Carl, carburetors mix air and fuel by venturi velocity. Air flowing through a venturi creates a vacuum- - - -the faster the flow, the more the vacuum. If the flow is divided between two carbs, the velocity slows down- - - -plenty of air, not much velocity, very low venturi vacuum, so little or no fuel gets mixed with the air. The ONLY solution is to put two SMALL carbs on the engine to try to keep the velocity in the venturis high enough to draw the fuel from the float chambers into the air stream.
Putting two of the same carbs that work as singles on an engine is a recipie for disaster- - - -it gets terminally lean and surges, backfires, hesitates, and generally runs like crap. Putting MUCH bigger main jets in helps a little, but the problem is too much air going too slow to create vacuum in the venturis.
Jerry
