In what order are the fuel tanks on the MiG-23 used? In other words, which tank is used first, then second etc.?
These guys could probably answer this question.
It'll cost you $20, but you can obtain a MiG-23 POH here which should have a systems description in it for the jet's fuel system.
There are different models of the MIG-23 with different internal and external fuel tank configurations. The usual procedure is to use external tanks first, then internal fuel. If you have tanks on the wings you use the outboard tanks first, then the inboard ones.