I have a YZ450 and it was sitting for a few years. Cleaned carb., but bike seems to only want to run with choke open. Also bogs at differing rpms. Figure its running rich. So removed air cleaner and it helped (not riding just idling in garage). I will remove the carburetor and work on that again. BUT.... I noticed after running the bike for a little while, that a small amount of fluid was dripping from the front of the head. I found a rubber plug that looks like it popped out from the head?? There is a small amount of anti freeze dripping from where the plug was. My question... is what is the small black rubber plug on the right side of the head, and is there anything that goes in behind this plug that may have come out with it, that I should be looking for? Any help is appreciated.