I have a Chevy 250 that came off a 1970 Camaro 4.1L. I recently switched to an HEI distributor and the car was idling good after timing it. But once I drove it around it died on me a couple of times and it's running rough, almost like it's misfiring. We checked the vacuum where it has always been hooked up to and it didn't seem tobe working. So we went to another port but I'm just not sure if it's correct. Can someone tell me where the vacuum advance line from the distributor goes to the carb? Which port? I wanted to post pics but I don't know how.

Did you switch to a new, used or rebuilt HEI?

I bought a new HEI distributor. I have been working on it with a friend. He timed it for me but where he located my vacuum advance, it makes the car idle rough and it vibrates the engine. When I re-locate it to a different spot on the carburetor it seems the vibration goes away but he's telling me I don't have any vacuum there when he tested. I guess I'm confused. I can't seem to find a diagram that will tell me what each ports are.

You kneed to establish whether you need full time vacuum or ported vacuum. Then you need to find the proper port for the application. Some ports on your carburetor will have vacuum on them all the time. Some will only have vacuum when the throttle is opened.

Find which is which and which you need and go from there!!

A port or fitting below the throttle shaft is full vacuum, with the same signal as any manifold connection.
Above it is ported. Ported has no vacuum at idle, but begins when the throttle disc's edge rotates above the internal hole visible in the throat wall.

Thank you both. I did try moving the hose from one port to the other and it did make some difference. On the port opposite from the valve cover, the engine was rough and it vibrated. But on the inner port closest to valve cover, it ran smooth. I haven't tried driving it though.

The idle speed will require a different adjustment for each vacuum source.

I will keep that in mind. Thanks

The car is running great guys. Thank you everyone who took the time to answer my questions.