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Carb throttle rod setup

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  • Carb throttle rod setup

    Topic originally created by Bryan on August 16, 2005 12:14 PM and viewed 3515 times in the old forum.

  • #2
    Originally posted by Bryan on August 16, 2005 12:14 PM
    Aaron Masters sent this to me and it has worked well. A friend of mine told me about a trick to synchronize carburetors - I used it and measured the result with mercury columns. It works well for setting up the throttle rods. First, on each carburetor, remove the screw that covers the idle circuit transition holes on one side of the carburetor. If you look down you can see the 4 little holes that make up the transition circuit. Shine a flashlight into the throat of the carb and move the throttle linkage. As the throttle butterfly moves, you can see it cross the hole boundaries. Push the throttle peddle by adjusting the stop or with a stick ... such the carburetors butterfly's are open equivalent to running on the transition circuit. Now you can compare the butterfly positions in each carburetor by shining the flashlight in the throat and observing the holes that are covered .... And best of all, the throttle rods can be adjusted on a non-running engine. When I did this, I adjusted the butterfly's such that they all barely uncovered the second hole from the fender. As I said, I check it with the mercury columns and the results were really good. I made no further adjustments to the rods. To adjust idle, release the throttle (remove the stick), warm the engine, re-connect the mercury column to set the balance with the idle crews, followed by setting mixture. From my first experience with mixture - if the idle balance is not set even across all the carburetors, cylinders that are not contributing equally to the engine power are difficult to adjust because they have little effect on engine RPM. Aaron


    • #3
      Originally posted by pauldridge on August 28, 2005 10:21 AM
      What a great hint Bryan! And great timing since I'm re-syncing my carbs this week. I was frustrated with the original Ferrari carb linkage rods having RH threads on both ends, thus limiting adjustment to a full 360 turn on the ball connector. I replaced all mine with beautiful units from for about $125 total. In addition, I purchased 3 "Carbsync" manometers... each has 4 vacuum columns rubber hoses, and nipples that screw right into the existing intake manifold bosses designed for that purpose, obviating the need to remove the air cleaners. This way, I can see the draw on all 12 weber barrels at the same time.


      • #4
        Originally posted by Aaron on August 30, 2005 10:38 AM
        Paul, I quit using the thread portion to adjust the rods for the same reason you described. The last time messed with it, I loosened the clamp bolt such that the levers can be rotated on the throttle shafts with a bit of force and dialed it in that way. – Still the tightening process moves the settings such that perfection is difficult to reach. I believe that you now have the ultimate setup! Aaron


        • #5
          Originally posted by Bryan on September 11, 2005 11:35 AM
          Originally posted by Aaron
          Paul, I quit using the thread portion to adjust the rods for the same reason you described. The last time messed with it, I loosened the clamp bolt such that the levers can be rotated on the throttle shafts with a bit of force and dialed it in that way. – Still the tightening process moves the settings such that perfection is difficult to reach. I believe that you now have the ultimate setup! Aaron
          My idle speed has been on the high side...1400-1500 rpm when warm and fast idle cams have moved to warm position. So I decided to go back to basics and re-set everything. There is not room to back off anymore on the idle speed adjustment screws. The mixture screws are 1.5 turns out. Then I took a look at the transition holes. Aaron: my recollection is that you mentioned that you hold the throttle open a bit with a stick on the gas pedal. You then adjust the length of the throttle rods so that you can see two of the transition holes with the light, i.e. these two holes are upstream of the throttle plate. I realized that, while all carbs are the same, my present setup has two of the transition holes open downstream of the throttle plate even with the throttle plates as closed as they will get. The gas pedal is not held open at all. I thought that, when the throttle is completely closed, that there would be light visible in 3 or even 4 of the transition holes. This seems to be a possible reason for the high idle, i.e. I am always pulling some fuel through the transition holes even at idle with no throttle at all; hence, adjusting the idle mixture or idle speed screw will have no effect. So, as near as I can tell, I have to do the same that Aarton mentions above, i.e. loosen the bolts on the throttle spindle, adjust the throttle lever arm a bit, re-tighten the spindle nuts so that 4 holes are visible, and re-adjust the transition holes. I am surprised that this appears necessary on all 6 carbs. Any thoughts?


