[aeroace]

Holley does make the vacuum pot covers with nipples for connecting them together. They should be available at any speed shop that sells jets, etc. I usually remove the original ones, drill a 3/32" hole in them, insert an aluminun line about 1" long, flare the backside of the line, and seal it with loctite quick set epoxy. This looks factory if done on the later Holleys that are silver, if done correctly. Just buy a pair with the tubes already attached instead; it's ,much easier.

As for secondary opening rates: it's a question of airflow. Too early and you are overcarburated, too late and you could go a little faster. Too little carburation is always more driveable and fun than too much carburation. Jetting is separate, as it's a ratio of air to fuel.