1993 249SX Conv. only hitting on 3 cylinders. Need help 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

[navysnail]

if you unplug the wires, when you unplug the one that is not sparking, the engine will run like it is now instead of runing on 2 cylinders. the easier way though, is just get a timing gun and put the clamp on each wire. you don't need to flash it at the crank, just make sure it is flashing when you put it on each wire.

the PCV valve is in a pretty bad place on the DOHC motor. 240sx.org has a writeup on replacement.