I have the 2015 Harley Street 750, and the coolant dumps out of the overflow tank, when its not moving. i.e. after I ride a few hours, and stop the coolant starts coming out of the overflow tank.

 

Sounds like the coolant system may be overfilled. What is the level shown in the tank when the bike is stone cold? If it's high you might drain a bit out. There may be air in the system as well causing it to "burp". Since the coolant is now going on 3 years old it may be a good time to do a drain and refill with fresh. This is shown in the service manual.