Lunkenheimer mixer

Discussion in 'General Engine Discussion' started by Gordon, Sep 14, 2019.

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  1. Sep 14, 2019 #1

    Gordon

    Gordon

    Gordon

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    Has anyone had experience with a Lunkenheimer mixer? I have two Breisch Olds engines with this mixer/carburetor. One my dad built in 1981 and one I built in 2005. The one my dad built has always started easily but I just got out the one that I built and I am having trouble with keeping it running. It starts but only runs for about 10-15 seconds and then runs out of fuel and quits. I am sure that it is related to the mixer. Compression and spark is good. I have tried lapping the valve seat, tried different springs and even made new fittings but nothing changes. I am not sure what I should be looking for. The needle valve adjustment is extremely fussy. 1/32 of a turn makes a difference between starting or not starting. Any experience?

    Gordon
     
  2. Sep 14, 2019 #2

    gbritnell

    gbritnell

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    Gordon,
    A Lunk mixer is used when the fuel supply is higher than the carb. That being said the poppet valve is used to control the fuel flow. When the engine is on the inlet stroke the vacuum pulls the valve open and allows the fuel to flow so the valve only needs a spring strong enough to assist in reseating the valve. Once the valve is open the mixture screw controls the amount of fuel flowing. Usually the problem is with the valve. At times I have eliminated the spring and just added a small circular weight to the end of the valve stem
    gbritnell
     
  3. Sep 14, 2019 #3

    Gordon

    Gordon

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    It is interesting that on my Dad's engine the fuel supply is lower than the mixer and that one runs. I think that I will play around with a higher fuel tank and possibly a ball check in the fuel line. Perhaps there is a check valve in the fuel line on my dad's engine. If there is a check valve it is hidden under the engine and I have to do some disassembly to find it. Thanks. I will do some experimenting with the height of the tank.
     
  4. Sep 14, 2019 #4

    Gordon

    Gordon

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    I did some looking for images and videos on this engine and it looks like most of them have the fuel tank mounted high or have a check valve in the line. That is probably the answer. Thanks again.
     
  5. Sep 15, 2019 #5

    Jasonb

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    The valve closing does the same job as a ball or flap in a check valve so should not be needed. I usually make my needles more pointed that most drawings show to give a bit better adjustment range but they are still more on/off than a steady adjustment.

    You should be able to see the valve lift on the hit strokes, if it is not then your spring may be too strong. see about 20 secs into video

     
  6. Sep 18, 2019 #6

    BobsModels

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    I have an OLD's also. the tank is lower than the mixer. I have a check valve in the tank since it is lower than the mixer. If it leaks I get the same results. Usually a cleaning gets it running. Not sure the valve in the Lunk can really hold fuel in the line like a check.

    Bob
     
  7. Sep 19, 2019 #7

    Gordon

    Gordon

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    That is also my concern. I think that it would require a highly polished valve and seat and something more than just slight spring pressure to actually seal fuel from a gravity feed when the engine was not running. I think that it either requires a shut off in the line or a check valve to stop fuel from bleeding through the mixer.
     
  8. Sep 19, 2019 #8

    gbritnell

    gbritnell

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    The purpose of the check valve (poppet valve) in the Lunk carb is to shut the fuel flow off when the tank is higher than the carb. If the fuel supply is lower than the carb then there's no reason to even put the valve in the carb except maybe to act as a check valve. All of my hit and miss engines have the tanks lower than the carb and I have never needed a check valve to keep the fuel from back-flowing during the coast cycles. The needle opening is so small that the fuel really doesn't have much time do this. This is my Little Brother engine running quite slowly with no check valve.
    gbritnell
     

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