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plug hole than the 1A. And, obviously, there are many possible variations on this idea, including threading a fitting
into the spark plug base and the use of a balloon instead of soap. Note that this method won’t work if you’re at the top
of the exhaust stroke instead of the compression stroke, so if you seem to be having trouble you might want to give the
crank one complete revolution and try again.
The ignition timing on the V12 is checked with the engine held at 3000 RPM. Although a pain, this method insures the
timing is accurate at operating speed rather than at idle, where timing is less critical. However, proper timing by this
method requires that the tachometer be at least reasonably accurate. It’s not very critical, since it would have to be in
error by about 300 RPM to cause an error of 1° in the timing. Nevertheless, if one ever finds his car hooked up to one
of those fancy, super-accurate computerized engine analysis machines, it is suggested that the location of the
tachometer needle be noted when the engine is running at a real 3000 RPM. That point can then be held whenever the
timing is checked in the future.
When checking the ignition timing on the V12, the vacuum line to the distributor vacuum advance must be
disconnected and plugged. The vacuum advance capsule is at the bottom rear of the distributor, making it quite
difficult to get to this line (the larger hoses connecting to the distributor cap itself are for the distributor ventilation
system, and do not affect timing). Do yourself a favor and cut the vacuum line somewhere convenient and reconnect it
with a small piece of tubing. From then on, all you have to do is disconnect it at the break and plug it when checking
the timing.
Since the battery is in the trunk, connecting the power leads of the timing light requires ingenuity. The ground lead can
be connected anywhere on the car. The positive lead must go to 12V, which exists at any solid brown wire. There are
two terminals on the firewall adjacent to the valve covers, and a terminal on the back of the alternator -- all difficult to
get at. Another solution is to use a pair of jumper cables from the battery in the trunk. Peter Smith: “I connect my
timing light to the 12V power source at the headlights fuse box.”
Since disconnecting the plug wire from cylinder 1A is difficult, disconnect the wire from the distributor cap instead;
make a small jumper from an old ignition wire to use to connect the timing light. Or, just buy an inductive timing light.
If it’s more convenient, you can also check the timing using the signal from cylinder 6A.
Since you must crawl under the front of the car while an assistant holds the engine at 3000 RPM to read the timing, it is
suggested you put an extra long ignition lead on your timing light so you can route it around the fender rather than
feeding it through the engine compartment. Interference with moving parts at 3000 RPM would be memorable. Cheap
ignition wire with metal conductor works well for making a long timing light ignition lead, and is available by the foot.
Steve Chatman came up with another idea: “When checking the timing on the XJ12, I discovered that I was either
going to extend the wire that clamps the plug wire or feed the light through the engine compartment. I didn't really like
those alternatives and instead simply ran the timing light over and down the outside right front wing. From there, I
could shine the light under the car, using my left hand. With my right hand, I held a mirror to reflect the light vertically.
Worked fine and seemed very safe. Of course, you should correct for the increased distance traveled by the light, but
you knew that. I was in a poorly lighted garage at night. Better lighting could make this approach difficult or
impossible to use.”
Michel Carpentier provides another way of dealing with the inconvenient location of the timing marks: “Make a
pointer out of sheet metal, which you fit under one of the water pump nuts (pointing from above towards the crank
pulley). Now position the crank so it is in firing position at 3000 RPM and put a dab of white paint opposite your
pointer. From now on you can check your timing from above.”
Timing is adjusted using a long screwdriver to turn a small eccentric cam on the side of the distributor underneath the
cruise control actuator. There is a locknut on the adjuster, and this is a place where a crowfoot wrench will come in
very handy.
The Jaguar V12 design eliminates most wear items that would normally cause ignition timing to vary, such as ignition
points or sloppy camshaft drive systems. The only remaining reason for the timing to change would be wear in the
timing chain itself, and the Jaguar timing chain normally wears so slowly as to be insignificant. If the timing in your
car is way off, it is highly recommended you determine why rather than simply readjust it. For example: If your
centrifugal advance unit seizes in the idle position, then when the car is revved to 3000 RPM the timing will be retarded