Bought my 98 4x4 4.6 with a blown out lower intake plenum .previous owner said he heard a loud pop then it ran terrible.he thinks it jumped time..i towed it home and tore into it .i found the lower plenum to be in pieces .bought a used one and proceeded to tear down all the way to the heads without removing them..i replaced every aspect of the timing ..chains .guides.tensioners.crank sprocket.did not change cam sprockets because my engine is a windsor and I read they are pressed on..oil pump.water pump .alternator .everything new.upon opening up the timimg cover I found nothing out of place .chain had what appeared to me as a lil slack but nothing out the ordinary..so I followedbthe step by step instructions ans replaced my parts ..i watched and read hrs amoungst hrs of info on how to do so and was very careful...now truck fires rite up and purrs .once I take off it backfire repeatively. the faster I go the faster the backfire .i beleive its a combo of intake and exhaust backfires..i also put shorty headers on and straight piped exhaust..no cats..im thinking I have a stuck valve or my cam timing is off.i did not use any locking tools .but again was very careful...is ther a way to check for a stuck valve by taking off valve/cam covers ?? My next step is to buy the degree wheel kit.pull the motor. put on a stand and start from scratch retiming everything .id like to avoid this if possible so any suggestions will be greatly appreciated ..not a mechanic this is my first endeavour aside from a simple starter or oil change..motor has 187.000 on it and I did change could plugs and wires also..thanks also truck is a manual .

 

Sounds like a cam timing issue to me.

Most of the folks that can answer this stuff don't frequent the "New Member Introductions" forum. Your best bet would be to post in the "1997 - 2003 F150" or "Modular V8 (4.6L, 5.4L)" forums.