Ignition Timing Question

[Tio Pancho]

Ok rookie question here. 1977 Cherokee Chief with 360, 4 bbl. with an HEI distributor. Engine is freshly rebuilt and starts fine. No real issues there. But I need to break in the cam. My question is with the timing. I set the initial at about 5*btc with the vacuum line disconnected. But when I connect the vacuum line back up it jumps to about 15*. I believe the timing should remain at 5* after connecting the line and only advance when the throttle is applied.
What am i missing here and how do I address this? Is this done with the allen wrench inside the vacuum advance canister?

Thank you in advance.