I had this same problem. I fixed by changing the synch of the carbs. It helps to have a vacuum gauge so you can see what you are changing, but in the event you DONT have one, trial an error is the only solution. the carb synch is adjusted with the set screws that sit between the carbs, you will see a screw running though a spring, thats how you know you are working with the right part :) this is where the throttle of each carb is attached to the throttle of the one next to it, on the intake manifold side of the carbs. these adjust the tension from carb to carb, so an adjustment on either of the screws will affect all three carbs slightly. What you are adjusting is the resting angle of the big circular flap in the carb. If you have a vacuum gauge, you will first balance the vacuum on the outside carbs, then you can tweak the idle down with tiny adjustments to one side then the other. without a gauge, you might never get them to balance, but you can still get the idle down. It took me about 3 hours WITH a gauge :)
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