Backfiring cly. 4

[Bad03gt]

So just bought a 92 50 with gt40p heads on it and cal. 4 is backfiring. Firing order is right and all spark plugs are new

 

[4valvemach]

Cap & rotor look good?
plugs wires are not damaged(burned,cracked,cross arc with other wires) and are sending spark to each plug?

 

[Bad03gt]

I have replaced everything