Weber mixture screws Options

[dumbjim]

Hi all, I spent yesterday fooling around with my weber idf 40's. by following mrbubbleheads instructions. With my snail, I got all four barrels pulling the same by adjusting the air bleed screws. I then moved on to the mixture screw. The mixture screws have absolutely no effect on how the engine is running. I can back them out or screw them in and there's not change in the idle. Is there something else going on here? In general, after adjusting the bleed screws, the engine is running pretty smooth. Any ideas? Thanks for reading.

[Charles Freeborn]

There are a LOT of factors in proper jetting, synchronizing and adjusting Webers. As said, adjusting air bleed screws can be masking other issues. Also as said, you could be above the idle circuit's range so the mixture screws are ineffective.
As far balancing the flow(s) first check the top end is healthy with a leak down test. If one or more cyl's has an issue it will draw less. Then look for vacuum leaks in the manifolds, carb bases, etc. . Any leaks below the butterflies will throw off readings. This can also be butterfly shafts, etc.
Double check fuel pressure. Too high can overdrive the needle valves and cause the floats to over fill / spill / over rich. For webers it should be between 2.5 lbs and 3lbs.
Then double check the idle jets themselves. They are the smallest jets in the carbs and plug easily. Then do a proper sync with linkage removed. Then and only then adjust air bleeds. One more item - the "mixture screw" is actually a misnomer. It adjusts the volume of air/fuel to the idle circuit, not the rich/lean ratio.