A narrow orifice often means that the wheel was originally designed to spin flax (linen). Not 100%, but often.
If you're on Ravelry, send a photo of this wheel to the group Antique Spinning Wheels. They'll not only identify it but probably give you the name of someone who can get it into tip-top shape *and* help you with ratios.
New spinners (I may sound experienced, but my physical skills are those of a beginner) often treadle too fast and make the wheel grab the fiber more quickly than they can draft it. Practice treadling without any fiber in your hands; maybe read a book or pet a cat/dog. A steady, regular rhythm is good.
But check things out w/Antique Spinning Wheels; it's truly an amazing group.