I just installed a mostly stock engine in my car and can't seem to track down this problem. At part throttle the car feels bogged down, sputters, and sometimes backfires. If I put the throttle down about 85% it will sputter, backfire, then take off and run normal. WOT seems to be ok, although maybe a bit under powered. I cleaned the MAF and installed new O2 sensors but it didn't help. Mods are a ported lower intake, shorty headers, o/r h pipe, cam installed at 112 ICL, new timing chain and lifters, EGR delete, all smog stuff deleted, a/c delete. I had a vacuum leak at my oil separator that I'm not sure I got fixed. I suppose that would be the next place to look Plugs, cap, and rotor are supposed to be new. They all looked fine so I reused them. Plug wires are good. New fuel filter. Fuel pressure set at just under 40 psi. Timing is around 12°.