Fuel enters the idle well through the metering rod jet. The low-speed jet measures the amount of fuel for idle and early part-throttle operation. The upper idle air bleed, fixed idle restriction, and lower idle air bleed are carefully calibrated and serve to break up the liquid fuel. The fuel is then mixed with air as it moves through a passage to the idle port and idle adjustment screw port. Turning the idle adjustment screw inward reduces the quantity of air-fuel mixture supplied by the idle system. The idle port is slot-shaped. As the throttle valve is opened, more of the idle port is uncovered, allowing a greater quantity of the air-fuel mixture to enter the carburetor bore.

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ROUGH IDLE

• Make sure the idle is at specified RPM when adjusting the idle mixture screw. Once the RPM gets over the idle speed, the idle circuit is bypassed.
• Take the screw out and make sure the hole is clear. Screw should not have any scoring.
• There are 2 holes in the bore, one at the idle screw and one above. The above hole should be clear.
• The idle tube should be clear. This is the tube that screws into the float bowl with the top off.
• There is an air bleed hole in the bowl close to the top of the idle tube. Make sure that is open.
• Perhaps the throttle shaft is worn and leaking too much air. Use carburetor cleaner and spray around throttle shaft. If the RPM smooths out, you found a vacuum leak and will have to re-bush the throttle shaft.
• Was the throttle plate removed & replaced? It could have been installed backwards.
• A strong smell of fuel could indicate a flooding condition.
• A rough idle can also be caused by a vacuum leak. Spray carburetor cleaner around the mounting gasket, intake manifold and any other vacuum source.

IDLE MIXTURE ADJUSTMENT

• Engine should be at operating temperature
• RPM should be at factory specification. Probably around 500 for the YF cars.
• Hook up a vacuum gauge either to the carburetor, or intake manifold port.
• Start with screw about 1 1/2" turns out.
• Turn screw in 1/4 turn, wait for the RPM to catch up (1 second).
• Keep turning in 1/4 turn until vacuum starts to drop.