Hi Mike,
The problem most likely is that the flyback voltage pulse from the coil
(when the spark extinguishes) is re-triggering the tach electronics,
causing it to read double at lower RPM and eratically at certain other
times.
The modules that I build to replace the OEM electronics (but not the
movement) allow you to keep the original current sensing mode of
operation and only use voltage triggering if you need it, such as with
MSD or Jacobs ignition units. My modules can be connected directly to
the coil negative but this causes the behavior you describe in certain
instances (with specific coil and ignition combinations). The fix is to
rewire part of the input circuit to make it preferentially sensitive to
the initial voltage rise caused by the coil firing. However, I have no
idea what MoMa did to your tach so I can't speak specifically to it.
Tom Hall may be able to look at your tach and make a determination on
whether it can be restored to the OEM movement operation (with one of my
modules) or else you can send the tach up here. You can also try to
filter the tach signal from the coil, and I can give you some ideas to
play with but it may take a lot of messing around to get to an answer
that works.
One thing that you should check is the condition of the generator or
alternator brushes. The symptoms you describe (tach goes erratic when
lights on or at initial startup) seem to indicate that the problem is
worse when the charging load is higher, and that could be indicative of
alt/gen problems.
Good luck,
Theo
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