Vacuum Advance: Good or Bad?

[Long Knight]

A shop told me that my vacuum advance wasn't working. I put an Ignitor on a little while ago and would have changed it then if I could have broken the screws loose and didn't need to drive my truck home that night. The vaccum advance can wasn't locked up (I could move the plate the points mounted to and it was still as tight as the new can was).

How do I know if my diaphram is bad? Would it be noticable on the highway? I really can't tell a performance difference on the road because I put a new tranny in right before I changed the ignition. Right now it runs better than it ever did, but I am always looking to improve when I can...(afford it).

Any input on this will be helpful. By the way, how do you rotate that plate to get to those screws?

Thanks,

Ro