Large RPM and Vacuum drop when shifting from park to gear

As others have said, apply a vacuum to the hose going to the can.

I use a vacuum hand pump. With the timming light you can watch the advance happen. You can also hold the vacuum and if the timming stays, the can is not leaking.

To other poster, there are adjustable cans, some have a screw to adjust the max advance some have springs and stops that can be adjusted to achieve the curve of your choice.