I purchased my 98 f150 4x4 cheap .previous owner stated he jeard a loud pop and the motor ran terrible afterwards .he thought it may have jumped time..i towed home and started dismantaling it while in the truck..upon pulling the intake I found the lower plenum was blown to pieces I assume by the pop or backfire the guy told me about .i continued to tear down to the heads..leaving the heads I replaced every aspect of the timing along with both oil and water pump.alternator plugs wires coil packs and so forth..watched countless videos and read many articles about the timing procedure..i was very careful..now upon start up the truck purrs .wen accelerator is pressed it backfires . in gear it repeatively backfires .the faster I go the backfire gets faster..im thinking one of two things or maybe both .stuck valve .is there a way to visually check for this by removing covers?? Or cam timing is out .i did not use any locking tools during process but again was very careful and patient..also I installed shorty headers and straight piped .no cats.im not a mechanic .this is my first attempt at wrenching aside from a starter alternator or oil change ..my next step is one id like to avoid if possible and thats pullinf motor and purchasing a degree wheel kit and re timing everything from scratch..if anyone has suggestions or can help id really appreciate it