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carburettor and metering needles

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    carburettor and metering needles

    Hi- when trying to tune the carbs on my 74 mk2 with standard 175 CD carbs I've found that that when lifting the air valves by a 1/4" the engine slows down slightly, indicating (I believe) a weak mixture. In response I have turned the metering needles clockwise (richer) but having turned as far as possible I'm still getting slight 'weak mix' symptoms. I changed the needles a little while back, having bought from one of the main stag parts dealers, but remember thinking the new ones were slightly longer. Is it possible the reason I can't get the mix any richer is because I have 'different' needles?

    As a slight aside (possibly), when synchronising the carbs with an air flow meter I can get the flow quite similar but the idle is more erratic on one of them when the meter is on the inlet.

    The cars running reasonably well but I can't help but think it could possibly be better.
    Any thoughts greatly appreciated.

    #2
    1/4” lift seems a bit much in my experience, you just need to lift them a tiny bit..

    On the needles there are 2 types commonly fitted, the standard B1AQ and the slightly richer at idle B1BF, you can see the type stamped into the needle if you get the piston out and pull the needle against the spring, no need to remove it. They are always the same length.
    Terry Hunt, Wilmington Delaware

    www.terryhunt.co.uk

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      #3
      Thanks Terry. It says 1/4” in the manual?

      Comment


        #4
        Originally posted by Pete 21 View Post
        Thanks Terry. It says 1/4” in the manual?
        It actually says 0.8mm or 0.31Inch in both ROM's .. I think there was a conversion error there as 0.8mm is 0.031 inch.. Others may chime in but 1/32 inch (about 1mm) is what I always went for.. which incidentally is what an SU lifting pin does..

        I just found this at Burlen.. it also says 1/32 inch.

        http://zenithcarb.co.uk/zencdcdsseri...__store=zenith
        Last edited by trunt; 23 June 2022, 23:48.
        Terry Hunt, Wilmington Delaware

        www.terryhunt.co.uk

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