I replaced the square tops on my 73 with the round tops off of my 72. Car runs when I shoot starting fluid into the carbs for 2 or 3 seconds then dies. The float bowls had fuel in them but I drained and recleaned. Still have same problem. I used a timing light to make sure spark was good and I'm on the money. Set the floats. Could the fuel pump be "weak"? I have the carbs at 2 1/2 turns out on jets, 2 1/2 out on screws. New fuel lines and filter. I'm sure it's something simple but my brain is stumbling.

It should have been an even swap. Make sure the suction piston isn't stuck. With the engine off, remove the air cleaner and piston dome dipstick then check for smooth easy movement of the piston by lifting it up through the air cleaner side. You should also here a solid clunk when the piston hits the bottom of the carb. Good luck.

Yeah, been there. Devils hide in the details, sometimes in plain sight. Maybe these ideas will restart the thought process--

Have you double checked all the vacuum hoses and manifold connection points? How about hidden vacuum leak points such as a failed vac. advance diaphram. Are the PCV hose connections tight at both ends? Is the EGR system hooked up correctly (or are the manifold ports sealed)?

Have you checked the "choke" adjustment -- at least 6-7 mm additional nozzle drop at full choke?

Is the gas fresh? The high vapor pressure fraction you need for cold starts evaporates easily.

Were the 72 carbs clean when they went on? Some gas can gel during storage, and a tiny lump in the nozzle will block flow.

I rechecked the floats and the rear bowl was dry. Readjusted that float and the car runs but barely. Can't give it any throttle or it sneezes and dies. I'll play with it some more tomorrow.

Also, check your fuel pressure going to the carbs. It should be about 4psi.