The vacuum controller is not a delay valve but rather a controller that supplies a steady minimum amount of vacuum at idle. Above idle, it lets ported vacuum take over when the port is above 6 in of vacuum for the XJ6 and 11 in for the V12 HE. At full throttle, there is not much vacuum so it drops off. This extra advance is to give the engine more advance ignition timing at idle to run cooler and use less fuel. This unit is also used on some American cars as one of the ports is labeled “Delay”. This unit does not delay vacuum but that is where it is hooked up on other non-Jaguar applications. Easy way to check is to measure the vacuum at the distributor with the above mentioned specs assuming it is hooked up properly. Delay=manifold vacuum , Dist=distributor vacuum unit and Carb=Ported throttle (not direct manifold vacuum)