It sounds as if you have solved your problem, but if a tach is what you
are after, I have had good luck with a $24 unit sold by Tower Hobbies
for tuning model airplane engines.
you would have to put a piece of tape on the flywheel to mimic an
airplane prop.

I did this on my yanmar diesel and it worked nicely. Discovered that
the tach in my instrument panel was off by quite a bit.

good luck,
Andy