Happy,
You have a 77 or 78 I would presume. At any rate, when you pull the knob, have you removed the air box to see if the choke plates are closing/opening? Could be routing of the cable or it may be pinched/kinked or not secure in its stay on the linkage. Look under the "Manuals" section off of the websites main page to see diagrams and explanations of certain maintenance items. Fun times are ahead when you start to do small service, or T/S things, such as a sticky choke cable.