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Thread: SU carb floats front and rear question

  1. #1
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    Default SU carb floats front and rear question

    Hello all,
    Spent the evening helping a buddy try to get his 72 240Z running better. The front carb was messed up and pumping fuel into the carb.
    We finally got the level set in it pretty good
    They carbs were recently rebuilt by a commercial shop that said they knew what they were doing. Not really sure about that.
    I know the f and r carbs have subtle differences and the rebuild kits do also. Ie the float needle and seat valve has different length stems coming out if them. One question I have- are the front and rear floats different? I am thinking the metal tab that turns off the fuel might have a different curvature to compensate for the needle difference. We only had the front one apart. It had the long needle but was hard to get the float to smoothly work as designed. I'm thinking the guy that rebuilt it may have switched the floats.
    Also when setting the mixture, I had to turn the jets almost all the way up to get the Colortune to just begin to stop being orange.
    I set the float levels so the the fuel was about at 10mm below the bridge in the carb.
    If all else fails we may lower the needles out of the Pistons a little to see if that helps.

    As usual, thank you very much for any tips or advice. All is welcomed and appreciated.
    John

  2. #2
    Registered User Stanley's Avatar
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    Floats are the same front and rear. The lid on the front carb has 2 mm longer "ears" and uses the longer float valve. The fuel level in the front bowl should be 2mm lower than the rear carb but you can disregard this if you're setting the levels by fuel height at the nozzle; it should be the same level at the nozzles of both carbs.

    I use sight glass to set fuel levels, but it's still a big hassle, worth it though to get it right.

    If the nozzles are worn, or if the carb rebuilder installed new stock nozzles but used something other than stock N27 needles, it might cause a stock engine to run rich.
    Also sticking chokes that don't go down when the choke is released.

    Sometimes a float will hit the side on the bowl (due to bad adjustment or manufacturing defect) and not go all the way up to shut off the valve; fuel can run out of the carb or the vent when that happens. A little sanding on the side of the float fixed mine.

    FSM table graphs mix nut settings for various altitude and temperature for stock engine.

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    Hi Stanley and thank you for the info. I'll know what to look for.
    Where is the marking on the needles indicating which ones? We didn't take them out as they were set properly and we didn't have any others to put in. I tried to find a chart listing different needles that are avail but could not find one.
    I remember back in the 70s there was a good availability and we still would raise the needles to get more fuel for our race engines.
    simple/cheap solution.
    Thanks again,
    John

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