s14 ka24 wont rev up quick

[thekidvic]

so a couple of days ago my car ran fine, pulled hard, everything fine. went up a parking structure, held the clutch a little and was gassing it to go up and then heard something give out, made it to the top. ok.. so now it wont pick up speed when i slam on the gas but will reach 70mph but takes forever ! idle is real low like 300 when at a stop and sometimes almost turning off. so.. exhaust leak.. replaced the exhaust gasket, and found some holes by it that i had welded shut, and my headers arent cracked. idle went up a bit to 600 but didnt fix my problem much but is accelerating much quicker but wont pull hard at all. (also) car began backfire alooooooot after the incident, car hardly backfired, ever. i rev it from the throttle body and its making almost like a a quick door creaking noise. shifts into every gear fine, revs up high but real slow.. help ???