Need help with my 93 240sx vert please

[Glyph]

I just bought it recently and I'm pretty sure it's only firing on 3 cylinders. I've replaced the plugs, checked the timing, replaced the distrib cap and rotor. The plug wires look brand new so all I did was put a meter to them and verify resistance was less than 30 ohms per spec. It causes my whole engine and car to shake while idling and there's a definite lack of pick-up and power. Any idea what the likely culprit is? I'm on a tight budget but I'd really like to get this fixed. Oh yeah, two other things: when replacing the plugs I noticed one of them was discolored (burnt?) on the end a gray carbon-like color so I'm figuring that might be related and on an unrelated note I bought a new pcv valve for replacement buit can't seem to find it in the car. I hear it's under the intake manifold but I've looked and looked with no luck. Any and all help will be much appreciated. Thanks

[180fan]

probably a bad connection on the injectors. clean the plugs on the injectors and try again

[JeromeS13]

And, to narrow it down do this: start the car and unplug the injectors one at a time. Whenever you unplug one and you don't notice a difference, that's the culprit cylinder.