With most non-stock Tigers, the vacuum advance found in the standard
distributors is a real pain. As normally delivered, it allows the
distributor to provide way too much advance at part throttle conditions.
Most of us have found that the Tiger likes a lot of initial advance, and the
combination of mechanical and vacuum advance gets the total way above
workable levels. The adjustable vacuum diaphram made by Crane (or the Ford
complement) is a very good solution. It is also advisable to modify the
mechanical advance and limit its total capability. The stock distributors
have a couple of orientations which can change this total, and you can
reduce this further with small pieces of shrink tubing, to decrease the
total movement.
By limiting the advance mechanisms, you can crank in more inital timing and
still not exceed workable full advance levels. I like initial setting in
the range of 12 to 16 degrees, with about 20 degrees (10 degrees
distributor) of mechanical advance. By limiting the vacuum advance to about
5 degrees, you can still get the part throttle benefits of vacuum and not
get into excessive advance problems under WOT.
I was able to use this early adjustable diaphram on a late model solid state
Ford distributor by bending the actuator arm to fit the later design. It's
a small change, but it no doubt contributes to "Blues" ability to get 24+
mpg (highway) with the late 302 engine.
PS. Always check your total advance by examining it with a timing light at
3000 to 3500 RPM, with the vacuum disconnected, to be sure that you won't
exceed 36 to 38 degrees total advance under power conditions.
Tom Hall
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