Addressing questions from Jay Quinn:
Definitions:
LBC: The BC is "British Car" and the 'L' depends on how
your car is treatig you today. It runs the gammit
from Lovely to Lousy.
MGBs and MGCs: The MGB is what everyone else has. The 'C'
model was made in '68 and '69 to fill the gap the
Big Healey was leaving and as an answer to the
horsepower race in the U.S. The 'C' was fitted with
a 3 liter 6 cyclinder engine of about 145 horsepower.
The engine is very heavy -- about 200 pounds more on
the front axle and is big so a torsion bar front
suspension was added. The six has a 7 main bearing
motor (after all the flack from 3-main early 'B'
motors) but all that internal friction keeps it from
reving very fast (in stock form).
The press was really ugly on the car (poor handling,
not enough speed) so after less than 9,000 built,
they quit.
The count is about evenly divided between roadsters and
GTs and, again, about evenly divided between coming to
the US and going anywhere else.
So, for my CGT, less than 2,500 were ever imported into
this country and that was 27 years ago. Nonetheless, it
doesn't hold it's value as well as a dirt-common TR6.
Mallory Dual Point Distributor:
This distributor has been around forever. It uses two sets of
ignition points and the breaker lobe has a modest bump. Thus
each point set has a modest jump but the two overlap to give
the correct dwell. This keeps "point bounce" from screwing up
your ignition and saves wear and tear on your points.
With computer points, you cannot adjust the dwell. You hope the
manufacturer did a good job. Sometimes the dwell is so short
the coil doesn't have time to fully drain. Also, if the trigger
isn't matched correctly to your car, the rotor isn't aligned to
the cap too good when the spark fires (read old Pertronix).
The rotor has a wide blade on it so that you can adjust your
timing to suit the motor and some part of the rotor blade is
still aligned to a plug wire in the cap. But, if the blade is
too wide, and the trigger too far off, and the distributor cap
to narrow, (e.g., LBC) then the spark can jump to adjacent plug
wires. Thus "modern" electronic ignition have relatively large
diameter distribtuor caps.
Centrifigal Advance:
The plate upon which the points mount is not permanetly mounted
to the distributor. It is allowed to rotate. There is a mechanism
of weights and springs under the contact plate to permit additional
advance the faster the distributor spins. This is a good thing as,
at higher RPM, the fire needs to be lit a little sooner while the
piston is racing towards TDC.
On the Mallory Dual point, there is a post that limits total
centrifigal advance and the post is nudged one way or the other
to set total advance anywhere you want. There is a hole in the
contact plate to get to this post.
The stock LBC didn't think this was any of your business an total
advance is not adjustable.
Vaccuum Advance:
Vaccum advance don't do diddly for performance. You only have
significant vaccuum when your foot ain't in it. And it your
foot ain't in it, your not having fun! Sooo... Vacuum advance
allows a motor to run smoother at cruise and, sometimes, will
improve gas mileage. But, thankfully, when you open up the
throttle plates, vaccuum subsides, and timing goes back to
a "normal" advance and the motor won't detonate.
Side note: Vacuum advance should always use "ported vacuum"
directly off the carb and should never be spliced into the
vaccum that runs other stuff -- like brake boosters. The
timing changes must be instantaneous while accessories may
try to store vaccuum for "average" use.
Bob Allen, Kansas City, '69MGC/GT
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