Hi, Bill...
The micro-adjuster was originated to
advance or retard the timing a bit for
different octane gasolines, though of course it can be used to micro
adjust
the initial timing setting. Using it to
micro adjust the basic setting can lead
to "retarded" innaccuracy as can setting
the distributor (for timing) going toward
"retarded" (CCW) rather than going from
"retarded" toward "advanced" (CW) due
to mechanical play or whatever within
the system (originally..points).
The micro-adjustor mechanically repositions the vac. advance "plate"
within the dizzy, which is connected
by a small spring to the vac. advance
plate pivot pin. If this spring is not present
or connected (or the diaphragm within the
dizzy's vac. "can" is deteriorated), then
no micro adjustment takes place in the
first instance, NOR vacuum "advance" "action" in the case of either
condition.
Not to important on cars wherein vac. advance was ported to the carb.
but
a required item on later emissions carbs.
wherein this feature was ported to the
intake manifold.
According to Bill Perry, the only way to
accurately set timing is by total timing
for this is the number that really matters
as all else falls into place in accordance
with this priority, all things being equal.
Considering most factory advance curves,
this can be up over 4k rpm.
There are reasons for this method, as parts wear over time, so the
actual curve
will change just a bit from factory new over time in terms of mechanical
dizzy's.
The curves (advance rates)
are pretty conservative from the factory
so the only number of prioritiy (the one
that can get you into trouble) is "total
advance" a la Colin's post on this.
The engine (early..vac. advance ported
to carb) uses vac advance under light
load (partial cylinder filling) for better
fuel economy, "performance" being dependant on the mechanical advance
rate. So vac. advance is optional, so to
speak...the engine can live with, or
without it. Under partial cyl. filling or
light load conditions, you can light the
fire earlier in the process (considering
burn rate of "partial charges)) without getting into
pre-ignition/detonation
problems which enables one to get more "bounce to ounce" per lb. of
fuel burned (specific fuel consumption
or "economy").
As the load increases due to
a heavy foot or desire to hold speed while
going up a hill, the vac. to the vac. advance source decreases and the
engine
works off the mechanical advance rate
accordingly (when vac. is ported to carb.).
Assuming an early Spridget, vac. advance
is ported to the carb. So It is disconnected
when "setting" the timing "dynamically"
(or the setting will be set "overadvanced")
the priority being the correct setting of the "total" (more than rate)
mechanical ignition advance, for this is the number that will get you
into trouble one way or
the other, per Colin's post on this subject.
For a stock Spridget, in accordance with
Vizard, 32 mechanical degrees seems to be the magic number more or less,
assuming premium unleaded (yecchh!) fuel and stock engine set-up.
If you can't get 92 or 93 unleaded prem.,
then 30 degrees of total mechanical
advance may work better for you. Back
in the day of "manly octanes" (I love
the smell of lead in the morning) 34 degrees of total mechanical advance
on stock Series A motors was not uncommon. Disgustingly, those
days are gone unless you're fortunate
enough to have a CAM2 distributor in
your area and don't mind spending $5 a
gal.+ for the stuff to put a little hair on your motor.
Do you have a copy of Vizard's bible?
If not, you really should pick one up. He
gets fairly well into this on his chapter
which includes "timing" so you know
the "mechanics" behind the procedure
(combustion burn rates, etc.).
For more on Lucas distributors, you might
want to visit www.teglerizer.com. Click
onto MG's and appropriate tech article
under distributors. :):)
Cap'n. Bob
'60 :{)
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