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  1. #1
    Maximiliano Galvez's Avatar
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    four strokes carb systems

    I think the most dificult part to solve is how to make work a gas carburetor in a 4 strokes engine

    first is imposible conect pump direct to crancase fitting. engine and pump stalls.

    Some engines has been conected with 2 fitins to carb, passing crankcase preasue into the pump, it is not to good because it stalls wit oil and sometimes heaty damage diaphragm

    walbro developed a system, using a small spring in pump diaphragm that letys work only with negative preasure, and it is usually conect to mainfold intake.

    I tested thuis system and get bad results, when engine is in idle this system works nice, but when engine is full trotle vacum is not enought to move pump.

    In fact we have been contacted for saito fg owners that says engine has problems with carb in this way.





    For me the best way to activate pump is the backplate p`reasure usisng a small valvle . I get some problems in ver 3 system just like some engines that passes to mcuh oil shcok the pump with oil.
    fianlly we get ver 4 Im proud to say is the best way to use gas carbs in 4 strokes

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    Special Tuned Engines (STengines)

  2. #2

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    RE: four strokes carb systems

    Four strokes do not normally require a pump because of the basic operation i.e. induction controlled by valves unlike porting in 2 strokes.
    Four strokes induction time is generally much longer and have smaller induction passage and therefore stronger vacuum draw.

  3. #3
    av8tor1977's Avatar
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    RE: four strokes carb systems

    I fitted a two stroke carb to a Honda 31cc four stroke once. I used a small spring in the fuel pump, and plumbed the pump pulse to the intake tract. I too was worried that the pulse would be insufficient when the throttle was wide open, but it worked. Pumped plenty....

    Stihl's answer to the problem on their Four Mix four stroke design was to use crankcase pressure, but with a booster chamber with an extra spring loaded diaphragm in it on the carb to help the fuel pump. They also used a large diameter hose on the pulse line.

    AV8TOR
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    If it is not SCARY, it is NO WHERE NEAR powerful enough!!
    All R/C planes have expiration dates---> It's just not printed on them anywhere!

  4. #4

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    RE: four strokes carb systems

    The best carb to use on a four stroke is the Walbro WY series using intake pulse.
    If it doesn't run right there are two things to look at one is pop off pressure and venturi size, you are wasting you time with butterfly carbs on four strokes but if you insist then the venturi size is even more critical than the WY.
    A stock 31 cc four stroke should have a venturi no bigger than 9mm

  5. #5
    Maximiliano Galvez's Avatar
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    RE: four strokes carb systems

    We tested WY series. usins the intake negative preasure is coul be the best if no other choise. In fact any walbro can be converted like the wy series.

    Wy series has some disventages first is dificult to find and expensive, the reemplacent parts are same problem.

    but for me the 2 most worst things of this carbs are size, tats is very bigger and ventury size tah is smaller than comon walbros

    Bigger ventury not let works well internal pump system.
    Special Tuned Engines (STengines)

  6. #6
    Maximiliano Galvez's Avatar
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    RE: four strokes carb systems


    in the next pics can se the sizes of carbs the wyl is almos 9mm ventury, the wyk is 10mm ventury and very big body. wyk is imposible to fit in any engine

    about the wyl has no "H" needle valve


    The ste 4 strokes WT kind walbro carb is 11mm


    There is too the saito 120 and 150-180 carbs we can see the 120 is 9mm ventury diamter and saito 180 is 11mm ventury diameter.

    fitting a same ore more ventury carb will always give same ore more rpm but return to the wt series carb, biguer fiting not works wel pump

    We use 11mm ventury in 1.20 in (20cc) to 36 cc working nice. We tested 13mm ventury in 1.80 (30cc) with same result. the diference is the way to motion the pump.

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    Special Tuned Engines (STengines)


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