I think your battery is a little on the low side, 11.8 will not get the injectors to fire.
You need at least 12.35 for the injectors to go, at least that is what I have discovered with my 1987 944 which has the same type of injection system as your 1984 911 Carrera.
But I am no expert, so maybe someone else will chime in with their thoughts as to what may be causing your no start.
Another thing to check is the ECU/DME brain. Lots of times these boxes get rattled around alot and the solder points begin to crack and get loose. So, if you know of someone with another car just like yours, same year/model, try and swap out the brain to see if the car starts up.
Hope this helps. Good luck.
