[Banzaidrift]

I have a 1990 180sx, was originally a ca18det car, swapped in a red top sr. The car sputtered and spit at first would break up. I re did the timing and reset my cas. Fixed it mainly, now it pulls hard till 5000rpm and from there on it struggles/spits a bit and barely makes it to 7000rpm in 2nd gear so on. I have changed the ecu, coil packs, re did cas/ engine timing, changed/ cleaned maf and changed/ re-gaped my plugs. Cylinder number 4's plug was black when I pulled the plugs. Others were burning normal. Have to check for boost leak still. But according to my boost gauge I'm always at 7psi when under full boost. Motor is stock, freshly rebuilt with 2200km on it. Anybody have an idea what it might be?