Like most carburetors, the center in a trio has all the normal systems including idle, transfer, main, enrichment, and choke. And in most three two arrangements, all the carburetors have the same CFM rating (some factory installations have center carburetors that were bigger than those on the ends, others used center carburetors that were smaller than those on either end). But while all the carburetors may look the same, there are normally significant differences in those used on the ends. Generally they lack idle circuits and the throttle valves are thicker to seal more effectively and ompletely close off airflow when running off the center carburetor only, choke butterflies are also eliminated.
One of the systems found in most carburetors is an accelerator pump. It provides that squirt of fuel when the throttle is opened and it helps with starting and preventing flat-spots in acceleration. When the throttle is opened suddenly, the airflow through the carburetor increases faster than the heavier fuel droplets can, and the engine will go lean momentarily causing a stumble. The accelerator pump provides an extra volume of fuel until the main system in the carburetor can "catch up" with the demand. With three twos, when all three carburetors are suddenly opened, the function of the accelerator pumps is critical.
Another important feature found in carburetors is the enrichment, or power system. When the manifold vacuum drops as a result of the throttles being opened, the power system supplies a richer fuel mixture. In some three two installations, the power system is eliminated and the carburetors are simply jetted appropriately.
Variations On The Theme
* Triple-D Six Shooter
It you're looking to add three deuces to you classic truck, there are a number of options. Two of the best come from Barry Grant. Known for race winning carburetors, Grant has also been paying attention to the needs of street driven vehicles, and addressing the traditionalist in that crowd, he recently introduced his Old School version of the popular Triple-D Six Shooter induction system.
Based on Grant's new Triple-D intake manifold, the carburetors used on the Old School system have a gold chromate finish, a chrome fuel log with red fuel lines, and traditionally styled Rush air cleaners. But don't let the old timey look fool you; this system incorporates the latest carburetor technology. Rated at 250-cfm each, thanks to the progressive linkage, the Old School system provides excellent low speed throttle response (along with surprisingly good mileage) from the center carburetor and plenty of performance when all three are wide open.
As you might expect, the center carburetorin this system is different from the other two; it includes an electric choke, an idle system with two external mixture adjustment screws in the metering block, and a power valve that opens when the manifold vacuum drops below 6.5 inches. Two vacuum ports are provided: one ported for the distributor's vacuum advance and one full time for a transmission modulator (if they are not used, one or both can be capped).
To keep things simple, the only external adjustments on the outer carburetors are for float levels. And although the end carburetors do actually have idle circuits, they are linked to the transfer slots and the mixture and idle-speed adjusting screws are preset (no synchronization needed). The throttle plates are almost .040-inch thicker than those of the center carb to reduce mixture leakage into the venturi bores at idle.