1987 Chevy Truck engine very lazy off the line

Engine Performance problem
1987 Chevy Truck V8 Four Wheel Drive Automatic

Okay, here is the problem. I took a TBI unit off an 87 chevy and installed it on an 85 chevy, and it runs pretty good with one exeption, when you hit the throttle from idle the engine really lacks performance to the point of backfiring sometimes, It is very lazy to get going.I can mask the problem by bumping the base timing up to 10 degrees and it runs much better, but still lazy. I just installed a brand new engine yesterday thinking that would solve all the problems, but it made no difference ( the old engine was wore right out ) Fuel pressure is not the problem, It has been checked and rechecked, I installed new injectors too. No fault codes. What I did find is when the engine is accelerated the timing is retarting, alot. The timing retards to after TDC for a short period od time and then will advance up to as high as 24 degrees total. With the base timing bumped up to to 10 degrees I get a total of 34 degrees and it runs much better, but still retards on initial rev up. I checked my buddies truck, and his timing retards much less and much quicker on his truck, and his will actually spin the tires. HELP, I am stumped and dont want to just start changing more parts at more expense that dont help, and dont really want to put an HEI unit in to bypass the problem.