Jaguar Repair: 1987 xjs vacuum, poor vacuum, vacuum lines

Question
I seem to have poor vacuum. At idle it shows about 7psi. When revved up and released it goes to a full 15 psi then returns to 7. This creates problems with cruise and also brakes. Ignition timing is 10 bdc at 750 rpm. There is also a miss at idle.

Answer
Hi Ron,
If it came to me like that I would first pinch and/or block off all vacuum lines, connections etc and if it is still low run a "Leak-down" test on each combustion chamber.
An engine is nothing but a pump and if it is not producing a good vacuum it either has a very wild cam or it has a leak somewhere, not allowing it to pull a good vacuum.
You never said anything about it's ability to rev and pull at high RPM. If that is down too, you might check exhaust back pressure for a possible exhaust restriction. If it revs good and pulls good at high RPM you can forget that.
Howard