Vacuum Advance Help

@sincity-Okay. I'm currently using the stock carb, so which port is "venturi" the one coming from the base of the carb below the throttle plates or one of the ones above? There is one just below the choke plate on the inside(valve cover side of the carb)I think.

Venturi vacuum comes from below the plates and the one you should be looking for is on the front of the carb.

Question. If the venturi port is below the plates and a manifold port is below the plates, then what's the differnce between them. Wouldn't they both pull constant vacuum until a near WOT where vacuum drops significantly?

Venturi port is best, closer to the cause of the vacumn (plate in the carb), for better response, i.e. greater, quicker advance movement.

Good question, but a misunderstanding of how venturi vs. manifold vacuum works. At idle, manifold vacuum is based on what is going into the combustion chamber as a function of intake stroke, while ported vacuum is near zero psi. Once the throttle plate begins to open, ported vacuum begins to rise as a byproduct of the now opened port. That is what sucks fuel into the intake stream as the throttle is opened.

Thanks guys both those reasons make sense. It's been a long time since I had to think about carb related systems and most of the info out in cyberland is geared toward the injection systems. This truck has been in my family since brand new and I just want to repair it and modify it slightly. I don't suppose my family can complain because the truck has I think over 420k on it. I say "think" because the odometer cable broke at around 335k and my dad didn't bother replacing it. These trucks will last if taken care of.