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Thread: Carb overflowing ?

  1. #31
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    Quote Originally Posted by W5R View Post
    if his carb is overflowing, wouldnt that mean the seat isnt big enough to take all the fuel it is trying to feed it?
    Nope, force=pressure x area
    What your trying to control is the balance of forces between the float arm closing the needle and the fuel pressure trying to open it. Either reduce the pressure with a bypass etc or reduce the area with the correct needle and seat for the application.

  2. #32
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    It would almost be the opposite off how we set pop-off pressure...
    Usually we have to get the pop off lower hence lighter spring bigger needle and seat...
    On these carbs meant for gravity feed... we are trying to raise the pop off pressure... again smaller needle and seat...

    FYI... on some FCR MX carbs the seat is pressed in... you may not be able to easlily remove it and replace...
    So much easier to keep it the same and run a dellorto pump with pressure relief built in...

  3. #33
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    Needs a 0.7 mm needle and seat

  4. #34
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    Quote Originally Posted by Yakattack View Post
    It would almost be the opposite off how we set pop-off pressure...
    Usually we have to get the pop off lower hence lighter spring bigger needle and seat...
    On these carbs meant for gravity feed... we are trying to raise the pop off pressure... again smaller needle and seat...

    FYI... on some FCR MX carbs the seat is pressed in... you may not be able to easlily remove it and replace...
    So much easier to keep it the same and run a dellorto pump with pressure relief built in...
    Sorry for not updating sooner ! The dellorto pump worked great . Thanks for the help .

  5. #35
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    Glad to hear it!!!
    The pumps are a little pricy but work great...

  6. #36
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    Sjg22 - is the .7mm needle what's recommended for 38mm carb? I am running a YZ250, and carb is also spewing a little fuel. Engine sounds good but I think its a little lean.
    I have fuel pump pumping to a can that drains to carb. Can has overflow outlet on top with returnline back to fuel tank. Got a part number for proper needle size? Thanks

  7. #37
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    Quote Originally Posted by cking View Post
    Sjg22 - is the .7mm needle what's recommended for 38mm carb? I am running a YZ250, and carb is also spewing a little fuel. Engine sounds good but I think its a little lean.
    I have fuel pump pumping to a can that drains to carb. Can has overflow outlet on top with returnline back to fuel tank. Got a part number for proper needle size? Thanks
    I know nothing about your carbs, I have no idea about needle settings or what needles or what pumps or anything. I do know about air/fuel ratios.

    You said "a little lean", and I don't understand what that means! In my world, "a little lean" means less-than-perfect, it may mean something else to you. In my world "a little lean" means the chance of detonation! My engine doesn't sound "good" when it's getting detonation. It slows down too.

    "Too lean", would you rather run it rich? I don't understand why, if it's not getting detonation, if it's not slowing down, obviously it's not "too lean", not in my definition of the phrase.
    From the desk of Al Nunley 512-630-6215
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  8. #38
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    Al all I have done is fired motor up on stand. When I first fired it up it took a minute before it wouldn't bog a little with the choke off. After it warmed up, I took choke off and it sounded good with no hesitation. This is a Mikuni 38mm slide carb so I really can't adjust carb on the track other than maybe air screw.

  9. #39
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    I have found that ya can't really tune them unless on the track...
    i have tuned mine using a AFR, rpm and timing... still on the track tuning is best...

    i use a YZ250f and tuned the factory FCR MX carb and a Mikuni SBN38...
    usually on the stand and when not warmed up they will bog a bit... not a lot but some...

    if if you get to test and tune... run a couple good laps and do a shut down on a new plug...
    then read it.... that is probably the easiest way to see where your running at for rich or lean...

    i know now on both the FCR and SBN I had to richen the pilot jet and lean the main jet...
    along with pop off on the SBN, and on the FCR I used a mini washer to fit under the needle to adjust it a half a clip to get best lap times...

    i like the FCR carb because of the TPS and 3D mapping on the computer to dail things in...

  10. #40
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    order a pump that they use on a wankel with a built in regulator. they are 52 bucks u have to get them from renntec as they were special made for the wankel. I use them on 450 an have no problems

  11. #41
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    Thanks Yak. I am new to tuning these slide carbs.
    So if carb is spewing fuel, a smaller needle or raising the needle up with washer will help overflow issue?
    This would also be raising the pop off pressure correct ? This would all cause the carb to run richer also correct? Thanks for any help!
    I bought this carb off eBay and I think the settings are..
    Main Jet: 170
    Pilot Jet: 55
    Throttle Needle: 48-DDJ

  12. #42
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    I believe the 2 stroke carb is different than the 4 stroke carb...
    The main needle controls fuel from 1/4-3/4 throttle raising it leans lowering it richens...
    Has nothing to do with overflowing...

    First get a manual and see what the float level should be set at...

    The needle and seat is in the float bowl... the float is hooked on the needle and open and closes by pressing on the seat... sometimes the needle gets worn and needs to be replaced...

    I would do this before anything...

    After that like Oldloco said a pump from a wankel, or I just used a dellorto pump from a shifter Kart

    Also has a built in regulator...

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