Ok so I have an 84 fxr. I've recently installed a new starter but it sounded like the shaft wasn't engaging with the primary. So I pulled the primary cover and it is engaging. But it sounds like the starter is just spinning freely when trying to start. Is there just not enough juice in my battery to turn it over? It holds a charge at 12 volts but once you hit the start button it drops dramatically to 5 volts. If so would the starter just spin freely like that anyway?

 

From this description, I suspect the starter drive gear is shot (item 9 in the illustration).

http://www.docshd.com/OEMpartfinder....230125-H070025

You should have about 12.8 volts if the battery is fully charged and in good working condition.

The voltage drop when the starter is engaged should not drop more than about 3 volts (this is called a load test).

Note: when doing the battery load test, the voltage must be read at the battery posts, not the cable terminals.

Take all your battery cable connections apart and clean them, this starting system is not tolerant of excess resistance.

You have two separate issues here, one is not causing the other.