The vacuum hole is used to lift the friction brake on the Passenger car version of the Lucas distributor. The Passenger type dizzy (as opposed to the Commercial type) has a rotating disc inside it against the edge of which  a light spring loaded leather disc bears. This to to prevent the dizzy over advancing. However when the engine accelerates and creates a high vacuum on the inlet manifold the vacuum reduces the pressure on the disc. The vacuum port is between cylinders 7 and 8 so if being used for a diagnostic vacuum gauge test it will only indicate the condition in those two cylinders. Ford flatheads have no timing marks. The Ford recommendation is that the engines be tuned to achieved the highest vacuum reading.
24 February 2021 at 3:13 PM #5792
Michael Knapman
Participant

The vacuum hole is used to lift the friction brake on the Passenger car version of the Lucas distributor. The Passenger type dizzy (as opposed to the Commercial type) has a rotating disc inside it against the edge of which  a light spring loaded leather disc bears. This to to prevent the dizzy over advancing. However when the engine accelerates and creates a high vacuum on the inlet manifold the vacuum reduces the pressure on the disc.
The vacuum port is between cylinders 7 and 8 so if being used for a diagnostic vacuum gauge test it will only indicate the condition in those two cylinders.

Ford flatheads have no timing marks. The Ford recommendation is that the engines be tuned to achieved the highest vacuum reading.