Sounds like something is stuck or not staying on in the fuel system.
My older bro had an old 320i that sat for 6 months before he bought it. At first it would only start with starter fluid. After soaking the fuel pump in e85 and replacing the fuel pump, it still would not fire. Rapping on the fuel distributor got fuel moving and the car ran fine for a 2000 mile trip. the system on the 633 should be similar to that 320's so check those places for fuel flow (crack a hose and see if it sprays) You could also jump the fuel pump relay to see if that is the issue. Maybe bring some starting fluid to test?