wrote:
It really sounds like the tach is off. An easy way to check (at least at
one
RPM) is to fire it up under some orange sodium lights at night. Run it up to
where
the prop "stops" by strobe effect of the lights. Assuming a 60Hz country, a
2-bladed
prop will stop at 1800 RPM, a 3-bladed at 1200.

Does this work at a big field with lots of lights? The power feed will
certainly be 3-phase, with various lights randomly fed from different
phases. Do you really get enough of a strobe effect at 60 Hz to make
this work?

On the other hand, if you've got a friend who's a musician (or a piano
tuner), they might have a electronic pitch meter you can borrow.