Rear carb flooding from overflow | The Z Club of Great Britain

Rear carb flooding from overflow

Discussion in 'S30 (240Z,260Z,280Z) Engine' started by dav118118, Oct 13, 2017.

  1. dav118118

    Z Club Member

    Hi there,

    I have a little problem. My rear carb is over fuelling from the overflow and hence the jet too. I've had the float bowl apart and found a little debris stuck around the float valve causing it both to stay open and also not to allow full fuelling.

    I have cleaned the valve out but it is still over fuelling out of the overflow. The front carb is fine. I did check the float heights when I had the carb off and noticed the rear float was a little sticky when fuel was being trickled into the inlet pipe. I have cleaned the float pin and thought it was fine but obviously not. I think there may be a problem with the weighting of the float as it appears a little reluctant to rise properly when fuel is entering the float chamber.

    I have looked at the stay up floats from burlen (su) and wonder if these will fit the carbs.

    Has anyone on here experienced this or have used the burlen stay up floats?

    My carbs btw are round top hitachi su's 3 screw variants. They have been vapour blasted and therefore immaculate.

    Any feedback would be most appreciated

    Thanks, luke
  2. jonbills

    Z Club Member

    Sounds like your float may have a leak Luke - not sure if the Burlen ones fit, but I’d think it should be possible to find the leak and seal it again.
  3. dav118118

    Z Club Member

    Hi Jon,

    Yes did wonder that but I've shock the float and cannot hear / feel any fuel in it
  4. Rob Gaskin

    Rob Gaskin Membership Secretary Staff Member

    Had flooding in the rear carb but a needle valve clean cured it.

    Try swapping the floats to see if the rear one is faulty.
  5. dav118118

    Z Club Member

    Hi rob,
    Yes I saw your roadside repair post and did the same myself but thinking it's a float issue myself. I will swap the floats over as an experiment to rule out that issue but did read that the tangs on the two floats are different to compensate for the level in the bowls, hence the taller valve in the front bowl. But as for a test only this should be ok

  6. dav118118

    Z Club Member

    Btw my needles and valves are brand new with no wear so it's not a seating issue!
  7. Jimbo

    Jimbo 1978 260z in yellow

    it can be as ive fitted new ones and had them leak in the past.
    if they are new send them back and ask for more.
  8. nospark

    nospark New Forum User

    I had a problem with the rear carb frequently flooding. Solved it by dismantling the metal fuel rail and blowing it through with compressed air (both directions).

    There was crud within the rail that often caused the needle valve to stick open. Also if the fuel return part of the rail is blocked there is nowhere for the excess fuel to go.

    Then fitted an inline fuel filter between the fuel rail and rear carb
  9. dav118118

    Z Club Member

    Ok so seem to have got to the bottom of it. I blew all air lines through, but no crud was present. I then noticed that the rear carb float was touching the top of the float bowl before seating and closing the valve. The float heights were set and thought the valve was seating.

    Anyway the rear carb was fitted with the smaller of the two valves, which would seem correct to the theory of the rear carb level being 2mm lower than the front carb. Nevertheless I tried the spare longer valve as per the front and all appears fine now, not a drop from the overflow pipe or otherwise.

    Don't know if anyone else experience the valve height issue before.

    Thanks for the advice of all those that have it, most appreciated
  10. toopy

    Z Club Member

    Ive never understood the reasoning for the different size valves, the carbs are slightly different heights yes, but the angle is the same no?, so if you think about it, one carb could be 2 inches lower/higher, but if they are both at the same angle the fuel levels are identical in both.

    Or am i missing the obvious?!
  11. Jimbo

    Jimbo 1978 260z in yellow

    its best not to think about it, i bought a rebuild kit for my SUs and i changed the valves as a matter of course and one of them started leaking.
    i ended up putting the old one back in :banghead:
    if you look on the su website theres quite a few different types of float tops so i would imagine that there could well be different types of needle lengths.
    you might even find a shorter length needle would fit into a longer valve (maybe different manufacturer for replacement parts).
    so to use the shorter needles would require replacement of the valve seats too.

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