running lean

I just put a new top end in my 94 kdx200.Everything went together pretty good except the gasket kit didn't come with a gasket for the power valve cover(the one that says KIPS)so I made one out of some gasket material untill the one I ordered comes in.The problem is the bike is running real lean now,it seemed to be jetted fine before, I've never taken the bowl off to check the pilot jet but the main jet is a 155.Could that gasket be causing my lean condition???

I checked the power valve again, apperently I didn't get that left-handed nut tight enough and it vibrated loose. So the power valve wasn't working at all,when I fixed that and did another spark plug test it was much better but still seems a little lean to me. Hopefully that new gasket will make up the difference. I checked the torque on everything like you suggested and it was fine,the local bike shop guys said to try spraying carb cleaner around the head and base gaskets etc. and listen for a change in the way the bike runs.but it didn't change at all,anyway thanx for the help!!!!