318 issues

So heres the deal. My engine starts and runs but with the timing set to factory specs it will just backfire out the carb once you hit on the gas. if you adjust the timing by "feel" the car does good down low but then starts pining and loses power higher up in the rpm range.

the engine:
73 318 stock, estimated at least 140 thousand on it
Carburator has been rebuilt
new plugs cap and rotor
new timing chain

I thought it was the timing chain so i changed that and it changed nothing. Im really pissed i wasted my weekend. any help would be GREATLY appreciated.