My buddy has a 2006 honda rancher and he just put a jetkit and k&n air filter on it but we took it out riding this past weekend and it rode fine for a while then all of the sudden it just started blowing white smoke out of the tailpipe and has an oil residue on the pipe...........could it be a gasket or valve if you have some help please let me know THANX  

White smoke usually means that you are either burning oil or coolant. Is this a liquid cooled fourstroke or air cooled? If it is liquid cooled you can just see if your coolant level is down. If air cooled than you should notice that your oil level keeps going down. Either way it is bad.  

yeah check the oil for coolant,  

I have the same problem on my 06 rancher400.
I did compression test and it within specification.
I removed the head cover to replaced stem seals
and did some cleaning out the deposite. Reinstalled
and still has smoke coming out.
I am thinking of replacing valve stem, the guide and the seats
I works on cars but atv is new to me. I am still experimenting.  

sounds like you are on the right track, check the valve seat to see if it is burned as well.