Later AFB's had a velocity valve but was a weight driven opening. The universal models were heavy where OEM carbs were tailored closer. A 273 had a very light opening where a larger engine might have a heavier opening rate. The only way to modify the opening rate on these carbs was to drill the weights or add weight to the valve. The purpose of these valves and the better spring adjustable valves is the size of the carb can easily adapted to a large range of engine sizes and performance levels. No power valves, no sensitivity to cfm for a specific engine.
