Hi all.

My 1988 XJS V12 (UK) idles far to fast, about 1400 revs.
According to my CD manual the first thing to check is the overrun valves by
blocking them both then unblocking one then the other and starting the
engine each time.
Doing this makes no difference to the idle speed. But if I block both of
them the car starts much easier and quicker, on the turn of the key instead
of a couple of seconds. unblocking one or the other or both then it's
starts, but only after a few turns.
To the best that I can tell (by putting my finger loosly over the hole in
the air filter housing) there is no air going through the valves at idle
(about 1400 revs).
Does anyone know how these valves do their job, and maybe an explaination of
why the car starts so much easier when the valves are blocked. And even
better! Why this engine idles so fast. I have checked throttle linkage and
butterfly valves and all seems as it should.

Thanks for any info

Ivar