alright so after rebuilding the heads on my 4runner and putting everything back together, im stuck trying to get it to turn over. it cranks but doesnt stay on, dies out immediately. im also getting a popping/backfiring sound as i try to start it up. i know what everyones gonna say on this.. that its most likely my timing is off. so here goes, i looked over my timing components four times today to see if any of my markings were off. im @ tdc '0', rotor pointing @ #1, all markings on the belt are lined up perfect. and when i say perfect, i mean that if i were to move the belt one notch to the left or right of both cam. pulleys then you would obviously notice a big difference from being off the marks. the crankshaft pulley and belt are also lined up perfect. am i missing something here. dist. cap and rotor, spark plugs all brand new. i had a friend run a diagnostic reading on it and im getting very low fuel pressure. fuel pump is working. taking my injectors tomorrow to get cleaned and tested. any suggestions please help

 

fuel filter is a good start. then fuel pressure regulator and pulse dampener, and fuel pump itself. battery voltage is important.
low fuel pressure could make you run lean and cause the exact symptoms you're experiencing.