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  1. #1

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    Walbro carb model number deciphering (GX25)

    For those of you that haven't seen my post on the Honda GX25, check it out by clicking HERE. I did plenty more experimenting today and I think it's a keeper! I tried many different carbs since I am so against the fixed jet type. The peak RPM I recorded was 6600 on a dynathrust 18x6, that was with no muffler, the stock muffler caused a 300-400 RPM loss. For those that have messed with 4-strokes, you know that the pulse holes in the carb are not in the same place, therefore a typical 2-stroke carb won't work on the 4-stroke carb mount. I contemplated making a new carb mount, with a relocated pulse hole, but I decided instead to install pressure fittings, and use a piece of tubing for the pulse. I plugged the pulse hole in the carb, made a new gasket, with no pulse holes, then drilled and tapped the side cover to accept a pressure fitting. On the engine, I just drilled and tapped into the intake manifold. The 4-strokes use the pulses from within the intake manifold, since there is no crankcase pressure, this provided a very easy place to tap, But I was concerned that the pulses might be weaker than from a 2-stroke crankcase.



    With all the carbs I tried, I was able to get it super rich, and slobbering, so I assume it's pumping plenty of fuel, and the pulses must be strong enough. Now here are my questions, I don't have much experience with these carbs. With all the ones I tried, if you richened the low needle enough for a clean transition without stumbling, the idle was very rich. If you set the low for a good idle, it wouldn't take the throttle, you had to accelerate it very slow. Does this mean the carb is too big for the engine? Maybe such a large bore carb requires a lot more volume of air in the venturi to draw more fuel (for the transition) and by richening the low end, you improve the transition, but then idle suffers. I tried 7 different carbs, they all acted similar on the transition, and provided nearly identical top end numbers, So the peak HP must be limited by the size of the intake port, not a restrictive carb. Based on this, I think all the carbs I tried were too big, But I have nothing to compare it with. After running the stock carb again, it didn't do too bad, the idle and transition is much leaner, the exhaust note is even different (open exhaust) The stock carb has an accelerator pump which helps for a second, but once it burns that fuel charge, it stumbles a bit before finally settling at max RPM. With the stock carb, when you close the throttle, it pops out the exhaust on decel, with the other carbs (running richer) It never did. So now I am wondering exactly what the differences are on all these Walbro carbs. Here are the ones I tried:

    WA 56A
    WA 56B
    WT 132
    WT 132A
    WT 37
    WT 160
    WYB 6B 320 (stock on GX25)
    WYL 97 835 (stock on GX31)
    ZAMA ?? (stock on 30cc Homelite)

    I think we have some Walbro experts here somewhere, The WT 37 and the WT 160 have an extra "jet" that is removable near the fuel inlet, it looks like a large brass plug, protruding out of the housing at an angle, but it's some type of metering device. What is this for? And why do most of them NOT have it? Are there differences between carbs for Saws and for weedeaters/blowers? I'm assuming to get more top end, you just need a larger carb, capable of flowing more air and providing more fuel (within reason of course) But do different carbs even of the same bore size, run differently, (on purpose) some idling better, or transitioning better, etc? Are there any general rules that apply, such as "chainsaws typically have better carbs" Or "don't use the WA series" or "the WT carbs make good power, but won't idle" I noticed the WYB and WYL (both of which have a rotary barrel) seem to run a lot cleaner than the butterfly type. Even on my ZDZ and Brison, they seem to dribble a little fuel at idle or low power settings. All the carbs I tried have a much larger bore than the stock carb, yet the max RPM was the same. So it looks like I don't necessarily need a bigger carb, I just want one with two needles, so It can be adjusted. Where would I find a smaller carb than what I tried? I'm thinking maybe off a little 18cc featherlite or something. Someone enlarged the jets in a WYB or WYL carb, what happens when you go too far, how do you go back once you've drilled it? Or how do you adjust the mixture when you change props? I guess back to my original idea, a new carb with two needles.

