ive never owned a TX model bike, but ive heard they (yamaha) had issues with them.
something to do with the cam chain or bearings or something.
the TX bikes started getting a bad reputation!
even after yamaha corrected the problems, people didnt trust the TX model bikes, thats why they went back to the XS designation for 1975.
i used to own a 1974 TX500 that bike was a twin cyl. with 4 valves per cyl.
it was a great bike. im not sure if the TX650 shared this similarity or if it was only the TX500.
im sure if you wanted to google it you could find out all about it
cheers - Tom