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SU carb set up 444-544

posted by  arcturus on  Mon Feb 10 14:21 CST 2014
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last visit: Tue Jul 8 01:55 CST 2014
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Hi, still having trouble with carbs.Can't stop them from running over rich. I have replaced the jet and needle,(weak needle)fitted a new needle valve to float chamber (the floats don't leak)and set up the correct distance,7/16th on float forks. The Color tune still shows orange through the range and the plugs are sooting up. What else can I look at that might affect the mixture this way?








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    SU carb set up 444-544

    posted by  Nahtanha on  Wed Feb 12 09:53 CST 2014
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    last visit: Tue May 20 10:10 CST 2014
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    Not sure, but if the carbs have an enrichment valve, it might not be sealing. Check the usual vacuum leaks. I use and hand held plumbers torch, gas on unlit, to check for leaks. Just pass the torch end around the intake manifold area and listen for engine speed changes. How's the ignition coil strength of spark?







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    SU carb set up 444-544

    posted by  BigA on  Mon Feb 10 14:55 CST 2014
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    last visit: Sun Oct 19 02:50 CST 2014
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    One of the first things I would have to ask, is the carb linkage properly set to balance the two carbs at idle and at x RPM...

    most folks have their preference for carb sync tools..I find the one below very useful for a number of smaller foreign cars..

    http://www.ebay.com/itm/EDELBROCK-CARBURETOR-SYNC-TOOL-4025-/140960963344?pt=Motorcycles_Parts_Accessories&hash=item20d1ed9b10&vxp=mtr








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      SU carb set up 444-544

      posted by  arcturus on  Tue Feb 11 02:48 CST 2014
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      last visit: Tue Jul 8 01:55 CST 2014
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      Yes,I use a sync gauge.The throttle adjustment arms are off the stops. Timing set at 5/6 deg.Engine still revs too fast on idle.








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        SU carb set up 444-544

        uid 5322's pic
        posted by  alschnertz on  Tue Feb 11 05:55 CST 2014
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        last visit: Fri Oct 24 10:32 CST 2014
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        What engine do you have?

        Regardless, unless I'm mistaken, your timing setting doesn't seem right. What rpm are you finding that 5/6 degrees? I think even static would be around 10 degrees.
        The timing setting should be around 22 degrees BTC at 1500 rpm for a B18.

        Check for an intake manifold gasket leak or some other form of a vacuum leak to address your high idle.

        --
        "Differences of opinions should be tolerated, but not if they're too different' - Sharon Craig








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          SU carb set up 444-544

          posted by  arcturus on  Tue Feb 11 07:23 CST 2014
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          last visit: Tue Jul 8 01:55 CST 2014
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          First it's the B16 twin carb'. Static timing given in manual 6 deg.
          I thought that if I was drawing in extra air from manifold or else where I would be weakening the the Mixture. Have I got this wrong? I believe that The test for manifold leaks is to squirt carb cleaner around it and see if engine speeds up. Is that so?








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            SU carb set up 444-544

            posted by  Volvo From Heck on  Tue Feb 11 08:14 CST 2014
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            last visit: Mon Oct 13 20:56 CST 2014
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            Since mixture is set at idle speed a setting that allows the motor to idle will have to be set too rich for normal running if there's a vacuum leak--or if the throttles are not closing completely.
            Carb cleaner will work as stated--or use a glass cleaner squirt bottle to squirt a mist of water (in which case the idle will stumble a bit if there's a leak) as a less flammable substitute :-) -- Dave








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              SU carb set up 444-544

              uid 5322's pic
              posted by  alschnertz on  Tue Feb 11 11:22 CST 2014
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              last visit: Fri Oct 24 10:32 CST 2014
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              Bear in mind that it can still be very difficult to pinpoint leaks on the underside of the manifold with either carb. cleaner or water.

              I experienced a fair sized leak that a casual spray did not detect. I found that you've got to get very close to the mating surface and suspect area with your detector.

              I replaced the intake gasket numerous times and it would be ok for a month or two. But then the high idle would return. Ultimately ended up using gasket sealer on a new gasket to cure the problem.
              --
              "Differences of opinions should be tolerated, but not if they're too different' - Sharon Craig








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                SU carb set up 444-544

                posted by  Boobtube on  Thu Feb 13 06:07 CST 2014
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                last visit: Wed Oct 22 06:19 CST 2014
                [RELATED]

                I think there can also be air leaks at the throttle shafts if they are worn where they enter the carb body which requires re-shafting and/or new bushings. I'm no expert, though.








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                  SU carb set up 444-544

                  uid 5322's pic
                  posted by  alschnertz on  Thu Feb 13 06:34 CST 2014
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                  last visit: Fri Oct 24 10:32 CST 2014
                  [RELATED]

                  Excellent point. I completely forgot about the shafts!
                  Probably more common than a gasket leak too.
                  --
                  "Differences of opinions should be tolerated, but not if they're too different' - Sharon Craig








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                    SU carb set up 444-544

                    posted by  votx on  Tue Feb 18 20:07 CST 2014
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                    last visit: Thu Oct 23 18:48 CST 2014
                    [RELATED]

                    Worn shafts & bushings can definitely cause problems. As a former aircraft mechanic I have overhauled lots of aircraft carbs, some with the shafts worn 1/3 of the way thru. Airplane engines are subject to a lot more shaking & vibration which may speed up the wear.








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        SU carb set up 444-544

        posted by  Volvo From Heck on  Tue Feb 11 05:53 CST 2014
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        last visit: Mon Oct 13 20:56 CST 2014
        [RELATED]

        I think the key bit of information here is that the linkage is off the throttle stops. So the next question is--where is the motor drawing its air from? You need to look for a vacuum leak (carbs to manifold--manifold to motor--cracked manifold--etc.) or throttle plates that are not centered. When you now set the mixture at idle speed you have to compensate for the extra air--so you richen the mixture--but guess what? Now the mixture is too rich above idle.
        BTW-- the tool pictured used to originally be sold as a Uni-Syn---advertised in every magazine that covered imported sports cars in the '50's and '60's. If you can't afford such a tool--a practiced and sensitive ear can do an equal job with a short length of vacuum hose. Place the end of the hose in the same spot carb to carb and it is like listening to musical notes--when they are right (equal) you'll know it. -- Dave







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