I have measured the vacuum at the ported and manifold vacuum ports. The manifold port had 50cmHg at idle, which went to 45 cmHg as soon as the throttle was opened slightly. The ported port has 0cmHg at idle, jumping to 45 cmHg as soon as the throttle was opened slightly.

Once you are driving (ie not sitting in the car with it idling), there is no difference in the performance with ported or manifold vacuum. They are the same. There might be a difference at idle between the two, which might have an effect on idle emisssions. I have no idea about that.

When I did the measurements on my carb, the front port on the carb mounting block was manifold and the second port from the front was ported. I don't know what year carb/block that is. When I checked an 84 carb, the two ports on the carb (not block) behind the idle screw were manifold in the front and ported in the rear. See the attached picture.

So necro thread and all that but what's so different about the rx3 manifold?

Also do I have this diagram right then? Whats with the other 2 in mine?
http://i2.photobucket.com/albums/y30...111_224540.jpg