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At this point I am assuming you read and followed Chapter One in this series and are ready to start the tuning process itself. With this carburetor I like to start with the idle circuit. It is advantageous to tune the idle circuit first because, although it obviously controls idle mixture, the idle jet also influences fuel mixtures throughout the rest of the operating range. It makes sense to get it right first.
To start with, you will need to locate the idle speed screw and the idle mixture screw. The idle speed screw is the one that pushes against the linkage on the back of the carb. The idle mixture screw is mounted horizontally on the base of the carburetor, on the right side, assuming the float bowl is mounted toward the front of the car.
With engine at operating thoroughly warmed up and choke disengaged, set the idle to your desired speed via the idle speed screw. Once your idle speed is where you want it, slowly turn the idle mixture screw in and out until you find the fastest idle speed. If you have a vacuum gauge, you can also adjust the idle mixture screw until you find the highest intake manifold vacuum reading.
Now give the engine a moderate rev to clear out any residual fuel in the intake tract, and readjust the idle speed screw as necessary to get back to your preferred idle speed. Again adjust the idle mixture screw as necessary to achieve the highest idle speed or manifold vacuum. Repeat the process until adjusting the idle mixture screw no longer increases idle speed or manifold vacuum, then set idle is to your preferred speed via the idle speed screw.
Next, with the engine idling, turn the idle mixture screw in (clockwise) about 1/4 turn. If engine speed drops significantly, bring the idle speed back up to your desired RPM by turning the idle speed screw. This condition is called lean best idle. If you are using a wideband oxygen sensor, look for a reading around 15.0:1 here.
Shut off the engine. Gently turn the idle mixture screw clockwise, counting the number of turns until it is fully seated. Be gentle; you only want to turn the screw until it has reached the end of its travel. Record the number of turns until the screw is seated. With a correctly sized idle jet, the idle mixture screw should be about 1-3/4 to 2 turns out at lean best idle. If it is less than 1-3/4 turns out, your idle jet is too big. If it is more than two turns out, the idle jet is too small.
The reason the idle jet size is so critical is that it also handles progression from idle to low speed. The idle jet needs to be big enough to feed a little fuel to the progression holes right below the throttle plate as the throttle starts to open, but not so big that it makes this transition overly rich. A lean progression creates a stumble or hesitation off idle. An overly rich progression runs lazy, wastes fuel, and stinks.
Take the time to try different idle jets and note the difference each one makes. When your lean best idle setting is achieved with the idle mixture screw between 1-3/4 turns and 2 turns off the seat, you have the right idle jet. Again give the engine a bit of a rev to clean out any fuel. Revisit the idle speed screw and idle mixture screw to verify that lean best idle occurs with the idle mixture screw between 1-3/4 turns and 2 turns off the seat.
With the correct idle jet chosen, you have created a solid foundation on which to tune. You can now move on to the primary and secondary circuits. To do this you will change the change the primary and secondary main jets as well as primary and secondary air correctors. We will take those on in Chapter Three.