SBC base timing

[Wheelie]

ported vacuum is above the carb throttle plate and is always on the carb. It slowly pulls vacuum as the throttle valve is opened. If you put your finger over a ported vacuum source at idle it will be barely felt if at all but as you increase throttle it will pull more vacuum. Manifold vacuum is exactly what it is. It is a vacuum source pulled off the manifold or UNDER the throttle plate. Its vacuum will be constant and you will feel it pull a lot more vacuum at idle and usually if you unplug a manifold vacuum source while running it will run like crap until you plug it back up.