  2. #2

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    RE: Walbro carb model number deciphering (GX25)

    Bummer, it's been several days and no comments, Someone please share your experiences. Does the 18cc featherlite have a smaller carb than the 21cc kioritz/echo/mcculloch ?

  3. #3
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    RE: Walbro carb model number deciphering (GX25)

    The brass plugon the WT series carbs is a speed governor, excessive speed and vibration causes a small ball and spring to open up a different nozzle and make it rich enough to keep it from going faster..I usually take it out and put a solid piece underneath it to block it off....
    RC Ignition

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    RE: Walbro carb model number deciphering (GX25)

    As you mentioned, sounds like it is overcarburetted. once you go over the size the intake manifold can handle, the idle suffers as you don't have enough vacuum to draw fuel efficiently. Seing that a 4 stroke draws air only every every second rev. it doesnt require near as much air. Now you know what the max revs is, get a cheap set of inside calipers and measure the the inside of the smallest part of the venturi and go down in sizes 1/16 in. at a time till the top rpm drops, then go up one size. that is optimum for that engine and revs. then you will find the idle mixture is more friendly. As RCIGN says, do away with the speed limiter. If it's rich in the midrange, let me know. Most 4 strokes I have worked on with diaphram carbs, have a pulse line into the crankcase, but if it works well with the way you did it, don't worry about it. It's pulsing pos. and neg. pressure that operates the pump, not straight pressure. Hope this helps.
    If there are two raised numbers on the face of the venturi, that is the throat size in 64 ths of an inch.
    Gord
    Dreamed I was a muffler. Woke up exhausted.

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    RE: Walbro carb model number deciphering (GX25)

    ORIGINAL: Flypaper 2

    Most 4 strokes I have worked on with diaphram carbs, have a pulse line into the crankcase,
    how does that work? 4 strokes have no crankcase pressure, since the crankcase is vented to the atmosphere. (OS, Saito, Ford, Chevy) (except YS, and I guess the new shindaiwa)

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    RE: Walbro carb model number deciphering (GX25)

    The crankcase vent is to let off the pressure from piston blowby. There is a relatively large pressure difference in the crankcase because of the volume, as the piston goes up and down. The small size of the vent pipe doesn't change the pressure differential very much.
    For a sidenote. Some Briggs mower engines the use a diaphram fuel pump operating on intake manifold pressure, have a spring on one side of the diaphram to return the diaphram when the vacuum is released.
    Gord
    Dreamed I was a muffler. Woke up exhausted.

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    RE: Walbro carb model number deciphering (GX25)

    In case you didn't see all of my pics, the GX25 and the GX31 operate the pump on intake manifold pulses too. (because there is no crankcase pressure) The term blowby describes what happens when the rings are worn and combustion gases leak past the rings into the crankcase, heating and contaminating the oil. Blowby is bad, the volume of air you describe is perfectly normal, with the piston moving up and down. Obviously it will move air under it, as well as on top. Crankcase vents are typically large, (to prevent pressure buildup) The one on the valve cover of the GX25 is 1/4" and the crankcase is tiny! Ever look at the "breather" on the valve cover of a small block V8, one on each side actually, they are quite large.

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    RE: Walbro carb model number deciphering (GX25)

    ORIGINAL: Flypaper 2

    As RCIGN says, do away with the speed limiter. If it's rich in the midrange, let me know.
    Interesting, I guess I'll just replace that with a plug. Although I don't think that is the limiting factor in this case, since the top RPM was almost identical with all carbs I tried. After examining the intake manifold some more, there is a molded in "bump" down in the intake. It's big on the carb side, and on the engine side, but necks down in the middle, I assume this is some sort of "rev limiter" just to keep the airflow down. I enlarged it, so I have a constant diameter all the way through. Although it will be a while till I can run it again, I am having a prop hub made, and in the meantime I'm grinding and sand blasting on the crankcase.













