Re: Merc 500
I believe you have 2 carbs both of which may need a rebuild with attention to cleaning.Does a tickle of choke affect wot speed?
You could also be missing one or more cylinders. There are those out there that don't realize their 2 cylinder outboard is on one cylinder except for the lack of power.You could be a cylinder short and not know it but for your speed.
Check that the throttle opens all the way.Be sure the timing is set right and advancing as it should.
What is the weight of your "speed boat"? Is it glass? Be careful trouble shooting the ignition.You should probably work with a factory manual.A 13" prop seems ok for a little speed boat.
My 65 500 has a 11" prop on a 375lb Crestliner Utility with about a 900lb all up weight.I don't have the specs on it but it has been setup since 65.
