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Weber carb problem?

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  • #16
    Re: Weber carb problem?

    Originally posted by AdminUser View Post
    Its a long long time since I had anything to do with this type of carb. After you have done the usual checks - start with ignition (sounds like you have done that, but do go back over it) then check valve clearances - if it was running ok before then this is unlikely to be the cause. I iwll put the instructions here for anyone who hasnt had a play with these carbs before - but dont forget I may be wrong about stuff - my brain is turning to mush as I get older.

    Disconnect any linkage joining the pair together - take this opportunity to operate the throttle and check it isn't sticking - its a common problem that either the linkage or something is sticking - this is most likely the case here - its tightened up too much!
    Close the air bypass screws - one for each barrel
    Set the idle speed screw so it just contacts the the throttle lever, then give it another 1/2 turn max - same all round
    Screw in the mixture screw so it just lightly seats, then undo it one full turn on both barrels
    try to start engine - may be rough but keep going
    Get the best idle you can wit the mixture screws (not idle speed screws). Do the left barrel on each carb first then go back and do the right. Turn in then back off as the tickover gets more lumpy.
    Check synchronisation using your favoured method (i have a gauge) and adjust with the idle speed screws accordingly - bring the volume down rather than up.
    Adjust mixture as required - may need to keep going round in a cycle to get it smooth.
    If the engine wont start then it may be the jet is wrong. If the mixture screw is about 1/2 turn out then the jet is to rich and you need to go down one size, if its out more than 11/2 turns go up one size.

    My money is on the linkage being too tight and binding somewhere and one carb not operating as a result
    As per above but before doing any of this do as Hugh says. Give both carbs a thorough clean.
    If these are the standard carbs fitted by factory then they come with the cold start device (choke to the rest of us). Check, then double check that this is working correctly. It is effectively a carb within a carb so if it is not closing properly (little gears open and close an internal sprung valve) then you will be over fuelling and this will make the soot you describe. A common modification was to use a deletion kit to remove this. If this has been done, then again check it has been done properly.
    The Weber DCOE has an idle circuit. The idle circuit has VERY thin passages. Make sure this is not blocked. If it is blocked then NO fuel will get into the engine at idle. Another issue you identify. The idle jets are the 2 on the top, under the little round cover, closest to the engine.
    The butterflies should be virtually closed at idle, letting in hardly any air. The idle circuit takes over providing the air-fuel mixture. Carb cleaner can be very effective at cleaning these tiny passage ways.
    Often people who do not understand these carbs will try to overcome the blocked idle circuit by opening the butterflies. This then creates loads of other problems as you are now exposing the emusion tube holes which control the off idle characteristics. This will identify itself as rich mixture which will also give you the soot.
    Float height is also a very important factor. Check this against the manual but usually set to 8.5mm closed, 15mm open giving a stroke of 6.5mm.
    Hope this helps.
    Last edited by jon_cox; 04-04-18, 01:28 PM.
    Mk2 SWB Marina Roadster with a 1800 Fiat Twin Cam engine and 5 speed Abarth gearbox built in 1987
    - I have no idea what I am talking about........ but my advice is always free! -

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    • #17
      Re: Weber carb problem?

      Wow, thanks guys. Lots to go with there.
      I am going on a short break so will not have any tinker time till later next week. I will defo check the linkage and the choke mech.
      I don't use the choke to start the car as I was told not to and use the 5 pumps of gas method instead. Could still be stuck though as history unknown.
      I have checked the float level and it is as mentioned above.
      I don't think there is a diaphragm in there anywhere.
      Don't think my carbs have an air bypass screws. Layout is as below.

      Carb.jpg

      Could easily be wrong though
      Attached Files
      Stumbling around in the dark
      Marina based SWB roadster
      Alfa Romeo 1750 with 5 speed box

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      • #18
        Re: Weber carb problem?

        Update, today I disconnected the linkages to check for sticking shaft and all ok, no binding anywhere. Before I removed the carb I read this;



        Thought it was worth a try before I got into a teardown.

        I did as described re balance and low and behold they were WAY out. The rear carb had the progression holes completely obscured. I adjusted until both holes were the same, about 1/2 a hole covered in both carbs.

        Result, the rear carb is now doing its bit and idle is a lot smoother. I am not completely there yet as the left barrell on the front carb ( No2 Cylinder) is now not doing much, it was before. A little more adjustment required I feel.

        So I did have 1+2 firing and 3+4 not and now 1+3+4 going good and 2 not so good.

        I am going to run her for a wee while and revisit this after a few miles as she has not really been anywhere yet.

        Thanks guys,

        Derek
        Stumbling around in the dark
        Marina based SWB roadster
        Alfa Romeo 1750 with 5 speed box

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        • #19
          Re: Weber carb problem?

          well done - keep going - nearly there! When you have finished perhaps a write up for pitstop???

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