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Carburettor Hoses

Discussion in 'Technical and Troubleshooting Torque' started by jd, Mar 1, 2008.

  1. Originally only wanted to replace the fuel lines but like all good bike designs the easiest way to do this was by removing the carbs. In doing so however I noticed the things circled in red in the picture which look like they're supposed to have hoses attached, but didn't (though the bike seemed to be running fine other than a slightly troublesome idle).


    Now I know where the fuel line and vacuum line for the fuel tap are and these aren't them. So what I want to know is what are they and more importantly, what are they supposed to be connected to?
  2. Should be connected to breather hoses, which possibly, because of the size of the tube, suggests that they went to the airbox.

    However they could have just been vented directly to atmosphere.

    The idea is to equalise the pressure above the fuel in the float bowl to that which is likely to be in the carb itself. This is very important in ram-air intake systems where the pressurised venturi will not be able to suck up fuel from the non-pressurised float bowl.

    Note that each breather tube is paired to the adjacent carb.


    Trevor G
  3. Is this something that's done on other bikes? Must admit I never thought of that possibility but it would make sense - there's certainly no spare hose attachment points on the airbox and it would also explain why the workshop manual doesn't show any hoses coming from those bits of the carbs (does show fuel and vacuum lines).
  4. All carbs have a vent to atmosphere above the fuel in the float bowl. Some use a very small hole halfway up the side of the throttle body, others use a pipe which sticks up in the float bowl area which also acts as an overflow, while still others use a method similar to what you have shown.

    Sometimes one of the 2 air passages in the venturi (as shown in your pic) will have an outlet into the float bowl area. One of those is for the cold start enrichening device, while the other will be an air bypass or "balance" for the idle/pilot mixture circuit, or the needle jet/emulsion tube circuit.

    The correct way is to run hoses which end up innocently out of the way somewhere. Some are fitted like overflows and end up down near the back wheel (a silly place for an overflow!) or you could just stick it up under the tank. The important thing is to put a hose of some sort on, to reduce the entry of dirt and dust.


    Trevor G
  5. Yeah I thought the open tubes seemed a little bit vulnerable where they are, should be easy enough to run a couple of lengths of hose to somewhere.
    Thanks for the help :grin: .