Comments on my 340 plug under mag

Mattax said

This is a little bit of a chicken and the egg situation. Not enough vacuum will mean less fuel gets pulled from the bowls, and what does get pulled into the intake isn't as well distributed and vaporized. Increasing the timing can help overcome that, and of course more spark lead is needed with slower burning situations.

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I just went through exactly this. Big RPM drop going into gear. Wound up using manifold vac to pull in a ton of advance to help. With the extra advance it took forever to warm up and needed a few minutes on the choke when to keep from dying when I put it in gear. The idle speed would also be too high once warm and if I adjusted it down then it was near impossible to keep running cold (or would stick once the carb cooled and shrunk!). Super annoying.
So I started over and went back to ported vacuum (static 12 deg at idle), then set the idle mix and idle rpm in-gear while warm. It wound up MUCH richer (2-2 1/8 turns out instead of 3/4 out) but now it fires off very easy when cold and can come off the choke in seconds. It also never dies going into gear or when coming to a stop while cold. The idle drop is also just under 100 rpm.

The two other hopped up engines I drive in the summer have gone through similar tuning routines. Lots of advance helped to mask fueling issues, but once I dialed in the fuel they simply run better with modest inital timing (18-22d) and ported vac advance. The most notable improvement is that they are no longer cranky when cold, but they simply run better overall.

My Fitech efi has 'idle stability spark control' to help control the idle, but for reference it adds 2~5 deg of advance when the idle dips (between 40 and 250rpm dip) and removes 1~2 deg when it surges a little higher (50-100rpm). I've actually reduced the numbers a bit to help keep it smoother so my idle advance stays about 20-24. It doesn't really take 10 degree swings to keep the idle stable when the fueling isn't super lean at idle.