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On '75-and-newer passenger car TQs, we find a flat boss at the front of the air horn, which may or may not have various holes drilled into it, often with confusing devices attached. Here's the deal: these carbs were equipped with an auxiliary air bleed circuit, and the devices at the front open and close this air bleed to the atmosphere. When open, the signal to the primary main jets is weakened, leaning the mixture. The setup normally runs open, and is controlled by vacuum to the closed (rich) position during cold start-up. Besides the vacuum control, some carbs carried a barrel-shaped bellows (shown here), which varied the opening in relation to air density, to compensate for altitude changes. A common TQ mod, the control valve can be blocked by replacing it with a fabricated plate (shown in the left foreground). Removing the control mechanism and blocking the air bleed results in the carb running at the rich setting full time. As we will see, this is not always the best way to go.