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Weber Carburetor Jetting

Created on 2011-03-04 by IPD Staff

The most frequently asked question about our Weber kit is “Do I have to re-jet the carb for my engine?”

Re-jetting is the last, and perhaps the most important function of install-ing the Weber conversion. Correct jetting can make the difference between excellent performance and absolute frustration!

While not exhaustive, here are the four major points:

1) A common misconception is that the secondary throats should have larger jets. In this Weber, it’s just the opposite. Usually, no more than 5 points difference in the size of primary to secondary jets works well. For example, a 145 in the main primary to a 140 in the main secondary. The same holds true for the idle jets, 50 to a 45.

2) There is no “standard” jetting, although common combinations do exist, like 140/135 main to 50/45 idle jets. Several things affect the “right” jets for your engine. They include, but aren’t limited to mileage on the motor, valves, ignition system and elevation.

3) The idle jets, especially the primary don’t affect anything except how the motor idles. Once the throttle position is off idle, the main jets do all the work. So if the motor idles fine, but the plugs or exhaust indicate a rich mixture condition, it will usually require a change in size of the main jets only. This will also show up as poor acceleration and or a stumble or flat spot.

4) When the idle jets are right-on, idle speed is controlled by the idle mixture screw, not the throttle stop adjustment. The mixture screw is at the carb base on the passenger side, pointing diagonally back at the firewall. The screw should be set by turning it in all the way (clockwise), then backing it out two full turns. From this point, it is then adjusted for smoothest idle by turning it in or out. (Note: The motor will stall if turned too far either way). Once set, the proper idle speed can be adjusted by the idle speed screw on the back of the carb, where it contacts the linkage. If the motor doesn’t idle well through these adjustments, you will have to alter the jet size, usually the main idle jet only. Don’t worry, any change in jet size will give you clear feedback, so you’ll know which way to go and how much of a change to make after experimenting a bit.


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