At 10:25 AM 2/20/2003, you wrote:
 > If gap is too short, spark happens early; if too long, spark happens late.

Sorry to be negative... but WHAT ABSOLUTE TWADDLE -
the spark plug gap does not affect the timing one jot - weak spark, splashy
spark, no spark, good spark, but as for when it comes, no.

Clive, I disagree with the above.
When the points open, the spark does not jump immediately.
The magnetic field takes some time to collapse and the voltage
across the secondary builds up over a brief time until it has
enough voltage to jump the spark gap. The wider the gap, the higher
the secondary voltage must rise and it takes a longer time to reach
this voltage. It's easily observable on an oscilloscope.
How long it takes the coil to reach a particular voltage is a function
of the inductance of the coil.

Find someone who's got an oscilloscope and play with an ignition coil
and spark plug and you can watch the secondary rise time for yourself.

At 3600 RPM one degree of crank rotation takes about 46 microseconds...
Changes in spark gap can definately make small but easily measurable
changes in timing.

Just my .02
Steve