Paul,
Since you don't seem to be interested in the obvious solution of buying an
older model, I will give it a shot.
The assumption that a 73 B tuned perfectly to stock spec may not be a good
one. Remember that emissions requirements were escalating in America at
this time and most cars had driveability problems. I have owned/driven
many 1974 cars and none of them ran well. As much as we trash post 75 B's,
the catalytic converter was a huge step forward as far as allowing cars to
be tuned to run better and still put out fewer emissions. The 73 B was not
tuned from the factory to run well but to run as well as it could and still
pass emissions.
If it were my car, I would first advance the timing until I either got a
kickback against the starter or knock on acceleration then I would dial her
back until the problem just goes away. This won't be correct for the spec
but it will be correct for your car. Paul H. times by vacuum. I have
never done this but it seems to work for him. I prefer to time by
performance.
Next, if advancing the timing doesn't fix the problem, I would start adding
fuel until it goes away. It may idle a little lumpy but that can be fixed
with different needles once you get the cruise/acceleration right. Of the
two, I would rather have the car run well at cruise.
If you are feeling very adventurous, you could check the cam timing. I
have heard that an early trick to meet emission limits was to retard the
camshaft. This would also effect driveability.
So, to summarize, a 73 MG was built by a company with no development money
to meet an emissions standard that they thought was stupid and unfair. The
stock needle and timings in your car were chosen to meet the regulations,
not to make it run well. They were also chosen considering a worst case
stackup of tolerance in a new car. When you use a Colortune or CO meter to
set the idle mixture, you have a perfect idle mixture. If you want your
car to run well, tune it to run well. Screw what the manual says. As
always, my opinion, YMMV, in God we trust, yield to traffic on the left.
Regards,
Bill Eastman
61 MGA with a rough idle but decent mid/high range.
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