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  9. #9
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    RE: Walbro carb model number deciphering (GX25)

    BBirBro,

    There have been many threads on carbs and 4-strokes. I suggest you search for and read this thread, as I've been where you are. "Ryobi 4 stroker, whos flying them and what do ya think". I'm following your two threads with interest and would also like to find a fully adjustable 4-stroke carb. Good luck!

    Doug

    P.S. can someone tell me how to post a link from another thread and from a website? Thanks.
    If it\'s interesting, I\'m interested

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    RE: Walbro carb model number deciphering (GX25)

    ORIGINAL: DougT

    BBirBro,

    There have been many threads on carbs and 4-strokes. I suggest you search for and read this thread, as I've been where you are. "Ryobi 4 stroker, whos flying them and what do ya think". I'm following your two threads with interest and would also like to find a fully adjustable 4-stroke carb. Good luck!

    Doug

    P.S. can someone tell me how to post a link from another thread and from a website? Thanks.
    I have done some searching, and most of the stuff I read was things like "the pulse holes don't line up" "2 stroke carbs are different" "most 4 strokes have to use the carb they came with" etc. I have never found any "good" info on which carbs work, or what to do to make the "stock" carbs work better. So I am just experimenting with what I have.

  11. #11
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    RE: Walbro carb model number deciphering (GX25)

    Please, let me rephrase my statement.

    I suggest you search for and read this thread, as I've been where you are. "Ryobi 4 stroker, whos flying them and what do ya think". You obviously haven't read it if you think there's no "GOOD" info. I swapped carbs, jets, drilled out jets, fiddled with spark timing, etc. Maybe that's not what you're looking for or maybe you don't want to spend a couple hours reading.

    Anyway, I'm sure there is info you can use there.

    Doug
    If it\'s interesting, I\'m interested

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    RE: Walbro carb model number deciphering (GX25)

    ORIGINAL: DougT

    Please, let me rephrase my statement.

    I suggest you search for and read this thread, as I've been where you are. "Ryobi 4 stroker, whos flying them and what do ya think". You obviously haven't read it if you think there's no "GOOD" info. I swapped carbs, jets, drilled out jets, fiddled with spark timing, etc. Maybe that's not what you're looking for or maybe you don't want to spend a couple hours reading.

    Anyway, I'm sure there is info you can use there.

    Doug
    Ok, I found that specific thread, ( to post a link, just click the link button and cut and paste the address in the window ) and I hope you were kidding about not wanting to spend time reading, All I do is read read read, I know my way around the search button too (ask my wife how much time I spend sitting here LOL Anyway, I see you did a lot of experimenting with drilling out the jets. I have no doubt, that you can improve the performance, but my question is, what happens when the weather changes drastically, (temperature, pressure, humidity, elevation, etc) or you just change prop sizes ? The jetting will only be spot on for one set of parameters, and with so many variables, I think that would still be a compromize. I REALLY would like to use a carb that has mixture screws !! I read about a 4 stroke carb having a weaker spring on the diaphragm, That makes sence, I'm sure the crankcase pulses are much stronger than in the intake. I'm assuming that since mine ran fine (very rich) with the 2 stroke carb, that the pump must be working ok, even utilizing the intake for the pulses. How does that work anyway? I'm trying to visualize it, and in my mind, with the throttle plate wide open, and no restriction in the intake tract, I don't see how there are any pulses to operate the pump. Strange. But anyway, I think I just need to find a smaller carb, 4 strokes don't have a wicked top end anyway, so I am more concerned now with a clean transition and mid range.

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    RE: Walbro carb model number deciphering (GX25)

    visualize it in slow motion. When the int valve opens, there is a large vacuum and velocity in the int. manifold. When the valve closes, the air hits the back of the closed valve, builds up pressure and reflects back out the manifold as pressure.
    Gord
    Dreamed I was a muffler. Woke up exhausted.

