400ex running rich or leaking a little oil

[yamahardr15]

I'm not sure if it is running rich a little or if thier is an oil leak inside the engine. Also my chain is rubbing still and I followed the directions on how to tighten it and it stays tightened for about 15 min of riding then it comes loose again so is thier any way to fix this for a longer period.

[honda400exr]

why do you say it's running rich or leaking oil? is the spark plug black? if you put your hand at the end of the exhaust and rev the motor quick and your hand gets a little black then you're running a little rich. if you're burning oil then it'll smoke a blueish color.
as for the chain make sure you tighten the bolts on the back of the swingarm when you're done adjusting the chain and if it still comes loose then I think there are rubber things inside the swingarm that squeeze onto the carrier when you tighten the bolts to help keep the carrier from turning they might be warn out and not holding the carrier too tight. might wanna check into that then if you keep having problems