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3528 Highway B
St. Charles, MO 63301

Phone: 636-723-4996
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Technical Information
Preliminary Tuning of a New
Racing Carburetor

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All C&S carbs start with throttle position as shown in Sketch A.

The front throttle butterflies are never changed from this position.

Disregard anything you may have heard to the contrary.

Adjust idle screws on side of metering blocks one by one (most have 4 screws, some only 2). They are very sensitive. Adjust to get the highest possible idle RPM even if this results in a higher RPM than you want. If RPM is too high, shut off engine, then open carburetor wide open. This will expose a small headless allen stop screw on the right side of the rear throttle linkage (Photo D).
While holding the throttle wide open, turn screw in (clockwise) about 1/6 turn to reduce RPM about 200 RPM. And of course, turn screw out to increase RPM. Stop screw has a nylon patch to keep it where it is set.

Then go through the process again until the desired RPM is reached.

You will not only have the best mixture at that RPM, but it can be readjusted if major conditions change or carb is installed on a different motor.

Take time to do this right! Proper adjustment is critical to sharp throttle response.

Sketch B shows butterflies set too low. This will result in poor idle or (most likely) no idle. There is only one correct position of front butterflies as shown in Sketch A. NO EXCEPTIONS. More or less idle air is delivered by adjusting REAR butterflies.

Sketch C shows primary butterflies set too high. This is usually done to try to get desired idle RPM. This will result in a stumble off idle.