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    RE: Walbro carb model number deciphering (GX25)

    All of the 2-stroke saw/trimmer carbs I've disassembled had no pumper spring in them. I thought they operated on + crankcase pressure. The only time I've seen a carb pump return spring is in the Ryobi 26cc 4-stroker. I called Walbro and the tech told me the carb operates off of negative pressure(vacuum) and a spring was required to return the flap to neutral.

    As far as tuning, it seems gas engines are much more forgiving than glows as far as temp, humidity, barometric pressure, etc.

    I agree that the 4-strokes don't have the top end power of a 2-stroke but you can put a
    higher pitch prop on to regain some air velocity.

    Doug
    If it\'s interesting, I\'m interested

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    RE: Walbro carb model number deciphering (GX25)

    ORIGINAL: Flypaper 2

    visualize it in slow motion. When the int valve opens, there is a large vacuum and velocity in the int. manifold. When the valve closes, the air hits the back of the closed valve, builds up pressure and reflects back out the manifold as pressure.
    Yup, thats exactly how I visualized it, and I can see it working with the throttle closed, or partly closed, as then the air that "bounces" off the closed intake valve will then "back up" against the closed throttle plate, but with the throttle wide open, and no restriction, it doesn't seem like it would work.

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    RE: Walbro carb model number deciphering (GX25)

    ORIGINAL: DougT

    I called Walbro and the tech told me the carb operates off of negative pressure(vacuum) and a spring was required to return the flap to neutral.

    Doug
    So , are you saying that the 4 stroke carbs and 2 stroke carbs operate essentially opposite? That the 2 stroke carbs pump with positive "pressure" pulses and the 4 strokes pump with negative "vacuum" pulses? in a way that makes sence, since a 2-stroke has stong crankcase pressure, and I guess there is an equal amount of vacuum in the intake manifold of the 4 stroke, especially with the throttle closed. What doesn't make sence is based on this, I'm very surprised the engine ran at all, in fact, I'm surprised the 2-stroke carb would even draw fuel , let alone pump enough to run slobbering rich. Had I known this first, I probably wouldn't have even tried the 2 stroke carb, figuring there is no way it would work when hooked to a vacuum source instead of a pressure source. I am getting more confused. I wonder if the Hondas and the Ryobis are that drastically different? You can't change physics, maybe my stock Honda carb doesn't operate the same as the one on the Ryobi 4 stroke, or are you convinced ALL 4 stroke carbs operate as you described above?

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    RE: Walbro carb model number deciphering (GX25)

    Doug:
    Your right, the one I was talking about above, with the spring, is a Briggs &Stratton 4 stroke lawn mower engine.
    BBriBro:
    Your own motor proves it. Doesn't take much pressure difference to move that rubber diaphram back and forth. As long as it pumps enough to keep the engine happy at full throttle. Another thought. At part throttle, there is less air going through the carb, therefore less pumping pressure. but then the engine requires less fuel, so it is not needed. Make sense?
    Gord
    Dreamed I was a muffler. Woke up exhausted.

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    RE: Walbro carb model number deciphering (GX25)

    ORIGINAL: Flypaper 2

    Your own motor proves it. Doesn't take much pressure difference to move that rubber diaphram back and forth. As long as it pumps enough to keep the engine happy at full throttle. Another thought. At part throttle, there is less air going through the carb, therefore less pumping pressure. but then the engine requires less fuel, so it is not needed. Make sense?
    yeah, I follow you, I was in fact getting too much fuel at part throttle, but I think my problem is that I was over carbureted, since I couldn't get the idle and and the transition good at the same time, it was one or the other. I never did feel that it was starving for fuel, so I guess the pump is not an issue.

