Are we going to jump to main circuit soon?
Well I decided that we just can't jump into the mains until we've had all the fun we could possibly have with the progression circuit; fully milking them for all they're worth, so here we go.
As good as idle jet changes based on mixture screw settings can be, if you're 95% nuts and 100% committed to getting every bit of mileage and performance out of a Weber, there's still some more fun that can be had.
For instance, if you have a synchronous 38 and you pay real careful attention to detail of the linkage geometry on both the carb and the throttle linkage, plus careful adjustment of the sector gears, you can get both throttle plates to close off to this position and move in perfect harmony with each other, returning to this exact position each time. You shouldn't have an issue but mine was stubborn.
Then you can drill the pressed idle air bleeds out and tap them for threaded air bleeds.
Once the passages have been thoroughly cleaned and inspected, the new idle air bleeds can be threaded in.
Now the door is wide open to fine-tune the fuel curve of the progression circuit with various idle air bleed sizes in addition to all of the idle jet sizes. Look at all of that adjustability.
With the throttle plates closed off per the first photo, like they honestly should be so that no fuel is allowed to flow from the progression hole during idle, should a higher idle speed be desired, little holes can be drilled in the throttle plates. This will bring the idle speed up a touch but, more importantly, will also serve to counter the richness of the larger first progression hole.
Don't go hoggin out your passages and throttle plates just yet, I've done this sort of thing before and this is my madness! This also isn't something a guy should do without an air/fuel monitor. These little tweeks go along way to stretching out the mileage and they give you the ability to tune out lean holes or rich peaks, should you have any.