Walbro carburator

Discussion in 'General Engine Discussion' started by vagpas, Sep 23, 2018.

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  1. Sep 23, 2018 #1

    vagpas

    vagpas

    vagpas

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    I have seen many multi cylinder gas engines using Walbro carbs.
    The question is how someone can use the Walbro without a pulse from the engine.
    It`s a mystery to me as without the pulse neither the pump works nor the regulator allows any fuel to pass.

    I have tried to fit one on my radials using pulse from the intake of one cylinder but the pulse is way too weak.
    I would appreciated if someone helped.

    Evangelos
     
  2. Sep 23, 2018 #2

    gbritnell

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    The pulse part of the carb is for pumping fuel when the tank is lower than the carb. Some Walbros don't utilize the pumping action when the tank is above the carb. The diaphragm on the bottom of the carb is used to open the needle valve to allow fuel to pass through the carb and jets, high speed and idle. The diaphragm gets it's vacuum signal from the air flowing through the carb (venturi action). There are several drawbacks to using a Walbro. The first is the smallest Walbro is generally too large for small displacement model engines. The second is the pumping action needed to supply fuel. To get around this problem some fellows used small pressure electric fuel pumps, (2-3 lbs.) The third issue is the needle valves are too coarse for fine fuel control. When oil is mixed with fuel for 2 cycle operation to get the same amount of fuel to air ratio (15:1) the needles have to be richened to compensate for the oil mixture (the oil doesn't burn) so therefore more gas is needed to meet the mixture requirements. People do use the Walbro carbs but not right out of the box. They make modifications to them to get them to work properly. The much simper route is to use an air bleed style carb found on RC engines. I have built many of these over the years and have them on all of my I.C. engines, 3 cyl, 4 cyl, 5 cyl radial, 6 cyl. and V-8's. I played with Walbros but found it was more work than necessary.
    gbritnell
     
  3. Sep 23, 2018 #3

    lohring

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    Walbro carbs are mostly used on single cylinder two strokes larger than 20 cc displacement where the crankcase provides the pressure pulses. We've run into problems with high rpm engines where the internal pump can't keep up. The RC engine carbs are a great choice. OS makes the best ones and has a variety of sizes.

    Lohring Miller
     
  4. Sep 23, 2018 #4

    vagpas

    vagpas

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    Thank you for your input.
    I`m using rc Super Tigre carbs with an electric pump for my engines but i thought with a Walbro will get rid of the pump/batteries etc. which is still not a perfect set up with an engine that flies on plane.
    The electric pump does not know what the engine actualy needs as the chanel that controls the pump pressure is electronicaly mixed with the throtle chanel and not the actual revs of the engine.

    It seems that we experience the same drawbacks. At least i`m in good company...

    I would be interested if someone succeded though.

    Evangelos
     
  5. Sep 23, 2018 #5

    WOB

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    I have never heard of an electric pressure pump for the simple carbs used on 2-stroke and 4-stroke model airplane engines. Some flying model installations do use muffler pressure to assist fuel flow to the carb. On my Hodgson radial, the fuel flow is by gravity to a float bowl cut from a Briggs and Stratton lawnmower carb. The float bowl is mounted about 1" below the fuel inlet of a conventional OS model engine carb. This gives a constant head feed to the carb and it works perfectly with any amount of fuel in the tank. I got the idea from Kurt Bjorn on his 5bears website many years ago and which seems to have disappeared. The same system would work with a low pressure pump as long as it does not overpower the float valve.

    WOB
     
  6. Sep 24, 2018 #6

    Cogsy

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    Floats don't work in planes as they stop working when upside down, which model planes often are.
     
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  7. Sep 24, 2018 #7

    bluejets

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    We use the Perry oscillating pump for most aircraft applications that require pumping, they work very well.
    Just fitted one to a scale Rivierra boat as well where the tank position was well below requirement.
    There is also a pulse pump available.
     
  8. Sep 24, 2018 #8

    vagpas

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    I have tried them both. The oscillating would work great on single or maybe two cylnder 4stroke engines. On multi cylinders with less than 360 deg between firings it starts to struggle. On a 5 cyl radial does not work at all.
     
  9. Sep 24, 2018 #9

    dkwflight

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    Most walbro carbs get the pulse from a port in the flange mount.
    Beyond that the pulse port is connected to the CCase through a drilled passage.
    Dennis
     
  10. Sep 24, 2018 #10

    lohring

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    We tried an electric pump, rubber bands around the fuel bag, and an auxiliary Perry pump. Our final solution was to lower the pop off pressure and use new diaphragms often.

    Lohring Miller
     
  11. Sep 24, 2018 #11

    stackerjack

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    Yes O.S. carbs are very good, but better even than O.S. carbs is one that used to be made by E.D. in England. If you can get hold of one they are excellent. You can leave the engine ticking over for 20 minutes and it will respond to Full Throttle instantly.
     
  12. Sep 26, 2018 #12

    bluejets

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    Have you tried a recirculating system?
     
  13. Sep 26, 2018 #13

    vagpas

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    If you don`t mean the overflow bypass for the electric pump i am completely unaware of such a system.
     
  14. Oct 1, 2018 #14

    Dociron

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    Floats will work on airplanes if properly modified . Float type carb's like Marvel / Schebler were used on real aircraft with check valves inserted in the float chamber vents.
    This allowed inverted flight for the duration of fuel in the float chamber.
     
  15. Oct 2, 2018 #15

    Cogsy

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    They don't work well in model size though, as modellers tend to spend more time inverted than full-size do. There's also the issue of the reduced inertia of the prop so rather than a cough from a full size engine as it momentarily starves of fuel, the model engine just stop dead. They just aren't suitable for flying models.
     
  16. Dec 28, 2018 #16

    vagpas

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    After a lot of effort i cannot make the walbro work on a radial.
    I admit my failure.
    I reverted back to the rc glow carb with an electric pump.
    Here is the link to the story of the engine and some good footage of the plane.


    Evangelos
     
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