Vacume Problem?

I have a 440 duster that I am almost finished putting together. I seam to be having a vacume issue. I thought I would pick your brains.

When I have been running the car it Idles ok, and you can punch the throtle with short bursts and it wil Back fire a bit ( open headers) but if I slowly bring the rpms up, It won't have any part of it, and all it wants to do is Back fire, and almost die, then come back then almost die, then come back, then....... When It is running like that i have all the ports on the carb pluged except for one, and I have it with a hose conecting it to the distributer, and it back fires when I have it like that. If I disconect it from the distributer it runs fine with no backfiring, I can hold it a 2500RPM nice and smooth, I can't do that if the distributer is conected to the Carb.

I can feal the end of the hose that is conected to the carb and it is sucking more and more as the throtle gets used. I pluged the distributer, and took the hose off and pluged the last carb port too, and it seams to run fine with NO problems now

Is that the vacume advance? if not, what is it?
and do I need to use it?
can I just plug it? seams to work ok that way.
If I don't need it, Why did it come with the distributer???

thanks

Phil