The Rochester 4 Jet throttle body has several machined holes and channels that each serve a specific function for fuel delivery, idle control, and vacuum operation. Here’s a breakdown of the key ones visible in your diagram and what they do:
Primary Idle Channels
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These supply the fuel/air mixture to the engine during idle and just off-idle.
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They connect from the idle mixture screws to the throttle bores, discharging just under the throttle plates.
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If clogged, the engine will idle poorly or require excessive screw adjustment.
Idle Mixture Adjusting Screws
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Control the amount of fuel/air mix entering through the primary idle channels.
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Turning them in (clockwise) leans the idle mixture; out (counterclockwise) richens it.
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They only affect idle and just off-idle operation.
Secondary Fixed Idle Channels
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Feed a fixed amount of fuel/air mix to the secondary bores at low speed.
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These are not adjustable — if they get restricted, secondary transition will stumble.
Idle Compensator Channel
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Helps stabilize idle under certain loads or changes in manifold vacuum.
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Often tied to temperature or accessory operation (varies by calibration).
Power Piston Vacuum Channel
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Routes manifold vacuum to the power piston assembly.
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When vacuum drops under load, the piston rises, enriching the mixture via the power system.
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If blocked, the carb may run lean under acceleration.
Choke Vacuum Channel
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Provides vacuum signal for choke pull-off operation.
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Ensures the choke plate opens slightly as soon as the engine starts to prevent flooding.
Fast Idle Screw
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Sets the higher idle speed when the choke is on during cold start.
Idle Speed Screw
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Sets base throttle plate opening for warm idle speed.
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Too far open can expose too much of the idle transfer slot, causing off-idle hesitation.
Tips for Checking Any Passage
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Always test passages after soaking and brushing to ensure they’re truly open.
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Use aerosol carb cleaner first, then compressed air to verify.
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If cleaner sprays out of more than one exit, it often indicates a shared or branching passage.
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If nothing comes out, reverse direction and test from the other end — some passages only flow in one direction due to check balls or restrictions.
