Thermostat, gauge, or what?

New ('84) 700.

The temp gauge barely moves. It'll go up about a quarter inch to a notch on the dial face, that's it. Checked external cyl temps after a ride and they were about 155. Seems like the gauge should move more or it's actually running cold.?

Stuck thermostat or possibly lack thereof?

 

Check under the rad cap while the engine is cold to confirm that the coolant isn't too low, cuz the sensor needs to be fully covered by fluid to be accurate. Yep, thermostats on these bikes do sometimes stick open, something obvious when you inspect it.

 

I'm betting lack therof! My 84 was the same way, sure enough, NO thermostat.
My 85 was the same way, thermostat looked like something found in a shipwreck, in three or so pieces!
Luckily they are super easy to replace.