uses two diaphragms both to pump fuel and to meter it into the carb jets.
    There are also two screws on the side of the carb marked L (for low speed mixture) and H (for high speed mixture).
    Inside the thing there is the usual needle valve, (no float obviously) but on the input side to the needle valve there is a very fine gauze filter that will possibly be gunged.
    Before opening the carb up, I'd be inclined to undo each mixture screw a 1/4 turn, just to see if that makes any difference.
    If you do want to open the thing up, its best to have a diaphragm and gasket set standing by as they are likely to tear - and an aerosol of carb cleaner is vital.
    John :)
     
  3. RickH

    RickH

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    Looks like carb rebuild kit is only £3 ... so if I have to strip down parts are OK.
    I'll try the needle valve filter .... is this under both adjustment screws ..... or if not can you point me right part ... only diagram I have is http://www.zamacarb.com/carburetors/1134
     
  4. Burnerman

    Burnerman

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    The strainer (filter) I'm on about Rick is actually part number 1.....fuel has to pass through this before it gets to the needle valve. Its pressed into place and not necessary to remove it for cleaning.
    I'd normally recommend compressed air to help clean the carb and I use it myself - but its easy to blow away welch plugs or whatever and unfortunately you wouldn't know they had gone :eek:
    A good blast with carb cleaner will do as well.
    One little point - before you remove the 2 mixture screws, make a note of how many turns there are to screw them fully in, and undo them the same when you reassemble. Don't mix them up, either!
    John :)
     
  5. RickH

    RickH

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    So if I understand the drawing (no experience of this carb) this would be under the bottom plate ? beneath the pump diaphragm ?
    If that's right, then I need to buy in a overhauls kit so I can change diaphragm and gaskets ..... kits seem to be only £3 which is extremely cheap.
     
  6. Burnerman

    Burnerman

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    Looking at your diagram, the bottom diaphragm is indeed the pump system, the top one is the metering diaphragm - which may, on some applications will have a primer bulb too.
    Where you need to be is under a one screw cover, but I'd recommend taking both covers off, removing the two mixture screws and the needle valve assembly. You can then squirt in your carb cleaner to blast things out. Watch for the tiny spring under the needle valve though, it is easily lost!
    Genuine Zama diaphragms / gaskets are much more expensive than this - usually around £15 or so. Replacement carburettors are available too, although they are pattern rather than genuine. Check out GHS Ltd for examples!
    On a couple of occasions when carbs have defeated me I have used pattern replacements and happily they've been fine.
    John :)
     
  7. RickH

    RickH

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    Tool finally arrived for mixture screws .... I have removed both mixture screws and cleaned them and the ports with carb cleaner.

    Is there a base setting for these screws ... on cars it often 'x' turns back from fully closed ...... anybody got a guide.
    I did count number of turns or removal - but they may be way off a good starting point.
     
  8. Burnerman

    Burnerman

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    On the Zama carb (unless its been modified for McCulloch) go for one turn out on each screw.
    Finer adjustments can be made once the thing is starting and running reasonably well.
    Dont mix the screws up!
    John :)
     
  9. RickH

    RickH

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