[I sent this yesterday but it never showed up]
1200 RPM is higher than necessary for the idle (burns gas). I keep mine down
around 850 (hot). Advancing the timing will speed up the idle (to a point),
so you will want to readjust the idle downward, then readjust the timing,
and so forth.
A correction: the vernier on the distributor does not adjust the dwell -- it
advances or retards the timing, by tiny increments. I seem to recall that
the total adjustment range of the vernier is something like 4-1/2 degrees,
so if the timing is way off, you will have to loosen the distributor to
adjust it.
The dwell is adjusted by setting the point gap -- it is just a different way
of measuring the same thing. The point gap measurement is the mechanical
process of setting the dwell -- the dwell meter tells you if you set the
point gap correctly.
The only way to set timing dynamically is with a timing light. I'm sure you
could find one closer to $40, but a good one will last practically forever,
and you will need to tune your MG several times a year, so it's a good
investment. While it is true you can set timing staticly with a test lamp,
it is inconvenient, and doesn't tell you anything about the state of the
distributor advance mechanisms.
I don't know why your new distributor is different than the old one, unless
you bought the wrong one, or the previous owner had installed the wrong one.
The vacuum advance unit (or "pot") always wobbles (if it is the adjustable
variety), so that was not a sign of failure. If the vernier was frozen, that
would be only a minor inconvenience, since you can always adjust the timing
by rotating the distributor anyway. The following would be signs of failure
in parts of the distributor: the vacuum advance diaphragm leaks (easiest
test is to suck on the tube and see if you get air); the vacuum advance
spring is broken where it connects to the breaker plate; the rotor has a
noticeable wobble when rotated; the breaker plate fails to turn smoothly;
the breaker plate ground strap is frayed or broken; the clamping surface at
the base of the distributor housing is cracked.
--
Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the primer red one with chrome wires
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