73 240z; backfires through carbs/won't start?

Throttles on both carburetors turning choke cables are working fuel hoses not dripping by moving those carbs you may have dislodged rust and debris in the bottom of float chambers. Some Friend Hey. Unbolt the air filter embly look down the throttle bores reach in with your finger lift up on the dash pot make sure they move smoothly. Look at the throttle erfly's. Back fire from the carbs almost always means the car is to lean. STOP don't drive till you get a Book and learn how to work on one of the easiest carburetor to rebuild clean and overhaul. But make sure you have the complete overhaul kits and always replace any old cracked fuel lines and if you find rubber bits in those float chambers also replace the fuel pump. (diaphragm may be coming apart). You should have left those parts sitting on the work bench. Library may have books on SU carbs. Over haul kits may have instructions. Float drop and float height is very important as is main jet height and needle wear. You need to find a old guy who remembers SU carburetors either MG Triumph or Datsun GOOD LUCK GET ER DONE