Can anyone tell me which vacuum ports on a rochester M4MC are for what... I have a 75 corvette that I just bought and nothing on the vaccum system really works and I have more lines with bolts on it then I have cylinders.. Its hard to figure out what is what because I don't have any emissions systems left on the car... So I figure if I can figure out what is what on the carb I can hook everything else up from there...

 

i have one of those on my 76 chevy, so i'll tell you what i know, which is not much.
there are some ports above the plate, and some below. i'm not sure exactly what this means, but i think it has something to do with when you have vac/not based on whether the throttle is open/not ??

on my truck, i have removed umpteen pieces of emissions, thermal, warm up, egr, etc that didn't seem to contribute anything.
the big port on the front below the plate is for pcv. i have moved mine around, to above and below the plate, while chasing stalling problems that seemed vac related.
the small port on the back is for the auto transmission, which is required for shifting.
passenger side has one for the choke pulloff. i've never understood this one, but its still on.
another one near the front is for the vac advance in the distributor.
... i have plugged up all the others...
hope this helps. let me know if you need any pics.