i think your surge protector is out of whack
18 hz just slightly above 1/5 of speed,
is your generator really running at only 800 rpm ? normal speed is 3600rpm big big difference, way beyond { i think its running slow } more like "it barely runs and voltage is 50v"
now IF one pole in the windings was dead (2) pole generator
full speed would show 30hz and halve speed 1800 rpm would show 15hz
yes vltage islow and speed probably needs adjustment
just plug in a box fan or something and see how fast it turns or doesn't
that will give you a working idea of whats going on
I can explain it to you.
But I Can Not understand it for you !
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1997 F53 Bounder 36s