need help with my ka24de, missing in the 4th cyl. and...

[nwdrfter]

okay, i recently got this 1993 ka24de motor, and it was rebuilt by a shop in portland oregon back in dec. of 2004. i guess, according to the spec sheet, that it was bored .060 and has some hp cams in it, and some other things. i really don't know. i'm new to all of this. only had my '89 240 for a month now.

anyways, for some reason it misses in the fourth cylinder at times. like when it's reving at around 2000 to 3000 rpms. i say the fourth cylinder cause i started messing with the plugs and pulled the wires as it was running and the 4th seemed to effect it the most. but i really am not sure. also, when i decided to try putting in new plugs, i found a little oil on the fourth plug. i was told that it could be a bad vavle cover gasket, but i've been checking it every couple days now and can't find any oil now.

i put the new plugs in, new plug wires, cap, rotor, and it ran fine for a day or two, and now it's doing it again. so now i'm looking for some advice and guidance.

also, what rpm's should it be idling at? i think it's about 800 right now. and it's running really rich. very noticable when smelling the exhaust. could this be an o2 sensor gone bad? and if this is a tps problem, how can i fix it? or adjust it? if that's even possible at all. i really am still learning a lot about nissans. i grew up around v8's and muscle cars, so everyone i know is completely dumbfounded and not at all knowledeable.

so any help, or even suggestions as how to help this engine become more efficient would be greatly appreciated. thanks.