13B street port with MSD direct fire ignition. It has Vacum advanced and if I unplug the vacum line and plug up the port when I put the timing light on it the timing continues to climb while I increase the RPMs. Is this correct or why would this happen/

 

Mechanical advancers. There is a little spring inside the distributer that moves outward after a certain rpm and advances the ignition.

 

At ~4000 rpm full advanced timing should be seen. This is due to the mech timing advance mechanism in the dizzy.

 

Thanks for the responses! Being this having Vacum advances I didn't think it also had Mechanical advanced.