This is going to be a longer post so please bear with me.I bought a supercharged(vortech) g35 coupe 2004 manual.The mileage is 111,xxx previous owner had told me he changed clutch at 95,xxx and put a stage 2 clutch in.Car was in Tennessee, I bought the car with a p0300 random misfire code.I put new plugs in Tennessee(big mistake) I didnt clean the plug wells(they were soaked) obviously no change in performance.Fast forward 1 month, I get the car shipped to California where I live.The car drives very slow and the rpms dont climb that much when I drive it around the block.There was a time that I drove it and the engine started smoking and smelled terrible, I concluded that the valve covers and gaskets needed replacement.Now I pulled the plugs and cleaned them after doing a compression test(lowest was 160 highest was 173).Some of the plugs were fouled.My first questions is once a plug is fouled with oil will it run like it did again after the tip is cleaned?Now fast forward to yesterday I spent time on the valve cover gaskets, I put rtv in the area where the fsm specified and in a ring around the tubes.The car was still acting up.The smell of oil burning on an exhaust manifold was gone but I was still missing alot of power, even shifting up I only hit 30 mph tops.Now orginally the car had a p0300, after the valve swap and without clearing codes it became a pending code then this morning I checked and there were no pending codes.I also pulled 3 packs this morning and the wells were dry.I used a all-pro gasket and some lightly used almost new nissan covers.Is this because of the spark plugs or is there a clutch problem here?

 

Get the car to about 20mph, shift to 4th and stomp it wide open throttle, do the rpms rise quickly but the car doesn't move faster in direct relation to the rise in rpm?

If YES then it definitely sounds like a slipping clutch.

Normally you would do this at about highway speed in 6th gear to reduce the gear multiplication from lower gears but it sounds like your vehicle might not be able to do this right now.

If it's NOT slipping then I would inspect the intake tract for possible a torn boot. If you're losing boost because of a leak it will feel a lot slower.

 

Also when you say it was "smoking", where EXACTLY did the smoke come from? Was it something you just smelled or did you SEE IT. Burning oil has a distinct smell like... well like burning oil but a burning clutch has this terrible chemical nasty smell that is completely different and almost unmistakable. If it smelled like rotten eggs then that's the catalytic converters that died.