I have no idea why your tach needle rests at 500.  Mine parks at zero.  

I do recall several instances where it parked at random locations when the power was interrupted before the tach could complete its initial power-up routine.   That was associated with those aux contacts in the starter button.  Those contacts gave me a lot of grief before I figured out what was goin on.  Those wimpy contacts have to continuously carry 5.4 amps.  While that's not a whole lot of current, it takes a toll when the contacts are always opening and closing, and the switch assembly is exposed to weather. Once it starts to arc and get hot, the heat transfers to the main contacts and those start to screw up. Then it won't crank. 

Toot the horn and the current increases.  Its resistance is 1.9 ohms so if it was just a static load you can add another 6.3 amps.  Now your up close to 12 amps total.  But the horn is a dynamic load.  No tellin how it behaves.