I had tried 15 initial before. Yes, rpms go down slightly. But I can't compensate this with mixture screws turning out. It won't respond. I'd have to open a throttle blade to up rpms. But I will give it another try.
Got it. Being able to open the primary throttle blades slightly more was all that might be gained by going to 15 degrees. Based on your rainy day experience below, it doesn't seem neccessary
Rpm drop between N and D isn't much. 50 rpms or so. No harsh engagement.
That's pretty good. With the 2.5 in drop in vac I thought rpm might be dropping a couple hundred rpm. Even though your meter reads 14.2 AFR, it seems there is enough power. Maybe you'd get a little more power with a richer mix, but some people are OK with near stoich at idle. Leaner saves fuel and if no misfires, less HC and CO out the exhaust. Only downsides are a little more NOx and the engine may be a little hotter than with a richer mix.
"mixture screws don't react as they should": I mean what I wrote above. Turning the mixture screws in or out doesn't do much to idle rpm or idle AFR (or idle quality). I can influence idle AFR by about +-0.15 by means of the mixture screws. Nearly nothing.
I agree with Dr Charles. If turning the mixture screws all-the-way-in does nothing of significance, then fuel is coming from somewhere else. The primary transfer slot is only the normal place it should be coming from in your current iteration. If it is, then my only suggestion is try one size smaller Primary IFRs.
Will try the IAB closing trick. Interesting!
But I'd say decreasing IAB size would destroy my lower part throttle AFRs.
IAB changes will not have as much effect on the mix as the IFR changes, but yes I agree it will normally make part throttle a little richer. (I keep saying normally because once the IAB an IFR sizes are outside a working range the fluid mix becomes inconsistant,usually too much air, and the effect is sometimes opposite from expected)
If you go smaller on the fuel restrictions, then going smaller on the idle air bleeds should help keep upper transition consistant.
Another option is to put a restriction in the passage leading to the transfer slot. My suggestion is to save that as a last resort.
If you find the idle/transition peters out too early with the smaller IFRs, the smaller IABs will help.
Yesterday I drove the car in slight rain, wet roads. Therefore I could use the throttle only very gently. Driveability was great, no bogs, lags, flat spots. Once I am off idle everything is responsive to changes (IFR, IAB, main jets etc.) and runs very well. Idle is the only thing that acts strange...
Well then it would seem to be rich enough through most of the idle circuit. If you left it as is, that would be an understandable decision.
If you want to try to improve it, my suggestion is try the smaller IFRs first. If you are willing to do the work, then consider placing them in the lower position.
With the smaller IFRs, you probably will then be able to use smaller IABs, at least on the primaries. Then the mix screws
may start controlling a bigger proportion of the fuel at idle.
