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balancing multiple carbs...

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  • balancing multiple carbs...

    The EDM topic has mention of the joy of synchronizing multiple carbs...

    Our family once included a Suzuki LC10W two stroke coupe which had three carbs which never seemed to be in synchronicity, so I had a try at it.

    Now the engine was in the back and with the engine cover open it was easy to see the dash mounted rpm instrument. I started the engine and drove around a couple of blocks to get it well warmed up.

    Armed with nothing more than a screw driver I set to work. First adjusting the throttle stop screw until the engine was running at 1500 or so rpm. Then I shorted out each plug in turn and the one with the least rpm drop was very slightly opened and the process repeated until every cylinder had the same rpm drop.

    Idle throttle stop adjusted and the job was done.

  • #2
    it's a "po pholk" way of going about it and actually compensates for other cylinders being a little stronger and such,

    it's dynamic as all hell, might leave some plates closed a little more than others and slightly different air flows and even vacuum draws to where if you through it on a multi-carb sync vac-meter things would be off some - but who cares, if the idle and low range is sweet the other areas will follow suit

    good duty...


    • #3
      Another no-tool approach is to reach in with a small screwdriver and slightly lift the slide on each carb. The idle speed should increase slightly for a second or so, then drop.

      For carbs with vacuum fittings in the manifold (motorcycles) I use a tool called carb stix. It's four columns of mercury - four mercury manometers side by side. Very accurate. Can't get them with mercury any longer but they're made with some special fluid.

      There are no stupid questions. But there are lots of stupid answers. This is the internet.

      Location: SF Bay Area


      • #4
        yup had the same tool when working on bikes - just hang it up vertically and go to work very nice...


        • #5
          Used to be Morgan Carbtune over here. Now the mercury has been replace by four little rods. I use a bank of four vacuum gauges.
          Peter - novice home machinist, modern motorcycle enthusiast.

          Denford Viceroy 280 Synchro (11 x 24)
          Herbert 0V adapted to R8 by 'Sir John'.
          Monarch 10EE 1942


          • #6
            I could tune those SU with ANY method including the manometer, and in 20 min driving they would be way off, and out of damper fluid. It would sit in there for a week if not driven, never leak a drop.

            Keep eye on ball.
            Hashim Khan

            Everything not impossible is compulsory


            • #7
              I hated brit carbs --- (please allow for slower acceleration times in colder climates due to our inferior carburetors and the brains that built them) should have been a sticker on every dash stating that - how many people could have been saved from "trying to outrun a train in the winter months"

              gawd im glad those days are over - eyes burning from all the un-combusted hydrocarbons let alone "feeling kinda funny" while hovered over a Maserati Quattroporte tuning in the 8pack of webers lol

              back in the day i worked on everything from around the globe - nothing will teach you more about the way different "groups of people" think than being a foreign car mechanic... and then there was my own home turf --- ahh yes the reason i decided to be tortured instead by the foreign car people...


              • #8
                Same kind of setup on Mercury outboards back when I worked in a marina in the early 70s. Vertical shaft six cylinder two stroke with three carbs. Real screamers when they were set right. The fun version was the late 50s/ early 60s "direct reversing" motors. The control had a starter button on the top of the single lever control. There was no clutch in the lower unit, so the "Quicksilver" lower units could be very small. Neutral turned the motor off, and for reverse you pulled the lever backwards, pushed the button, and started the motor running backwards.


                • #9
                  I haven't seen that subject mentioned since the 60s I believe. :-) I remember the twin SUs well.


                  • #10
                    I've rebuilt several sets of motorcycle carbs installing new jets and needles, float valves, bowl gaskets, etc. Using the exact same number of turns for seating jets, I've never had to balance them after they were installed back on the engine, but I did buy one of those 4-gauge vacuum balancers expecting to need it. I also couldn't tell if they actually needed to be balanced afterwards either; but I also remember it would have been a real PITA to make adjustments while it was running if I actually did need to balance them. I took pictures of the last set of carbs I rebuilt from the GSX750 engine I used on my buggy. I guess I've just been lucky or simply couldn't tell they needed to be balanced. Today, I wouldn't touch anything that's not fuel injected.


                    • #11
                      Ahhh, fond memories! I synched the dual Webbers on my VW using a Unisyn nearly 20 years ago, haven't had to touch them since. Back in the day, my MGA had me under the hood nearly every day. Never did get the hang of those SU carbs. Can anyone explain to me why Volvo would have chosen to put those things on their engines?

                      Q: How do you know your English car is out of oil?
                      A: It stops leaking.

                      For bonus points, what does SU stand for and why?
                      It's all mind over matter.
                      If you don't mind, it don't matter.


                      • #12
                        Yes, balancing can be challenging......


                        • #13
                          They still make Uni-Syns, albeit in in China.

                          Brings back memories. Thank God for fuel injection.


                          • #14
                            Sid have you ever tested that engine (on a very clean track) w/o those tiny little filters and screens sitting right on top of the velocity stacks? if not you will be surprised at the extra power,,, they do look cool but worse place going to have a filter if performance is a concern...


                            • #15
                              Originally posted by A.K. Boomer View Post
                              Sid have you ever tested that engine (on a very clean track) w/o those tiny little filters and screens sitting right on top of the velocity stacks? if not you will be surprised at the extra power,,, they do look cool but worse place going to have a filter if performance is a concern...
                              Yup, sure enough, I agree. But on the street, they are a necessary evil.
                              Let's face it. More engine than tire anyway. Maybe when the snow melts and the salt is all washed off the roads this year, I'll give it a go. I do know this, when I had 50mm bodies on this engine WOT was scary.
                              The 40's tone it down a bit for the street.