I recently ran what I believe is a much more relevant test for determining the best idle jet size and I thought I'd share it.
To give you an idea why, and I'm not trying to undermine Redline's tuning methods at all, it didn't make any sense to me to use some arbitrary mixture screw setting with the engine idling to accurately determine the best idle jet size the engine wants when it's actually running down the road.
: After driving the Jeep around so it's good and warm, wheel back in and do your best at finding the lean-best idle as described earlier. Using the speed screw, increase the engine speed to 1,400 rpm. This forces the carb into its progression circuit where the engine spends 90% of its time, not at idle. Now, turn the mixture screw(s) in a half turn and note any change in rpm and/or engine sound. Return the screws to where you started and then turn them out a half turn. Again, noting any change in rpm or engine sound. Take the screws back to their starting position and return to idle.
If the engine responded with a cleaner sound and/or more rpm when the mixture screws were opened, you may benefit from a larger idle jet. If the engine responded with a cleaner sound and/or increased rpm when you turned the mixture screws in, a smaller idle jet may be in order.
To quantify that idea with actual findings, I started with 45 idle jets and 1.85mm idle air bleeds. Don't let the air bleeds throw you off too much; it's basically a leaner 50 idle jet that pulls richer towards the top end of the progression circuit than a 45 would. At 1,400 rpm, turning the mixture screws out a half turn really had no impact. However, turning them in less than a quarter turn resulted in a quick drop in rpm and the engine basically choked. Had I not been maxed out on my idle circuit, I'm positive an increase in fuel from the mixture screws would've made a difference in rpm. This particular test shows that the engine wants more fuel. My wide-band air/fuel monitor showed full lean, over 16:1 before touching the mixture screws.
: Installed 50 idle jets and went through the lean-best idle procedure again. Brought the engine back up to 1,400 rpm and took the mixture screws out a quarter turn. No change in rpm or sound. Mixture screws back to the starting position and then in a quarter turn. No change in rpm but a slightly cleaner sound. Took them in another quarter turn and the rpm started to drop and the engine sounded rougher. This tells us that the engine has enough fuel by it not responding to giving it more with the mixture screw and it also tells us what our window is for going leaner by it dropping rpm at a half turn in on the mixture screw. The air/fuel ratio before touching the mixture screws was 14.2:1. Good! Now we know where the lean spot is and where it no-longer responds to more fuel.
: Installed 60 idle jets and went back through the lean-best idle procedure. Again, 1,400 rpm with the speed screw. Take the mixture screws out a quarter turn, no response in either sound or rpm. Turn them in a quarter turn from the starting point and only a vague improvement in sound, being a little cleaner. Another quarter turn in showed another barely detectable improvement in sound. This tells us that the engine is getting more fuel than it wants by not responding to additional fuel from the mixture screws and barely a slightly better sound from reducing the fuel with the mixture screws. Obviously, the volume of fuel being delivered by the 60 idle jets was so much that a smaller, controlled amount being added to or removed from the total by adjusting the mixture screw had very little to no impact. To visualize the scenario, adjust the little flow gate on a river dam that's already overflowing. The engine sounded sluggish and 'heavy' with these jets. Air/fuel ratio before touching the mixture screws was 11.8:1
The whole point with this test is that not everyone wants to spend $300+ on a wide band air/fuel monitor and the typical "X amount on the mixture screw at idle is perfect while driving" setting leaves a little to be desired. It does take a good ear and "feel" for what the engine is telling you but it's pretty damn accurate. Hopefully this helps bridge the gap by providing a more accurate means of sizing your idle jets.
Any and all thoughts are greatly appreciated, of course!