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

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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
     
  9. Oct 23, 2019 #9

    Gordon

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    Still trying to sort this out. I have been playing around with this thing and I still cannot get it to run well. I have determined the the problem is that it floods out after a short run. It runs for about 20-30 seconds and then quits. When it quits there is too much fuel in the mixer. If I release the valve, fuel runs out the bottom. It is just not atomizing. I have reworked the valve and seat and I have a good seal. I have tried restricting the valve travel so that the valve cannot open too far. I have tried different needle valves and seats. I have sealed all joints with Teflon tape to eliminate air leaks. I am not sure what I am missing.

    Anyone familiar with the principle on this thing and what I should be looking for?

    Gordon
     
  10. Oct 23, 2019 #10

    BobsModels

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    Gordon


    I had a similar problem when I first made my ¼ scale Lunk Mixer. What I found was the valve was being sucked up and cutting off the fuel flow to the engine. What I mean is the valve went up high enough that the intake pipe from the mixer was blocked. Upon looking at a full size Lunk I discovered they had a little pip on the cap that kept the valve from coming up too high. I added a bar across the cap and then filed it to give a nice lift clearance. See pdf


    Bob
     

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  11. Oct 23, 2019 #11

    Gordon

    Gordon

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    Thanks but I have already tried that. The mixer I have has a center section with holes for air flow so I tapped the cap and put a 2-56 screw in there so I could adjust the lift. I have tried several different lift distance without any change. My theory was that by opening too far it was letting in a stream of fuel instead of hitting the valve and atomizing.
     
  12. Oct 24, 2019 #12

    gbritnell

    gbritnell

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    This may be redundant but let's start off with some basics. Every carburetor that I'm familiar with has a venturi. This is to speed up the air flow thus causing a low pressure area (lower than atmospheric and also referred to as a vacuum). Being as the atmospheric pressure (14.7 lbs) is pressing on everything, including the fuel in the tank it forces the fuel toward the low pressure area (the venturi) When a fixed throttle carb, like that on a hit and miss engine is operating the fuel needle is adjusted for the amount of air flowing through the carb to provide the proper air/fuel ratio which is actually 14.7:1. If a hit and miss engine wasn't governed the rpm would be very fast due to the size of most of the carbs.
    Now onto the Lunk carb. The nonpareil generator which is what the Lunk carb we're talking about is named has no basic venturi. The venturi is formed by the reduced area formed when the valve lifts from it's seat due to the vacuum created when the piston is on the downward stroke. With this in mind the amount of lift of the valve needs to be controlled to form the venturi area for a given needle setting. The lower the valve, the smaller the needle setting and vise versa until it opens too far and then there is no low pressure area formed. I have never tried to operate a Lunk carb without the valve but I would assume it would be hard to draw fuel because of no venturi. As I had stated in another reply there would be no real reason to have this type of Lunk carb on an engine with the fuel level lower than the carb although I would suspect it should still operate properly. If the fuel source is higher than the carb and you would start opening the needle valve the fuel would start flowing due to gravity and weep or run out of the needle depending on how the needle was adjusted. I can't swear that I know what the original design intent was for this carb but I'm sure that the valve is to shut off the fuel flow when the engine isn't running otherwise it would run out of the needle valve.
    I would say to adjust the valve travel to a minimum amount, .025-.03 off of the seat. Start with the needle just cracked. Turn the engine over and see if it starts. Probably not. Choke the carb while turning the engine over. It should hit or try to run. If it does but won't keep running open the needle a little more and repeat the steps. At some point the engine should continue running and then it's just a matter of fine tuning the needle for that particular valve setting.
    It's surprising how small of a venturi opening is needed for slow speed operation.
    gbritnell
     
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  13. Oct 25, 2019 #13

    Gordon

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    Thank you for the input. I will try restricting the stroke of the valve more than I have done presently. One of the things which have kept me baffled is the fact that I have an identical engine with the Lunkenheimer mixer made by my dad back in 1985 and it consistently starts and runs. The fuel tank on my dad's engine is lower than the mixer and does not have a check valve. In looking at internet pictures and videos some have the tank lower and some have the tank higher. The valve on my dad's engine has about 1/16" travel. I have had both mixers apart and I cannot determine what makes one work and the other one very marginal. I had my engine running several years ago and I decided to tweak things and have had nothing but problems ever since. Guess that proves that if it ain't broke don't fix it.
     

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