Meh, all throttle bodies get dirty, every one, and all throttle bodies will have to be celaned at some time.
The Z uses a common crank breather system where any blow-by gases are fed back into the intake to burn them off. In the old days they were shunted out to the fresh air, but of course that eventually stops the air being fresh for everyone.
Slowly but surely oily crud will build up, especially if you happen to use higher revs regularly on an engine that is no longer in the first flush of youth (or you have a HR engine which IMO are practically diesels with the amount of oil they get through . BIg up the DE's )
Basically, clean it or at least whip the pipe off and visually check it
Just wanted to say, I haven't got a clue where you got your idea about HR engines??
They burn NO oil whatsoever. I think the revup engines do, but being an HR owner, I can say mine doesn't burn a drop, and neither does a lot of other HR owners on here.