Paul,
my wife's GZ goes much faster than 50. medium saddle bags, a small windscreen and my 5-10, 215lb self can see 70+ on flat ground. fastest i've riden her bike was an indicated 75-ish. her carb gummed up the first winter we had it. i removed it and cleaned it, but it still had idle issues with some loss of power. i adjusted the pilot screw and its a whole different animal now along with the new front sprocket.
you should read this thread:
viewtopic.php?f=8&t=617
my thread where i had a similar issue
viewtopic.php?t=3888