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  • Weber carb problem?

    Hi, I have a 1750 Alfa Romeo nord engine in my Roadster. The idle is really rough and appears to be only running on cylinders 1 and 2. Seems ok once I open the throttle a fair bit.
    I have pulled the plug leads on these cylinders while running and it makes no difference to the idle. There is a good spark when I put a spare plug in the lead.
    I have also tried blocking each inlet with my hand, no difference.
    I have tried spraying starter fluid in also, no difference.
    Adjusting the mixture screws does nothing.
    Plugs on all four cylinders will eventually foul with soot and rear wheel behind exhaust outlet will get a bit sooty after a run.
    The carbs are 40 DCOE 32.

    Carbs are a still bit of Black Magic to me so any suggestions welcome.
    Stumbling around in the dark
    Marina based SWB roadster
    Alfa Romeo 1750 with 5 speed box

  • #2
    Re: Weber carb problem?

    if you have the same as mine check the rubber connections between the carbs and the engine. the rear one was split and letting air into engine. Which would run on choke but was backfiring. Plugs were black and sooty.
    I am just about to call Alfaholics who have an aluminium fix for the splitting problem

    Comment


    • #3
      Re: Weber carb problem?

      Do you use Misab plates or "O" rings. I've always used Misab on my 2l Fiat with DelOrtos without problem. It's been known for the "O" rings to get partially sucked into the inlet tract.
      Have a look at https://www.demon-tweeks.co.uk/motor...er-misab-plate

      Comment


      • #4
        Re: Weber carb problem?

        I have twin Delortos on my Alfa engined Roadster. If the float chamber valve gets stuck or is leaking the fuel overflow runs into the carb which could have the effect of making the two cylinders fed by that carb run rich (or not al all). Also the vacuum feed for the brake servo is taken from the inlet manifold by cels 3 & 4. If thats leaking it will affect the running of this cylinders although you would probably notice the effect on braking performance first. I also agree with Hugh's comment regarding the "O" rings.

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        • #5
          Re: Weber carb problem?

          Thanks for all the suggestions.

          Rubber connections are all ok
          There are only paper gaskets fitted between carb to rubber mount to manifold. Misab plates are new to me, should I be using those instead?
          I have removed the servo pipe and blocked the outlet at the manifold, good suction but no change to idle.
          I still have to take the top off the carb again to check if fuel is leaking past the float chamber valve but I have previously checked the float level and it was ok.

          The hunt goes on
          Stumbling around in the dark
          Marina based SWB roadster
          Alfa Romeo 1750 with 5 speed box

          Comment


          • #6
            Re: Weber carb problem?

            I would do a compression check to compare the four cylinders.
            Ben Caswell probably not the last word on anything here!!

            Comment


            • #7
              Re: Weber carb problem?

              Originally posted by b_caswell View Post
              I would do a compression check to compare the four cylinders.
              Will double check compressions tomorrow, hopefully, however engine was partially rebuilt approx 2000 miles ago .

              Have the Alfa engine guys fitted a fuel pressure regulator. I haven't but it was suggested on Alfabb that I should have.

              The engines with mechanical pumps have filter/regulators. If you do not have one then the pump will overpower the needle valves in the Webers and flood the engine, causing the problems that you have.

              I would have thought this would have affected both carbs, or at least the first one. My problem is with the second. I have tried temp swapping the tops / floats with needles etc. to see if the fault moved, but no.
              Stumbling around in the dark
              Marina based SWB roadster
              Alfa Romeo 1750 with 5 speed box

              Comment


              • #8
                Re: Weber carb problem?

                What pump do you currently have installed, mechanical or electrical?

                Comment


                • #9
                  Re: Weber carb problem?

                  I use a Facet solid state pump with no pressure regulator fitted with my 2 litre Alfa engine. It has been on the car now for over 20 years and works fine. From memory I think it is the road version as opposed to the fast road or competition ones which deliver a higher flow rate and pressure.

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                  • #10
                    Re: Weber carb problem?

                    Mine's a genuine Mini electrical one. Been on the car since I built it and survives two different engines and is still going strong. Shouldn't have said that, tempting fate.

                    Comment


                    • #11
                      Re: Weber carb problem?

                      Originally posted by dogoncrazy View Post
                      What pump do you currently have installed, mechanical or electrical?
                      Hi, I have the standard mechanical pump.

                      I took her out today for an " Italian tune up" and she went really well, easily going up to 70+.

                      I checked the compressions again afterwards and was supprised to find the were all around 125 PSI. I wonder if my cheap guage is ok as this seems low. I did have her on a rolling road last year and she was producing 112 bhp at the flywheel, I believe the quoted figure is 122 bhp.

                      Anyway I digress, the idle was better but still rough and plugs 1 + 2 are nice and normal looking but 3 + 4 are still sooted up

                      Still suspect an issue with the rear carb, something common between the two chokes perhaps.
                      Stumbling around in the dark
                      Marina based SWB roadster
                      Alfa Romeo 1750 with 5 speed box

                      Comment


                      • #12
                        Re: Weber carb problem?

                        In your first post you mentioned that pulling the HT leads made no difference to the idle but placing a slave spark plug gave an indication that all was ok. I experienced a similar issue with my last Cabrio and as my mate had a spare set of leads tried them. Got all six pots up and running properly. Just a thought .

                        Comment


                        • #13
                          Re: Weber carb problem?

                          Originally posted by dogoncrazy View Post
                          In your first post you mentioned that pulling the HT leads made no difference to the idle but placing a slave spark plug gave an indication that all was ok. I experienced a similar issue with my last Cabrio and as my mate had a spare set of leads tried them. Got all six pots up and running properly. Just a thought .
                          I need any thoughts I can get at the moment . I have tried an old set of leads but no change. She does run well at wider throttle openings, it just seems to be the rear carb that does little or nothing at idle
                          Stumbling around in the dark
                          Marina based SWB roadster
                          Alfa Romeo 1750 with 5 speed box

                          Comment


                          • #14
                            Re: Weber carb problem?

                            My tuppence worth. Since it seems to be rich, ie sooty plugs, I'd take the rear carb off and strip it down to the last nut and bolt. Check all the passageways are clear, no cracks leting fuel through where it shouldnt be, gaskets all good, and jets tight. Is there a diaphragm in these like the DelOrto's I have? That could be leaking.
                            Best of luck you find the problem.
                            Hugh

                            Comment


                            • #15
                              Re: Weber carb problem?

                              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

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