Large RPM and Vacuum drop when shifting from park to gear

1. Set timming and idle speed with vacuum advance disconnected and plugged.
2. Put the vacuum advance on the correct ported vacuum port on the carb

What I suspect is happening is with vacuum advance attached to manifold vacuum you have high vacuum at idle and that is pulling your vacuum advance to full, (let's assume 15 deg.) So you set the static timming to 10 BTDC. You put it in gear the engine rpm decreased and now you loose enough vacuum that the vacuum advance drops out and
now you are at 5 ATDC,