Carb tuning, larger accelerator pump nozzle?

yellow rose said
Maybe @Mattax will see this and post a link to the paper.

Lower IFR papers ? That would be NACA 49 http://naca.central.cranfield.ac.uk/reports/1920/naca-report-49.pdf

Most relevant conclusion Mark quotes at RFS is..
"Submerged metering passages are free from instability and irregularity of discharge when the head is very small".

I'm going to cut and paste the rest of Mark's summary..
"What this mean is that on a performance engine with low idle vacuum the idle characteristics will be improved with the idle feed in the lower position. Because the idle circuit can impact the the air bled into the mainwell at high speeds, it may be that someone in the past saw an improvement on a specific engine at high speeds by moving the idle feed up, however with the advent of metering blocks with more emulsion holes to chose from making changes to the main metering does not have to be done by manipulating the idle feed. This was also done without regard to how it affects the idle and transition metering, both are improved significantly when moving the idle feed back to the original location."

and from Tuner
"
Locating the Idle Jet below float level is more important than many people realize because cruising at a low RPM and steady speed the fuel delivery is nearly 100% through the idle circuit (T-slot), and with the jet above float level the AFR oscillates a lot, as much as 2 AFR or more. Smooth idle, clean sparkplugs and oil, and good fuel economy, all depend on a consistent idle-low speed circuit.

An un-damped vacuum gauge will clearly show engine manifold pressure (vacuum) oscillates constantly and as valve timing becomes more radical the increased overlap causes larger pressure swings and more intense oscillation.

An un-damped gauge clearly shows engines are not a steady flow device. The intake pressure (vacuum) is oscillating with each intake suction stroke and overlap reversion pulse. Because passages and cavities in carbs and manifolds have individual resonances at particular frequencies, the resultant effects are more pronounced at some engine speeds than others. Because air is compressible, it is elastic, it can rebound like a spring and as a result it bounces and reverses direction in small passages in carbs.

With a high (above float level) idle jet, air can easily get upstream behind the jet when the idle fuel is pushed backwards by the pulsing.

When the jet is submerged, the air cannot get back upstream of it as easily because the much greater viscosity of the liquid fuel (than air) on both sides of the jet impedes the backflow and the air cannot get back through the jet unless the flow is reversed for a longer period of time than the frequency of the pulsations.

Another undesirable phenomenon caused by the high idle jet is the larger oscillations in the idle circuit causes a pumping effect on the main well that results in the booster nozzle dripping in spurts and blobs before it would normally start to flow if the venturi air flow were the only excitation and the idle circuit flow was more regular. With a submerged jet the metering reversion isn’t as severe or does not occur at all unless the pressure reverses for a longer time period than the manifold pressure (vacuum) pulses at a cruise.

This submerged idle jet phenomena was thoroughly studied and well understood 95 years ago during aircraft carburetor research for World War One."

Above information is from here: Relocating the Holley Idle Jet

I have too many irons in the fire so my apologies for being a little less involved in the forums, even this one.