Oh...Im not sure, looking at the diagram it appears theres a nut holding the orifice. The other end of tubeing you removed is screwed to the orifice. I thought you loosened the burner (53/54), slide it over, remove the nut 59 holding orifice 56...

But in looking I realized all of this is inside the lower sheet metal box?...no access?
If thats the case how is one supposed to access? umm, how do you get to thermocouple? thru the lil round cover...bizzare
If all your doing is cleaning the orifice-since you have line loose and assuming you have compressed air is pour a little alcohol down tube-let it set a while, dribbling out orifice. Then blow it out throughly? And chimmny/burner area.