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Discussion Curious carburetor question

Discussion in 'Yamaha 250cc Singles' started by GeorgeT, Sep 24, 2021.

  1. GeorgeT

    GeorgeT Active Member

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    Has anybody ever replaced the double carburetor arrangement on, e.g., Yamaha SRX250 with two say PZ30 or other slide carburetors + short pod filters? I am thinking of doing this while rebuilding the standard arrangement that will take some time as the rebuild kit is coming from the US?
     
  2. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Sounds perfectly doable. I would use two small Mikuni VM or make a new manifold and use one carb.
     
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  3. DannoXYZ

    DannoXYZ Well-Known Member

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    You can do it, but it'll make absolutely zero difference in output. On small single/twin bikes, air-box size, air-filter size, carb-venturi sizes aren't constricted and limited like trying to cram 4x of those things on larger 4-cylinder bikes. I measured airbox on my EX250, it's same size as on my CBR600RR!!! That bike has 5x power and 5x air-flow, yet it gets by with same size airbox as EX250. Meaning, EX250 has zero flow-restrictions with factory airbox.

    Finally, 25-years of EX250 racers have gone through tonnes of trial & error testing on what works and what doesn't. Racers are cheating bastards who'll try anything and everything to get an unfair advantage. They'll do what works and improves their changes even if it's illegal by rulebook. So... millions of them have tried to improve intake-airflow by removing airbox and going with pods. Nope, didn't work, that's why not single one uses pod-filters on their race EX250, even though it's allowed by rules. And backed up by thousands of dyno-tests.
     
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  4. 2valve

    2valve Well-Known Member

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    Have seen a single Dellorto carby with a custom made inlet manifold fitted to a SRX 250. It looked pretty neat , but no doubt a lot work went into it to make it fit / work. Cant remember if the O.E airbox was used or not.

    Pretty sure i have some picture's of it on the old PC.

    You can purchase a comp carby set up out of Japan , but the price was eye watering and the way it looked , no option for a air filter.
    The original carby work's pretty well when set up properly , but like everything , they do need rebuilding when they get tired / worn.
     
  5. GeorgeT

    GeorgeT Active Member

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    Are you suggesting by "changing or making a new manifold" I still send the fuel-air mixture through both inlets or should I block off one inlet?
     
  6. GeorgeT

    GeorgeT Active Member

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    I have checked out The Mikuni VM26 and feel 2 150cc might do the job albeit the mixture screws would probably need to be screwed in more than the usual given they would be supplying ~20% too rich at the normal setting. If they actually fit directly it can be done for around AU$150 by the look of things. Maybe a little more if I need a one-to-two throttle cable arrangement?
     
  7. 2valve

    2valve Well-Known Member

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    The custom manifold that I mentioned still uses both inlets to a single Dellorto carby.
    If I can find the pictures of this set up , I'll post what I have.
     
  8. GeorgeT

    GeorgeT Active Member

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    I think I have found a Mikuni that I could use two of that may actually fit nicely (eBay code 233760603927). I just need to confirm the dimensions. The filter end of the one on Ebay is 50mm od, the cylinder head end is 34mm, while the length between rubber mounts is ~85mm. They have a lever choke that is sort of accessible. I would need a one-to-two throttle cable to operate them. I would be interested in the Dellorto arrangement. According to the math, the single Dellorto would be a 32mm carburetor as far as my calculation goes.
     
  9. 2valve

    2valve Well-Known Member

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    I've found the pictures of the single carby set up. I'll transfer them over tonight and post them for you.
     
  10. 2valve

    2valve Well-Known Member

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    Sorry for the late reply.
    Here is the picture of the single carby set up , this is the best picture i could find , but at'least it may help you.



    IMG_20170310_103055501.jpg
     
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  11. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    That would be on a race bike and most likely a 350cc barrel and head given the oil feed into the head. That doesnt work on the 250 unless you do stupid stuff (like I did) and machine out the casses to take a 350 piston, barrel etc.
    I really dont see any gain going to a single or even twin carby's. The Factory setup is perfectly good if cleaned and set correctly.
    The SRX isnt a powerhouse (unless you do the 350 conversion then it would be a slightly better non powerhouse) and is a commuter bike... light, nippy etc.
    But then who am I to talk after what I am doing to mine...
     
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  12. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    What is wrong with the standard set up? Is it worth spending $150 over fixing the original carbs?
     
  13. GeorgeT

    GeorgeT Active Member

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    That is a great idea. solves issues of adjustment beautifully by having everything easily accessible. I would definitely do that if I could get hold of the one-into-two manifold. I have attached a picture of what seems an alternative albeit a bit of a hassle for fine-tuning


    Mikuni FromChina$92 60 5.Code 132699025918jpg.jpg .
     
  14. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    I am not sure why you are waiting on a rebuild kit from the USA? Most people use a Keyster kit..and they are normally pretty easy to get.
    What needs rebuilding on the SRX Carby? You change some gaskets / oRings, the float valve and clean the rest... If the diaphragm is torn you are screwed so you just buy a 2nd hand carby hoping it is OK... you are far better off keeping the original Brass (jets) than using aftermarket ones and then set it up as per the manual... They are not a complicated carby at all...
     
  15. 2valve

    2valve Well-Known Member

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    Hi George
    Personally i'd get a rebuild kit and try that first. The factory carb's work great , they do need looking after like other part's on our bike's.
    But like any carby , keeping clean fuel through them is alway's a good thing , but your aware of this.

    The factory fuel filter in the tank has fine mesh , maybe a good idea to check that also while you have the tank off.

    Is there a issue with your stock carby set up ?.

    I have a spare carby kit if you're after one in a hurry. PM me if you are.
     

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