  19. #19
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    RE: Walbro carb model number deciphering (GX25)

    I'm not sure if all 4-stroke carbs operate the same. That's what the Walbro tech told me and I've never seen a pump diaphram spring in a carb except on this engine. I am considering T-ing the crancase vent and using one of the sides to operate a +presure 2-stroke carb that way there would be unlimited possibilities. I was told that a WT456 carb would work and had double needles, which it does, just haven't got around to making an adapter carb spacer as the pressure holes aren't the same pattern.
    If it\'s interesting, I\'m interested

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    RE: Walbro carb model number deciphering (GX25)

    ORIGINAL: DougT

    I am considering T-ing the crancase vent and using one of the sides to operate a +presure 2-stroke carb that way there would be unlimited possibilities. I was told that a WT456 carb would work and had double needles, which it does, just haven't got around to making an adapter carb spacer.
    I like that idea, sounds good. But with the crankcase vent being fairly large, (compared to a pulse hole) Do you think the pressure would just vent by taking the path of least resistance, and not operating the carb? I guess the diaphragms are real sensitive and don't take much pressure, but I'm not sure it it would work with it "open" to the atmosphere, might take a check valve in there too. Why do you need a new carb spacer? I'm assuming the crankcase vent on the Ryobi is external, in which case you will need to connect the carb using a piece of tubing, rather than holes lining up in the mount.

  21. #21
    DougT's Avatar
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    RE: Walbro carb model number deciphering (GX25)

    I would use a converter carb spacer if I use the different 4-stroke carb and use the intake pusles because the holes don't line up.

    If I use a 2-stroke carb I would tap the case/carb and use a tube.
    If it\'s interesting, I\'m interested

  22. #22
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    RE: Walbro carb model number deciphering (GX25)

    Hi Friends
    we have convert saito 150 and saito 180 fom nitro to gas. We got same rpm as nitro fuel. We make all adaptations, for walbro carb, hall sensor, and the micro magneto that activates the hall. Te ignition is auto Advance and its more eficientry than the saito ignition, thats why gives same rpm wit nitro and gas

    saito 180 with master prop clasic series 18-8 84000 rpm gas and nitro


    http://www.amderc.com/nitrogas/index.htm
    Scale Team Mexico
    Special Tuned Engines (STengines)

  23. #23

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    RE: Walbro carb model number deciphering (GX25)


    ORIGINAL: Maximiliano Galvez

    Hi Friends
    we have convert saito 150 and saito 180 fom nitro to gas. We got same rpm as nitro fuel. We make all adaptations, for walbro carb, hall sensor, and the micro magneto that activates the hall. Te ignition is auto Advance and its more eficientry than the saito ignition, thats why gives same rpm wit nitro and gas

    saito 180 with master prop clasic series 18-8 84000 rpm gas and nitro


    http://www.amderc.com/nitrogas/index.htm
    Scale Team Mexico
    How about a video??
    Fly and let fly

  24. #24
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    RE: Walbro carb model number deciphering (GX25)

    ORIGINAL: Maximiliano Galvez

    Hi Friends
    we have convert saito 150 and saito 180 fom nitro to gas. We got same rpm as nitro fuel. We make all adaptations, for walbro carb, hall sensor, and the micro magneto that activates the hall. Te ignition is auto Advance and its more eficientry than the saito ignition, thats why gives same rpm wit nitro and gas

    saito 180 with master prop clasic series 18-8 84000 rpm gas and nitro


    http://www.amderc.com/nitrogas/index.htm
    Scale Team Mexico
    Show us some pictures of your Walbro adapter

    What brand ignition are you using
    Attended the CutFinger Institute of DirtNap University for years but never did graduate....
    Recipient, Mangledhand award August 2008
    Club Saito Member #7
    Original AMA #31261

  25. #25
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    RE: Walbro carb model number deciphering (GX25)

    Dear Friends
    Sorry about not get a fast response.
    this are some pictures about the convesrions

    saito
    http://amderc.com/saitoconv.htm

    os

    http://amderc.com/osbgxconv.htm

    this is the link
    http://amderc.com/conversion.htm

    Imake a very big mistake not giving the correct linto to the english page of the nitro to gas conversion I hope it be solved in the next week
    Best regards
    Maximiliano
    Special Tuned Engines (STengines)


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