Decompression valve

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This topic contains 12 reacties, has 5 deelnemers, and was last updated by SuperSi 1 month, 3 weeks geleden.

13 berichten aan het bekijken - 1 tot 13 (van in totaal 13)
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  • #804547

    Bagzy
    Topic starter
    Berichten: 5

    Hi all, does anyone have any knowledge/experience ref the design/fitting of the decompression valve.
    We are currently building a Malossi deps engine and have carried out an air seal pressure test of the assembled engine before fitting into the frame. We found that the Air pressure is leaking out through the decompression valve.
    Is this normal? does it need to seal when engine not running and the valve designed to only seal when the engine is running?
    Any help appreciated

    1
    #804548

    zevenenergie
    Forum Goeroe
    Berichten: 3262

    I tink you are one of te first people to performe a pressure test on the engine. Malossie is known for inperfections . I don, t know hou long the engine stays on pressure. You can hit the valve in to his seat to make it close better.Or lep it in.

    1
    #804549

    Semmad
    Bijdrager
    Berichten: 815

    The valve stem is not bent?
    Is the spring strong enough to pul the valve back?

    And do some valve grinding with grinding paste.

    And while the engine is runnig there is more power on the valve
    so the valve wil be pushed in his seat.

    But some grinding is always better.

    Gr. Wim.

    • Deze reactie is gewijzigd 1 month, 3 weeks geleden door Semmad.
    #804559

    Bagzy
    Topic starter
    Berichten: 5

    Thanks Seven/Semmad
    Yes the spring is strong enough. We have hit the valve to the valve seat and we’ve used grinding paste to lap in the valve also.
    We build many lambretta/vespa engines and always perform a pressure leak test after assembly.
    Air is escaping out through the decompression valve when engine not running, i guess we will just have to hope that the valve seats properly when the engine is running…..dunno,,?

    Many thanks

    1
    #804560

    SuperSi
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    Berichten: 1854

    Helllo,

    Are you also using a malossi cilinder head?
    I know there is a different stem that can be used for malossi cilinder heads.
    Our house retailer sells them here.
    https://www.jmpbonderdelen.nl/decompressiestift-veer-citta-ciao.html

    #804586

    Bagzy
    Topic starter
    Berichten: 5

    Yes, we are using the stem that Malossi supply with the kit. Why is there a different stem available for Malossi heads other than the stem Malossi supplies? Is there a need/requirement for an alternative? Many thanks for the link 😉

    1
    #804630

    Seru.ch
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    Berichten: 46

    That leak is normal. The air coms true the little hole in the cilinderhead and ends behind the valveseat.
    When you whil make a leak test you must close this hole between the head and the Cylinder. You can do this with a self made headgasket without the small hole. After the Test change the headgasket wit the normal one.
    When you Maka the leak rest look good between Cylinder and Carter.

    1
    #804639

    SuperSi
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    Maybe i dont understand seru completely. But the leak is not normal

    #804646

    Bagzy
    Topic starter
    Berichten: 5

    Thanks again
    We expected the decompression valve to seal (even when engine is not running) but alas it does not seal.
    Yes, we could block/blank the hole/cylinder head with a gasket to leak test the rest of the engine…but that surely that defeats the object of achieving a pressure seal test…?
    The aim is to pressure test the completed engine with a successful test and then leave the test proven engine assembled and fit to the bike.
    The question we have is……by design, does the valve seal when the engine is running? but not necessarily when engine isn’t running?
    Hope this make sense 🙂

    #804659

    Seru.ch
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    Berichten: 46

    I try to explain better.
    When you make a leak test the air go’s thru the small hole (blue arow) and ends behind the valveseat (yellow arow). To make the sealtest you must close the small hole or keep the engine under pressure during the test with a compressor (1-2 bar). So you can test all seals without desasembling the engine again.
    The Valveseat itself is difficult to test. Maybe with a compressiontest it can be proof.

    Screenshot_20201202_180607

    #804669

    zevenenergie
    Forum Goeroe
    Berichten: 3262

    I totally agree.With Seru.ch

    There is always something leaking down the stem of the valve. so a pressure test cannot actually be performed. And if you’ve ground the valve, it should be good. I would pressurize the crankcase and use soapy water to test for leaks if I were you.

    Isn’t 1-2 bar a bit too much?

    • Deze reactie is gewijzigd 1 month, 3 weeks geleden door zevenenergie.
    #804685

    Seru.ch
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    Berichten: 46

    0,5 bar ist es auch OK, ich glaube nicht, dass der Druck im Gehäuse 2 bar erreicht, wenn der Motor läuft. Aber mit 2 bar sind die Blasen größer. 😎
    Die Dichtung der Kurbel springt bei mehr als 4 bar heraus. 😀
    It’s bether to make the Test without the ignition, so you can check the Crankseal too. 

    • Deze reactie is gewijzigd 1 month, 3 weeks geleden door Seru.ch.
    #804703

    SuperSi
    Bijdrager
    Berichten: 1854

    I just put my lips around the valve, give it a kiss, and try to blow air through the seal.
    Than tap the stem from the “outside” to open and close the seal.
    If it leaks, you’ll notice

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