power loss and back fire at hard trottle

[blk8950]

i have a 89 gt with 75 mm 24 lb mass air meter 24 lb injectors bbk ssi intake bbk 75 mm trottle body 14 degree timing and 40 lb of fuel pressure if i barly give it trottle it runs great but if i give it half trottle of more it falls on its face and starts pinging and backfiring like it isn't getting fuel with the spark it also has a msd and a dyna mod distributor
i didn't have this problem until i tryed to advance the timing and my engine light comes on right when i start my car after i reset the computer im thinking maybe a map sensor but my hands are up in the air please help

 

[82CapriRS]

you sure its set at 14 now? what was it set at before you changed it? did you pull the lil plug by the distributor before you changed the timing?