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Lawnmower "pulsing"

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  • 04-03-2020 1:34pm
    #1
    un5byh7sqpd2x0
    Closed Accounts Posts: 1,862 ✭✭✭


    Hi all,

    Not sure if this is the right forum. I've a strange issue with a lawnmower I bought a few months ago. When started, it runs fine for about 30 seconds or so, and then starts pulsing (see the video for what I mean)

    Anyone have any idea what's causing it? Seems fuel related, but I've drained the fuel tank just in case and replaced it with fresh petrol.

    https://www.youtube.com/watch?v=OIcfeqbRKOk


Comments

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    #2
    CJhaughey
    Registered Users Posts: 14,907 ✭✭✭✭CJhaughey


    Sounds like the governer spring is just pulsing with no load, those chinese engines will do that try holding the governer spring steady and see if it settles down.
    What is is like under load?


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    #3
    un5byh7sqpd2x0
    Closed Accounts Posts: 1,862 ✭✭✭un5byh7sqpd2x0


    CJhaughey wrote: »
    Sounds like the governer spring is just pulsing with no load, those chinese engines will do that try holding the governer spring steady and see if it settles down.
    What is is like under load?

    It's a Briggs & Stratton 500e engine which is American as far as I am aware :p

    Same under load


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    #4
    Duke of Url
    Closed Accounts Posts: 1,662 ✭✭✭Duke of Url


    Did you clean the Carb and Sparkplug?


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    #5
    un5byh7sqpd2x0
    Closed Accounts Posts: 1,862 ✭✭✭un5byh7sqpd2x0


    CJhaughey wrote: »
    Sounds like the governer spring is just pulsing with no load, those chinese engines will do that try holding the governer spring steady and see if it settles down.
    What is is like under load?

    I had a look again at the governor spring, and it looks as if you're right. It is pulsing, and holding it still sorts it out. It is the same pulsing under load however.


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    #6
    TheBoyConor
    Closed Accounts Posts: 3,292 ✭✭✭TheBoyConor


    Petrol these days goes stale and clogs up carburettors in short order if left sit in the tank for any length of time at all.

    Yeah holding the spring will settle the revs but that spring nor the governor is not the actual problem.
    Because of a slightly clogged carburettor, the jets to be exact.
    Did you clean the Carb and Sparkplug?
    I see some peoples take on cleaning a carburettor is just spraying carb cleaner down the intake. Useless. It is not the throttle body that gets clogged, it is the delicate needle valve, the pilot and main jets and the associated fuel passages. And usually it is not clogged by actual visible dirt. Rather, it is stale petrol which forms a layer of transparent gum or varnish which attaches to the orifices in the jets and reduces the opening. It is rather like atherosclerosis of sorts. To clean them you need to disassemble.

    The engine is starved of fuel. The When the engine then drops below the desired speed the governor then responds by opening the throttle more, which gives the engine a bigger charge of fuel and air. However, the crude systems on these small engines are relatively slow and imprecise and end up giving too much fuel and it overspeeds a little, then pulls back, then is starved and revs up again. This is why it suges up and down rapidly.

    I have had this problem umteen times in recent years. It is always fixed by draining, flushing and carefully dismantling the carburettor and removing the various jets and valves and giving everything a judicious clean and then reassemble. Runs good for the rest of the season then.

    You really ought to be mechanically minded to tackle it though.


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