          • #6
            Originally posted by Aaron on September 12, 2005 6:26 AM
            Over the years I have picked up the following opinions regarding idle - DCOE38s on C4 – 1. Proper idle can only be achieved when the butter fly is closed past all the transitions holes on all carburetors. 2. The butterfly stop screw on the side of the carburetor is not for idle adjustment. It set ½ turn beyond the point where the butterfly valves hit the barrels of the carburetors, and at this setting the butterfly valves cover all the transition holes 3. Idle speed and idle speed carburetor balance is set using the air bleed screws on the sides of the carburetors. Idle speed is set to 950 RPM with all barrels drawing the same amount of vacuum. 4. If a vacuum balance is not achieved, it will be very difficult to set idle mixture because a week or “less contributing” carburetor will have little effect on idle speed or smoothness as the mixture is adjusted. As idle mixture is adjusted it will likely be necessary to go back and rebalance the carburetors – it is an iterative process where the goal is – with all the carbs in balance, quarter turn either way on any of the 12 idle mixture screws will cause a detectable degradation in engine smoothness or idle speed. 5. Last time I set idle mixtures, I used the initial value in the manual, 2 turns out from lightly seated. Because of use and slight variations in castings I have found that the difference in final idle settings can vary up to ¾ of a turn for my carburetors. So, based on your information, If it were me, I would get my carburetor balance gear together and hook it up. I would fire up the engine and start one carburetor at a time, disconnecting the throttle rod, apply light pressure on the throttle lever to close the throttle, back out the throttle stop screw until I could feel the butterfly contact the carburetor barrels, then turn in ½ turn. If the carburetor is mechanically sound, this position will cover all the transitions holes. I would they loosen the lock nuts on the bleed screws, turn them in until lightly seated, and then back them out until the I achieved something close to balance with the rest of the carburetors. (don’t bother tightening the lock screws until you are finished balancing all the carburetors and have your mixture settings dialed in. Note – because of the massive accelerator pump spring that came factory in these carburetors, sometimes they don’t close all the way by them selves. If you find the carburetor will not stay closed by itself ( with the throttle rod disconnected) find a rubber band and use with something in the center of the engine to assist the throttle return spring. I would follow this process for each carburetor until I had all 6 throttle rods disconnected. Then, again using the air bleed screws, I would dial in the idle speed and the balance. If Idle was too high, I would close off the bleed screws on those carburetors that were drawing more air (had lower manifold vacuum) and visa-versa. If I achieved a point where all the carburetors were balanced and the idle was too low – I would go back and open each bleed screw ¼ turn and then rebalance them. With the correct idle speed and a set of balanced carburetors and by now, a warm engine, its time to start idle mixture adjustment. You need to listen closely for both speed and smoothness. I will usually go about ¼ and then wait a few seconds for the engine to stabilize. - (Another note - it is really hard to hear when the rediator fans kick in - I have found a small garden sprinkler inserted to spray the radiator will solve this problem) As said earlier, if the idle speed changes with a mixture change – you may need to check/re-adjust idle speed and carb balance. With all this done – it is time to reconnect the throttle rods – I will assume that the basic linkage is symmetrical and the rods coming off the bellcrank area parallel. As a side note – good working bearings for the throttle rods make a big difference. They are standard bearings; I think I paid about $5.00 each for a set (after I ruined a couple with paint stripper.) They press out and in – just be really careful to press on the bearing holders and not depend on the cover structure. The first throttle rod on the first carb will set the throttle linkage position. All the rest of the throttle rods should be adjusted such that they slide over the balls on the carburetor levers. This is where I would initially loosen the clamps on the levers and adjust them. If you have all the throttle stops set as described and all the rods fit nicely over the throttle level balls, you will find that the off idle rod adjustment will be very close. I’m out of time for now – Aaron