Question? Is it timing or Fuel?????

chevy 350 with HEI dis. two barrel carb.
There is a mid range fluster ?? The engine runs fine but when you get into the half way point in the throttle it back fires through carb then when you hit about 2500rpm it comes on . It will cherp second gear shift .
I have tryed another carb and it does the same thing
I replaced the mechanical advance and tryed diffrenet springs I ve checked the vacuum advance it works.
On these HEI distributors when the coil is bad will it do this???? I thought that when a coil went out it will not work at all. Help please if anyone has had this problem .

 

you could have a lobe down on your cam.