Greg,
You're right. The carb maintains the same fuel air mix regardless of the throttle plate position (except at idle). The difference lies in the large balance tube. The balance tube equalizes manifold pressure AFTER the fuel air mix has left the carbs. If you have one carb that is open more than the other, you will be feeding more air, and thus more fuel through one carb even though the balance tube is keeping the manifold pressure equal. The carb that is closed (less fuel/air) causes higher vacuum in that manifold and pulls the fuel/air mix from the other side through the balance tube thereby equalizing the amount of fuel/air in both sides. With no balance tube or a too small balance tube, The open side would run slightly rich and the closed side slightly lean because the carbs are unequally open but all the pistons are running at the same speed.
The existing balance tube is adequate at very low RPMs but really does very little at higher RPMs. Actually, if the carbs are perfectly balanced, the balance tube is not even necessary. In real life however, that perfect balance doesn't exist.
Biggest advantage to a large balance tube is to keep the fuel/air moving. In a boxer engine at certain points in the cycle, all the valves on one side are closed and the carb airflow has to stop completely. When those valves do open, they have to start that airflow up again from a dead stop. With the large balance tube, the airflow on the closed side continues to move through the balance tube to the side that is calling for air. Much more efficient than stopping and starting the airflow.
In cars, crossover pipes are often installed in exhaust systems for the same reason. To keep the exhaust airflow moving smoothly and induce better scavenging.
In normal applications with a rotax, where you are operating at consistant and high RPMs, a large balance pipe is not necessary, even though it would offer advantages. In my situation however, I must operate at the lower RPMs (4200 cruise) and the large tube has made a considerable performance difference.
Bill.
