My car started to stumble at WOT so i figured i needed a new fuel filter so i changed that it helped a little but it still would miss sometimes when i hammered on it. So i changed the spark plugs,wires,cap, and rotor and ran even worse when in gear, so i checked for cracked plugs made sure all the boots were on good (its hard to get the msd boots on with slp headers). So i try to start it after all that then it is hard 2 start then it backfires through the intake and i didn't . What could be causing this i know my vavle seals are worn (smoking on cold start ) could that be causing a intake valve to stay open ?

 

sounds like its out of time man.. that would be what i check first thing

 

I agree. Sounds like timing. Check fuel second. Lean pops "up"Rich pops out the exhaust.

 

Are your plug wires in the right firing order?

 

I had the same problem along time ago on a high milage 350. it turned out to be a worn exhaust lobe on the cam. It wasn't opening the exhaust valve fully, and at WOT it would pop out the carb.