It is apparant that the Triumph engine with (more or less) stock
compression will tolerate a wide range of ignition timing, especially if
it is fed a diet of fuel richer in octane than normally required.
Here are some numbers to be considered when setting the timing:
The centrifugal advance mechanism has 20 deg. of timing built into it,
unless the limiting peg is modified. (More on this, if requested)
The 20 degrees of advance starts just off of idle and continues for
nearly 3,000 more rpm. (Based on info taken directly from the Lucas
Ignition Book)
The vacuum retard has 16 deg. of retard built into it. Timing set at 4
deg. ATDC will move to 12 deg. BTDC when the retard is interrupted by
throttle opening.
When the throttle is opened, no retard takes place. The timing, now at
12 can advance to as high as 32 deg. BTDC.
This would be a good place to mention that those using the retard should
have it connected to only one source, that being the port on the
carburetor designed to pull vacuum when the throttle is closed. Using
the manifold, or other carb vacuum sources, will allow retard to be
taking place at all speeds except WOT. This could make your timing as
much as 16 deg. late!
Having the advantage of both an in-cockpit vacuum gauge and timing
control allowed me to see where my engine was the happiest. The highest
vacuum signal will be given with what the engine feels is the right
timing. This will be true at both idle speeds and road speeds.
(Air/fuel ratios come into play here also, but that's for another time)
A vacuum gauge is inexpensive, and I recommend one for those who like to
tune their own engines.
Dick Taylor
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