1990 FLT Keihin CV carb overflows

Discussion in 'Engine, Fuel and Exhaust' started by toyz, Jul 3, 2012.

  1. toyz

    toyz Member

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    Replaced all essential parts in 1990 Keihin CV carb, but didn't solve the overflow.

    With float bowl off, the float (checked for leaks), the new float valve shuts the supply correctly; float level set according to manual at 18mm, later raised to 20mm. The float chamber drain seals well.

    Mounting the float bowl again causes overflow. Using transparent overflow tube, it appears that the level reaches the tip of the overflow tube, causing the overflow.

    Can't find the cause for the float needle not shutting off supply with chamber installed, unless float is restricted in movement with chamber on, but doesn't look that way.

    Any suggestions other than the usual? (new needle valve on clean seat, new gaskets, o-rings, accelerator pump diaphram, etc.) Any way of checking float movement with chamber installed?
     
  2. SledDog

    SledDog Senior Member Staff Member Moderator

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    Is the float bad? It may be getting some gas in it causing it not to rise completely. That would stop it from closing the needle valve.

    Double check the needle valve to make sure it's not stuck. Also I have found some of the "generic" rebuild kits needle valves don't work very well. Is the needle valve 3 sided or 4 sided? The 4 sided ones work better and give a better fit in older CV carbs.

    You could use water to flow into the needle valve (tube connected to the normal gas supply) with the bowl off to check the operation of the float. I've done that a few time to check operation. Since you will be checking the needle valve and float only you can let it dry of flush it with carb clean after the test.
     
    Last edited: Jul 3, 2012
  3. toyz

    toyz Member

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    Float checked for leaks, submersed for minutes, definitely ok. New 4-sided needle, shuts correctly when testing without chamber installed. Checked indeed as you suggested with water (carb on the bench), with and without chamber installed, with same results as on bike: actuating float by hand is OK, with chamber installed is NOK.

    Can it be that float gets stuck with valve open, when chamber is attached, and that such problem occurs from one day to another?

    With the correct settings for float level (as per manual), what is the actual fuel height measured from the chamber/carb interface? With the transparent tube used, it looks like level gets up to overflow tube, which is approx. 9mm (about 1/3") above chamber gasket level. I would assume that normal level would be somewhat below the float chamber gasket, which seems to correspond with the float height as per the workshop manual.

    Thanks for your reply and suggestions.
     
  4. SledDog

    SledDog Senior Member Staff Member Moderator

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    Ok... Let's see. It's possible the clip on the needle valve is causing issues with the float hanging it up. You can try and reverse the position, rotate it 180 degrees. Or use the clip from the old needle valve. Does the float float when you put it in water?

    Even though you have set the float per the manual, you may need to adjust it down a little. Have you tried to do that?

    Or try installing the old needle valve and clip, unless it's bad. Then see what happens.

    I re-read your first post again. This was a rebuild for the same issue you are having now... Have you checked where the needle valve seats to make sure there isn't something stuck in there?

    Verify the O ring on the bowl is not moving out of the groove when you install it so it does not hit the float or needle valve.

    Can you take a few picture and post 'em? Maybe an actual look at the carb's insides will help us track down the issue.
     
  5. tourbox

    tourbox Senior Member

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    On the new float needle there should be a small pin in the center of it.Located just below the spring catch where it attaches to the float. That pin is spring loaded and you should be able to push it down and it should spring back up.I usually use an exacto knife to check it. I have free'd them with carb. cleaner. I think it acts as a buffer for the float.Maybe someone here knows for sure?
    tourbox
     
  6. fin_676

    fin_676 Experienced Member Staff Member Moderator Contributor

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    Float bowl should be about 3/4 full when checked with a clear tube
    some care is required when setting float height that the height is set with the float needle just seated and the spring plunger on the float needle not depressed
    Check the tip of the float needle for any nicks marks or bubbles will prevent proper sealing under pressure from the float
    The float will only apply light pressure on the needle tip and it may be a bad tip as you can get a lot more pressure using your hand

    Brian
     
  7. toyz

    toyz Member

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    OK, ongoing experimenting results:
    1. float movement in chamber is OK: shaking carb reveals movement
    2. different combinations (needle, needle spring, float height settings), result = overflow
    3. float height per manual, shows needle closing with float center about 4mm below chamber/bowl seal, and about 1mm with needle internal spring compressed (see pics). So Brian, fuel level in bowl is indeed below seal, but higher than the 3/4 full.
    4. in my opinion the spring loaded needle pin, prevents the float to push to hard on the delicate needle seal, caused by vibration and bouncing fuel levels. Pics included show closed needle/uncompressed spring and closed needle/compressed spring.
    5. determining the amount of overflow was tested (with water) by waiting from needle closing to reaching bowl seal, which was between 5 and 10 minutes.
    6. it must be the needle seat causing the (small) problem and it may have been there for a while. Because I always shut the fuel supply I may have never noticed it, as the fuel leakage passed the closed needle is possibly slower than the consumption of the running engine. And yes, it seems that closing the needle by pushing the float by hand, is significant more force than the float itself can establish.
    7. as the specific gravity of fuel is only 72% of that of water, the float force on the needle will be about 30% less when using fuel instead of water, so leakage bypassing the needle/seat is expected a bit higher with fuel than with water.
    8. because the seat cannot be replaced, I'll try operating it for a while and monitor.
    Thanks for suggestions.
     

    Attached Files:

  8. TQuentin1

    TQuentin1 Well-Known Member Staff Member Moderator

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    Did you clean out the tank? Just want to make sure you are not getting a bunch of trash out of the tank that is keeping the needle from seating. Doesn't take much!!

    TQ
     
  9. fin_676

    fin_676 Experienced Member Staff Member Moderator Contributor

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    It may be that the valve seat is the problem area but it only takes the smallest amount of grit to prevent the fuel from weeping past the needle tip so a final check may be worth doing

    The long term solution may be to replace the carb i personally think the mikuni hsr is a much better carb and have them fitted to the 2 big twins the float needle comes with a new seat so the whole lot is replaced when needed

    Brian