what is a overspeed assembly on a 1994 hp500 do? its on the back of the motor buy the ignition system

It is the rev limiter used on TB IV ignitions on Merc racing/blue engines.

What Griff said.
it's # 41 in this picture
http://www.mercruiserparts.com/Show_...CAL+COMPONENTS

Yup, and when they act up they cause all kinds of weird problems.

i must be missing one then b/c my starboard engine does not have one. thanks guys

They can be bypassed.

You just get the module harness from a "Black" engine
TB IV ignition system, plug it into the ign module, connect it
to the dist, and coil, and you're back in business,
but, without a rev limiter.

i think thats what someone did. the starboard one must have went bad and they bypassed it. they both still run fine. just the one without a limiter.