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How does 52cc Brush Cutter Engine Work

Discussion in 'In the Garden' started by yottie, 3 May 2020.

  1. yottie

    yottie

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    I purchased a wheeled brush cutter some 3 years ago, it has had little use since then.

    When I came to start up recently it refused to “get going”, on examining the 3 pipes, coming out

    of the tank grommet, I found the black one had multi splits in one side, seemed as if it had been wound tightly around a drum at some time.

    Have replaced the black pipe which I assume is the feed to the engine (there was a filter on the tank end of the pipe).

    The other two transparent pipes consist of a small unattached one with what appears to be a “breather” on the end (??).

    Am intrigued to know how this engine functions. As a novice “engineer”, am I correct in assuming

    that, as the tank is below the carb, the carb relies on pressure from somewhere (?) to get the fuel uphill.

    This would mean the engine wouldn’t function if there were any leaks in the feeds (pipe splits,

    tank stopper loose etc).

    Am sure this is basic stuff to a lot of people but it would be helpful to know what is happening with

    each piece of gear, the start-up routine is:

    prime fuel bulb (fuel to carb)

    activate the throttle lever 3 – 5 times (what does this do)

    choke lever set (OK).

    Many thanks
     
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  3. Burnerman

    Burnerman

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    With these engines, the fuel is lifted up into the carburettor due to pulses of differing air pressure as the engine ingests air. In the carburettor are 'flap valves' that put these pulses into good use and lift the fuel into the carburettor. There is no float chamber in these carburettors to speak of as the engines are intended to be used at any angle - even upside down.
    A priming bulb is used to manually fill the carburettor up and allow a cold start - after that, the pulses from the engine take over.
    Coming to the fuel tank now, several systems are used. One pipe coming out of the tank means there is no priming bulb, the engine has to pump the fuel on its own - common on chainsaws. The fuel tank has its own breather to allow air into the tank as the fuel is used up.
    On a two pipe system there will be a priming bulb. One pipe draws fuel into the tank, via the filter on the end. The other pipe returns excess fuel to the tank and often terminates just through the grommet.
    On the 3 pipe system, its as before but with another breather to help things along.
    The systems do vary from manufacturer to manufacturer but generally that's about it.
    So - to start up.....maybe 3 to 6 pumps on the primer bulb, depending on how dry the carb is;
    Apply full choke
    Apply full throttle (there's no point in working the throttle a few times)
    Pull the engine over very quickly until it fires once (pops, in the trade) then:
    Choke off, but retain full throttle
    Pull the engine over smartly again and it should start. If it starts then stops, its full choke again but only for one pull, then choke off.
    Don't try to start a two stroke on low throttle, they don't like it.

    Maybe a pic or two of your machine would help if you can.
    John :)
     
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  4. yottie

    yottie

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    That's great stuff John, just what I was after. Am attaching pix, if you want a different aspect please let me know.
    regards Brush Cutter Fuel pipes.jpg Brush Cutter xx.jpg Brush Cutter.jpg
     
  5. Burnerman

    Burnerman

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    Looks like a useful bit of kit - I guess the cooling cowls are left off for my benefit!
    Top pic.....left to right pipe wise -
    Tank breather (that type can be used because the machine won't be inverted - intentionally :p)
    Fuel return from the carburettor
    Fuel supply to the carburettor (there should be a filter on the end of that one.)
    John :)
     
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  6. yottie

    yottie

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    Thanks again John, it's very good for neighbor annoying in early mornings!
    Now, as I can't get the spouse interested in machinery, will have to have a go at those weeds!!
    regards
     
  7. Nige F

    Nige F

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    Fresh petrol/oil mix is important.
     
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