Mike Harrison wrote:

Hello group, hopefully a simple question:
For a four-barrel carb, at what RPM setting should the 3&4 barrels be
set to open at? For example should they open when attempting to climb
onto plane, and then once on plane, throttle back and plane with only
1&2 open?

Drag boat / jet boat? Snap the secondaries open off the line, these are
suitable applications for mechanical secondaries.

The vast majority of recreational boating applications are best served
by vacuum operated secondaries. The primaries are used at lower engine
speeds to keep the intake velocities up and provide good mixing &
atomization.

Draw the parallel: a boat is a big, heavy, sluggish vehicle with a WOT
top speed well below any comparably powered land vehicle, and with only
one gear to compound this!

The secondaries should be opened slowly and late, based on demand...
typical for a recreational boat would be starting to open around
3200-3500 rpm, and rarely fully open(probably about 1/2) as a top rpm of
4800-5200 is way too low to demand the entire 600-650 cfm from a small
block / up to 350ci engine.

Rob