I just finished up a project on a set of 46mm "American" Webers and noticed a machining error which is worthy of note to highlight idle adjustment procedure.
When at idle the progression circuit holes should be completely blocked by the throttle plates leaving only the mixture screw hole to provide fuel at idle. Both pictures are of the same carb, freshly remanufactured with new throttle bushings and new butterflies and taken after idle adjustments completed.
The next picture shows the same carb with the throttle plate closed but exposing the first progression hole. Note that the mixture screw tip is projecting into the bore effectively blocking fuel delivery.
Since air flows and mixtures were carefully set before I took this picture (I use a STE synchrometer and six Gunson Colortunes for getting idle mixtures spot-on) the final set-up is not ideal for progression circuit operation. This is due to fuel delivery for progression being provided by the holes of the progression circuit (seen under the large, brass screw in the side of the Weber throttle body) and by the fuel delivered from the idle mixture screw port. If you set idle mixture with the progression circuit activated (throttle plates opened too far) you will not have the same fuel delivery during progression as you would if the mixture screw was set properly; a lean progression.
Worn Webers have enough air leakage to keep the throttle plates closed enough during idle adjustments to avoid activation of the progression circuit. I recommend purposely opening idle air screws one turn as a starting point for idle adjustments to assure your throttle plates are as closed as possible to preclude progression leaness issues (which may lead one to installing a larger idle jet than required.)
Another comment: Use of 40mm Webers on large displacement engines will demand larger initial throttle openings at idle to provide enough air...obviously you will need to be careful to avoid opening the throttles too far. A remedy for this problem is to drill a small hole near the edge of the throttle plate adjacent to the progression holes to bleed more air.
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