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CBR250RR Fuel Tap

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by Nathanael, Jun 28, 2008.

  1. Hey guys, after a day of mucking round and working out my bike wont start because it keeps getting flooded i narrowed the problem down to the fuel tap.
    When the bikes not running the diaphragm doesn't go back into place and restrict the fuel coming out. I took it apart and it a looks fine except for a little rubber washer which is acting like a one way valve. It sits on the end where you plug the vacuum hose in. I tried it without the rubber washer and it doesn't keep the fuel coming at a steady rate (it pulses with the vacuum). The washer has a little bubble in it so i flipped it over and still it keeps the diaphragm open allowing fuel to run out freely and flood the engine even when there is no vacuum on it.

    I was wondering if any one has any experience with this same problem and if i should replace the whole tap or just the diaphragm and washer etc. If so where would i be able to get my hands on one, i live on the border of NSW and VIC (Finley) bout 1.5 hours from Albury and don't really have access to Honda bike shops and the ones round here have nothing.

    Cheers Nathanael
  2. I doubt that is your real problem.

    Most fuel taps can stay on forever because it is the float needle and seat in each carb which actually keeps the fuel at the correct level in the carb.

    If you're sure your carb(s) are flooding that will also occur while you are riding, so the needles and seats will need cleaning at the very least, or replacement at the very most.

    How did you identify that it was flooding?

    When the diaphragm valve is working correctly, you check it by sucking on the vacuum line to the tap, while watching for fuel to come out of the hose to the carb.

    If fuel comes out only when you suck, the tap is working correctly.

    If you lock the end of the vacuum line against your tongue, the vacuum should remain indefinitely. If it dies away in a few seconds you most probably either have a hole in the diaphragm, or a hole in your tongue.

    Your comments on flooding suggest that you have either misdiagnosed the problem, or that your carbs actually need the real work.

    A faulty tap diaphragm usually stops fuel from flowing altogether - if you re-examine the way the diaphragm works, you will see that fuel can only flow when vacuum is applied. If the diaphragm fails in any way, fuel stops flowing.

    The other problem that can occur is if the tap shut-off valve is no longer closing because of dirt or perished rubber. Then, and only then, the fuel can bypass the valve and flood the carbs, but only if the carb needles and seats (as already explained) are dirty or worn.

    Another issue occurs when the diaphragm develops a hole which allows fuel to flow through the vacuum line into the manifold and thence into the engine. Once again, this usually won't occur unless the tap closure system is also not seating correctly

    NB A hole in the diaphragm small enough to stop the tap working can be invisible to the naked eye.


    Trevor G
  3. hey thanks for the reply, the problem is that when the vacuum is applied and then released the valve stays open and allows fuel to run with no vacuum, found the carbys were flooded by taking out the plugs and turning it over heaps of fuel came out, couldn't find the screws people talk about to drain them though. left it over night with out the plugs in it to let it evaporate and when i turn it over now it sounds like it wants to start. If thats not the problem it could be the carbys not to sure though.
  4. I'm not certain as to what the problem with your bike is but if it is a faulty fuel tap you can get one on this ebay link -

    CBR250 Parts

    I got one for my cbr250R cause my fuel still ran through the fuel lines with the tap in the 'off' position. Never caused starting issues though, it was only a trickle running out.

    Goodluck dude.
  5. thanks mate i will see what i can work out cheers
  6. Fair enough.

    Does any fuel come out of the vacuum line?

    If it does, that could be the source of the fuel flooding the engine.

    If it doesn't, then you really need to do a carb strip and cleanout, and then check the condition of the needles and seats.

    if the needles have a circumferential ring around the V groove at the bottom (the part that does the sealing or shutting off of the fuel) then they will need to be replaced.

    Sometimes just that "shellac" appearing muck which remains after fuel evaporates, or fuel is used from a dirty storage tank, is enough to cause the problem.

    Maybe even just undoing the drain screws at the bottom of each carb will be enough to clean them out.

    The real problem is not the tap, if the carbs are actually flooding via the fuel line.


    Trevor G
  7. thanks for that, i will strip them down and have a look. Hopefully its something simple like that and will be easy fixed cheers