Vacuum secondaries will come on when you demand enough to cause them to.
Mechanical secondaries will come on based on the the throttle position. RPM
has nothing to do with either.

"Mike Harrison" wrote in message
om...
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?