Good idea, but I don't think it's that. I checked it when I had the tank off and it seems to operate fine and the engine reacts to changes in the choke whilst running. What I was thinking is that the plugs aren't coming up to temperature, so they aren't burning off all the deposits. Which would fit with it running a bit cool. Though I'm no expert.

You could have resistance in the loom from the sensor to the guage, the guage is basicly an ohm meter because the sensor reduces resistance as it warms up. So cold on the guage is high resistance while hot is low resistance.

Directly shorting the wire from the temp sensor should to the frame should swing the guage to hot.

On most honda's the temps guage runs through the fan loom too so high resistance in the fan brushes can cause bad reading leading to overheating issues even though the guage is reading below the red.

I've wondered about the temperature on mine as the gauge only just lifts off the bottom mark. Since there is no "Normal" mark and the bike runs well I've been assuming that it will probably go higher in the summer (we're in the middle of winter here at the moment).