The easiest description is to close the throttle plates on the end carbs completely (also unhook end linkages)and adjust the center carb to allow proper idle/acceleration. The idle circuits should be defeated (on end carbs) but not the accelerator pumps. Then hook the end linkages and adjust for desired cut-in (opening) at whatever RPM/ road speed desired.
If using Rochester carbs, follow any GM TRI-POWER instructions as the engine does not know what type of carb it has.
The other poster by describing end carb throttle plate adjustment is to allow for proper transition from idle to power circuit.
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One misconception of the FE tri-power is it is over carbureted for 390 and smaller displacement FE's. One must realize 2v and 4v carbs are flowed differently. 2v carbs are flow tested at a pressure of 3" and 4v carbs are flow tested at 1.5". To make an accurate comparison of a 2v VS 4v carb take the total cfm of the 2v and divide by 1.414. Example: Total cfm rating of most FE tri-power carbs (List #2436/2437) is 1030 cfm. 330 cfm primary + 350 cfm each outboard = 1030 cfm divided by 1.414 = 728 cfm.
1030/1.414 = 728
As one can see if comparing the tri-power system to a single 4v intake system the tri-power is equal to a single 4v intake with approximately a CJ carb that is 730 cfm. Ford did the homework, and rather well at that!
From HP TRENDS-
http://www.hptrends.com/carbtech.htm
Source of 2GC Information-
http://cgi.ebay.com/ebaymotors/Rochester-TRI-POWER-Carburetor-Service-Repair-Manual_W0QQitemZ260132277676QQihZ016QQcategoryZ34198QQcmdZViewItem