Plugging Timed vacuum advance hose

It might be helpful to keep in mind the goal of timing with respect to vacuum. Vacuum is a good indicator of load and throttle position. More manifold vacuum indicates lower load, leaner mixtures and therefore more time required for the flame to grow.

However at idle, using manifold vacuum is not always the best indicator of mixture. This can be due to exhaust dilution and related factors. So the way to decouple the vacuum signal at idle is to internally place the vacuum port so it is just above the throttle blade at idle position.