I have a 04 450, and it has started running rough when you hold it at a constant speed, at first i thought it had carb issues so i cleaned that and put it back on and it still does it. So looking further into the problem I noticed that it had ALOT of blow by coming threw the vent tube that comes down the side of the motor from the head, but the machine doesn't blow any smoke and still starts right up? I have the head off off it and im putting in a new piston , rings, and going threw the head with new valve stem seal. Also the head is getting ported and polished will i have to re-jet after that? How big of a difference is the head work on a four stroke? I know a 2-stroke it does alot. Hopefully im going in the right direction any help would be great.