You really should not have needed to do any experimentation, because on page 55 of the ‘1011 Captain’ MANUAL Part II – Aircraft and Systems, it tells you that with  the NAV Source Select Switch set to FMS, it will follow INS route under the following conditions:

FMS - autopilot in HDG mode tracks CIVA-INS flightplan.
       - HDG Switch should be engaged
       - NAV Switch will be automatically deselected
       - Heading Bug on Captain's HSI will be controlled by navigation system.

Except that it says that the NAV Switch will be automatically deselected, while I think it really should say that you need to turn it off manually. ;)