I did a lot of head scratching with VM with air screws (side of the carb) and fuel (pilot) screws (under the carb) working out what does what. I found 'juggling the screws to get a smooth idle' with four carbs pretty much impossible. But I finally got it figured out. Understanding the operation of the pilot circuit is really important as things only make sense when you get this. Sorry if you already know this stuff. But was a lightbulb moment for me.
The basic operation is air enters the pilot intake situated on the edge of the bellmouth. This air flow is metered by the air screw (rather than the air jet depicted above) which then goes to the pilot jet. The pilot jet is a combination fuel metering jet and emulsion tube. The hole in the bottom of the jet meters the fuel the pinholes on the side of the jet allow air from the air screw to mix with the pilot fuel to form an air/fuel mix (emulsion) that is fixed for the entire idle/pilot system. Being like a fuel/air 'froth' this mix is more easily atomized into a mist as it it drawn into the inlet tract.
This pilot circuit emulsion A/F ratio (mix) is set by the pilot jet size and air screw position. Screw in less air = richer, screw out more air = leaner. This is the mix for the entire idle/pilot circuit.
This pilot mix enters the main airflow through small orifices drilled on the floor of the carburettor venturi one on the engine side of the throttle slide, the others are located upstream and are under the throttle slide, downstream from the needle and needle jet.
The one closest the engine is the pilot orifice and this is metered by the fuel screw. The other orifices are called by-pass orifices and have no adjustments.
With the throttle closed at the lowest idle there is a tiny amount of air flowing under the slide and not enough vacuum to draw fuel out of the needle jet or pilot circuit by-passes. Engine vacuum is focused on the pilot orifice controlled by the fuel screw. Turn the screw out allows more volume of the pilot mix into the airflow passing under the 'closed' throttle slide. Turning the fuel screw in decreases the pilot mix joining this air. The idle fuel/air mix is the sum of both the main air flow and pilot mix passing the fuel screw. So turning the fuel out adds more pilot mix = richer combined A/F ratio, and turning the fuel screw in makes the combined ratio leaner.
What is important to realise is that the pilot orifice and fuel screw only have a strong influence on fuel mixture when the throttle is shut at warm idle and the pilot orifice is the only game in town. As soon as you lift the slide the by-passes start to flow pilot mixture into the increased air flow passing under the slide and this greater quantity of pilot mix is not metered by the fuel screw. I proved this to myself turning the fuel screw at a high idle and getting no colour change on a colortune plug - so this adjustment was making a negligible difference to the mixture.
When the throttle is cracked open above idle the engine is running on the by-passes. When you are cruising on a just open throttle you are on the by-passes. This fuel mixture is set by the pilot jet and air screw.
Because the air screw is upstream of the fuel screw, any adjustment of the air screw affects both the by-passes and pilot. The fuel screw only fine tunes idle mixture
My method was to set the air screws at a high idle of around 3,000 rpm using a colortune plug. So I was reading the mixture while the engine was running on the by-passes. Once that was set the idle was dropped to as slow as it would steadily tick over and the fuel screw adjusted to give a Bunsen blue flame. That set the base line. At that point I left the fuel screws alone and tweaked the air screws for the best throttle response.
Doing this way you are setting for correct mixture. Not a fan of arbitrary screw settings across all carbs. The adjustment is there to compensate for the differences between the carbs.
