Why would it backfire from TBI?

I have a 1990 gmc k1500 5.7 tbi. I had the engine rebuilt, bored .040 over
and mid range cam installed. I have replaced every item that touches the
engine already. Map, IAC, TPS, rebuilt the TBI, EGR system, Fuel tank, fuel
pump, fuel lines.
I sent this truck in once to a mechanic already to fix a lifter problem i was
having (for the life if me i could not set the lifters correct). After he returned it
back to me it drove fine. I decided to upgrade the Oil pump in the block to
ensure better oil pressure due it is going to be used alot under heavy loads.
After i swapped out the oil pump i had to reset timing on the truck and when i
set it at TDC 0* and hit the gas from a idle, it will backfire through the TBI
unit. I was first told that with the increased oil pressure that the first round of
lifter adjustments may have been to much and needed to re adjusted. After i
had then re adjusted again and checked compression on each cylinder and
all came back fine. i still cannot figure out why i have a backfire if i do a
wholeshot from stop from the TBI and if i try to drive it at all it is really
sluggish. I have tried to set it advanced between 12, 10,8,6,4,2 and back to
0* and each one is having the same problem.
I did a full tune up again with new plugs wires and cap, same problem. Dont
know if i missing something.