mixture screws on OEM type 4 1700cc carbs?

Discussion in 'Mech Tech' started by grub, Jun 24, 2018.

  1. Hi all,

    My bus runs ok, but running a bit lean & a bit low on power when warm.

    tried everything else, and all looks ok, so thinking that it may just be running a little lean.

    is there just a simple mixture screw that I can adjust a little richer - didn't want to mess with anything else.

    Idles fine when cold & war.

    Standard PDSIT carbs which have been rebuilt.

    Thanks
     
  2. You can’t adjust the main mixture for running at speed, only on idle. But if you are able check the idle mixture, do so. They say turn the screw out to 2 1/2 turns, (this is the smaller of the two on the top of the left hand carb) but mine was massively rich at this setting and needed turning in to about 1 turn. I’ve got an ancient Gunsons Gastester. Although the numbers are hopelessly inaccurate, it does tell which way to go to reduce the CO. Mine had been at 9.5% when I took the MOT. It failed as the limit is 4.5% but I got it down to 1.9% and it runs great now.

    Since rebuilding have you accurately set up the throttle stops on each carb? It needs each lower butterfly open to just 0.1 mm at rest, and a minor adustment when running to synchronise using the right hand threaded adjustment rod. It needs a carb balancer for this to check the flows are equal.
     
  3. when you have rebuilt your carbs did you change the throttle bushes? I have not yet seen any with bushes not worn - they let false air past throttle causing all sorts of issues most have problem diagnosing.
     
    snotty and Lasty like this.
  4. also there are 3 mixture screws, idle on the top left carb (idle setting) and one each in throttle plate for main jets (throttle partly or fully open)
     
  5. I did not rebuild the carbs myself - only going on what the previous owner told me.

    Can anyone show me where the main mixture screws are located (i.e. non-idle).

    Sorry for my lack of knowledge, but carbs scare me!

    Can I just try adjusting these to richen the mixture a little?

    I don't have a tester, but the plugs are white, so defo running too lean & lacking power at higher revs. If you feather it, it runs ok, the minute you bury the throttle pedal, it seems to be holding back.

    thanks
     
  6. marked as volume mixture screw
    [​IMG]
     
  7. The mixture screws adjust the mixture strength through most of the lower throttle range, it increases or decreases the fuel added to the main fuel circuit.
    The adjustment is 1.5 turns out from when the engine starts to falter when screwing in the mixture screw, but aim for the smoothest running position around that point.
     
    andyv likes this.
  8. Poptop2

    Poptop2 Administrator

    Just a note that may help others reading this for answers

    A few things people tend to overlook when setting up the carb. One is compression it is important each cylinder is in the appropriate compression range and that both valves are fully closed at the top of the stroke, so as the fuel burns correctly, this leads on to checking tappet settings ( again!) and the ignition timing being correct. Warn or badly seated valves, rings or cracked heads will always mess up the running of an engine and everyone automatically thinks it's the carb.

    The HT leads, plugs, points, rotor arm, dizzy cap and condensor should all be in good condition, with correct points gap set, and there should be no leaking in of air from the exhaust manifold, or more commonly, the spindle bushes, as mentioned above by Nobody. Any of these not functioning properly can sometimes make people think ' carb ' too. So check these things first and then tuning the carb will be the icing on the cake.
     
    Last edited: Jun 26, 2018
    Valveandy likes this.

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