Hello, I have a siple question, I think. I have a reman Rochester 1 barrel for my 66 C20 with a 250, and I can't seem to find which port I can use for my spark advance. The model is 7040011, it has three ports on it, one in the base plate and two(one on top of the other) on the side of the throttle body. My understanding is that the port should have no vacuum at idle and should then increase to something above 16 inches once opened.....it has a manual choke and no other control devices on it. Which of the three should I use? Does anybody happen to "know" this carb and what each port is for? Thanks, Josh