I'll add my $ 0.02 and cure to the backfiring discussion.
My impression is that backfiring when you shift was a common
problem on all MGBs with air pumps. My wife's
cousin had one back in 1973 or 74 which did this and her
mechanic told her that "all MGs do that".
In any event, my solution to the problem was the anti-backfire
kit from VB (part # 2-750). Their instructions call for removing
the gulp valve and plugging the port on the intake manifold
and air pump. The new valve from the kit is connected between
the airpump and the air injector manifold and actuated by
a vacuum line. It completely cured the backfiring problem.
Good luck,
Peter Schauss
1980 MGB
1963 Austin-Healey Mk II
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