The Allison ignition is a rats nest of wiring, but the ignition originally
fitted to the car had the amplifier mounted right between the banks of
cylinders -- eventually the amplifier would burn out from the heat. There
is a Lucas kit to relocate the stock ignition amplifier, this is preferable
to the Allison in my mind. The tach needle probably does jump because of
the electronic ignition, something in aftermarket systems seems to break
Jaguar tach's. The tach will most likely have to be rebuilt even if you
remove the Allison, expect something like $50 for the rebuilt tach on an
exchange basis.
I had a similar problem when I replaced my Europa's mechanical points
ignition with a Piranha Electronic ignition. Turns out the Smiths tach
is, in essence, an ammeter - it just measures the current flowing through
the ignition circuit. The new-fangled electronic ignition pumped a lot
more current through the plugs (a Good Thing), especially at higher revs
(at 1000rpm the difference was barely noticible, 3500 rpm was the knee in
the curve, by 5500 (or so), the tach was offscale (pegged) at 8000rpm).
There is a company called "Nissonger" (or something like that), based in
New York (a few years ago) that deals with Smiths instruments. They are
alleged to be able to recalibrate Smiths tachs.
-RDH
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