Ok, I'll give this a try:
The seconaries open due to air volocity, meaing that at a certain air flow/throttle position the secondaries open. Greg is right in that the rods stage between "cruise" and "power" modes. The step up rod is this carborator has 2 different size steps. The small diameter part of the rod rests inside of the jet and meters idle fuel and light cruise fuel so long as vacuum is high. When vacuum drops below ~ 8", the spring on the rod pushes the rod up out of the jet and provides more fuel. This makes sense since the motor is getting more air and thus needs more fuel. The point at which the rod moves up is controlled by the spring tension (and vacuum). Edelbrock provides 5 springs: blue-3", yellow-4", orange-5", pink-7" and plain-8". Most of the carbs that Edelbrock sells come from the factory with the orange spring.
If you want to delay the opening of your secondaries, then you must increase your primary jet size of go with a smaller tip rod. This will give you more gas but will make the motor run rich.
As for the delay or bog from idle, the Edelbrock rep was almost right. By switching to a blue spring, you would delay the trasition from cruise to power mode until you vacuum was next to nothing. This indicates that you are to rich on your current calibration. A larger tip rod would help your situation. You can also move the pump arm from the center hole to the top hole and see if this helps (less pump volume). Sorry about being long winded.
Brian
