[ceslaw]

Could the length of the connection between the bell crank and the carbs be slightly different, left to right? This could result in exactly equal flow at idle when checked with the flow meter, but under throttle the flow may no longer be equal left to right causing one side to run richer than the other. Perhaps checking flow at higher RPM, say around 1500 RPM, near the top of the scale on the flow meter, will give you some indication. Since the problem involves both carbs on one side it suggests a linkage issue.