3.2 - XK Engine Timing information ( Paul Saltwick,  June 7, 2005 )

Jaguar's ignition timing settings for Saloon's are static numbers,
so there is no vacuum when the engine is off.  When you check
the timing dynamically with the engine running using a timing light,
you have to disconnect and plug the vacuum line and make sure
the idle speed is not higher than 500 rpm (usually not the case).

If the idle is high or the springs in the distributor are weak,
(they usually are), the advance will affect the idle setting and you
will wind up with retarded ignition timing.  It is better to look up
the Max advance number, in your case 5 + 26 = 31, and have someone
rev the engine to 3000 rpm while you check the timing with a light
(vacuum disconnected).  You can use an advance light or mark the
damper by measuring the 10 degree mark and doubling and tripling
it.  After you set the Max advance, if it is showing more than 5
BTDC at idle, say 12-15 degrees, then your springs are worn. You
will often hear people on this list saying their car doesn't run
right unless you set the timing 10 degrees advanced, it is usually
because the distributor is worn out and the idle is high enough
to bring in the advance. 

There is 4 crank degrees of slop in the spec for a ''new'' Lucas distributor, so the spec is really 27-31 BTDC. 
I can tell you from experience a hemi head does not make best power at 27 degrees. 
If your engine runs hot and you can only get lousy fuel, you may not be
able to run the optimum timing. 

The XK engine is a knock limited design and the optimum advance
figure varies with engine temp, air temp and octane.  The engine
will keep making more power if you advance the ignition, but you
will more than likely run out of octane before you get there.

The E-Type used the same 13 degree (dist) advance mechanism with a 10
degree static setting.  I have used those cams in the past and
they have a little more duration, which in theory should allow more
advance because they bleed off low speed compression.  If you
have 180 psi of cranking pressure, your milled head may be offsetting
this a bit. Cold air to the carbs is the best modification you
can make to allow you to run the best timing.