Inlet manifold and reed valves

  • Hi There, I found some small tuning info from a French dude and I was wondering what you guys think of this?
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    I adjusted the inlet manifold where it goes inside the reed cage (made it YZ style, see pictures ‘these pictures comes from the French site... yam2stroke’).
    I also want to adjust the reed valve stoppers, so that more air will come through. Is this a good idea? Or should I leave it original?
    I am going to give my bike a lot of air so that means I will need to adjust the main jet. But the main jet only is useful when I go full throttle if I am correct?

    I have been looking around and I only get info about main jets (that I need to get a bigger one) so was thinking putting in a 250 and maximum a 260 main jet.

    But what about the pilot jet (it’s a 25 according to the Haynes manual). Shouldn’t I get a bigger pilot jet?
    And what would be the ideal size?

    Also where can I get good main jets and pilot jets? ( I have a Mikuni TM28SS). I would like some different sizes so I can test away.

    Then there is the oil matter… The DT has a oil pump that is automatic. So I guess that it’s set to the original factory settings (240 main jet, etc.)? Do I need to give more oil? Since I am going to give the carb more gasoline and air? Or do I just leave it as it is?

    Any help I much appreciated!

    Regards, Timo

  • Administrator

    I wouldn't recommend butchering the inlet.manifold. The YZ is cylinder inducted not crank inducted. The differenxe i massivr and would explain why it's done the way it's done.

    Unless the guy modified one item at a time, dyni'd the results, I would take it with a pinh of salt.

    By all means do modify ig, just know that it may or mau not give the desired results.

  • The reed valve in a DT125R requires no modification at any point for what you need from the bike.

    Boysesen Reed petals are there to reduce 'flutter' at high RPM, Probably the best thing you could do for the Reed Valve as Standard Reeds suffice unless you want to constantly ride it between 10-12k RPM like a race bike.

    'The inlet tract into the crank case is more than double the size of the carburettor bore, this is due to the Reed valve section having to be quite large.

    So the 28mm carb is a good size for the DT because it's smaller bore increases the speed at which Air can pass through, The Reeds are more sensitive to a higher pressure and operate quicker, Good for Low end to Midrange torque, which is what you want with the standard Cylinder barrel and head.

    So these protruding 'stuffer' mouldings on the Reed valve are there to decrease the size of the inlet to match it with the size of the carburettor bore to minimize the pressure loss between the two.

    By cutting the stuffer apart , your effectively enlarging the inlet bore in the crankcase and slowing the Air speed, by increasing the pressure difference from the carb.

    This in theory, will make it slower until potentially much higher RPMs

    More air is good, but you will begin to lose much of the Midrange torque that makes a DT so easy to ride.

  • Administrator

    Sorry I typed this on my phone at lunch time in a rush lmao

  • @Calum Crap! I already did the modifications on the inlet manifold. I hope i didn't mess up my bike.
    @Darty Thanks for your explanation! I will stop 'abusing' the reed valve stopper. I ordered the Boyesen dual stage reed valves so I wont need the stopper anymore. But like i said to Calum, I already did the modifications to the inlet manifold. Maybe I was a bit naif.
    But I have read on 'scrimsmustangs' (when it was still online) that one of the restrictions on the 2001 bike is that 'square rubber nose' and that you had to cut off the whole part. So cutting away some parts seemed not so drastic as removing the whole part.


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