1972 Pontiac Firebird my car won't start

If the timing chain is original the camshaft gear will have plastic teeth that wear down, then break off. When the chain jumps a tooth or two the engine will be hard to start and have low power.

Also be sure there's fuel spraying in the carburetor when you look into it and work the throttle by hand. If the fuel spray is weak, try starting the engine with starting fluid. If that works you might suspect a plugged pickup screen in the gas tank. Usually the symptoms are less severe so check the timing chain first.