Coincidentally, I 'cooked' my new one at the weekend. It won't fully compress the spring, as long as you have 8mm between the ring above the spring and the main body where it seats when cold, it will be fine. That assumes it shuts fully when it cools down, 'cos if it doesn't the bike will take ages to warm up. It also assumes it started opening at the right temp, which off the top of my head is around 80c. Again, if it doesn't start opening at the tmep listed in the Haynes and doesn't reach 8mm by the temp stated, it's shot.
If the stat operates correctly, and the bike is still running hot for no good reason unless you're stuck in traffic on a hot day then there potentially are several causes:
Air in the system, although it's probably unlikely.
Dodgy water pump leading to loss of circulation.
Radiators need cleaning on the outside, or have lots of flattened fins reducing the airflow.
Radiators and engine waterways are full of crap and the system needs flushing to improve the flow.
Radiator cap is knackered and you're not getting any/much pressure leading to low efficiency and maybe boiling.
Fan is knackered.
It may have been you who posted about cooling recently, so apologies if you've already done/heard these before, but if it was my bike I would:
Make sure the rads were cleaned of all crap stuck between the fins. If necessary, remove from the bike and attack them under the shower with one of those brushes for cleaning wheels. You can jetwash rads, but you need to be VERY careful as if you spray from too close, and at an angle, you can damage the finds very easily, so it's better to use elbow grease. Also, if you take the rads off, you can see what the coolant looks like. If it's dirty, chances are the inside of the system is too. I flushed mine with RadFlush last year, and the coolant I drained at the weekend was crystal clear still. The downside was that my waterpump was shot and only the grot in the system was sealing it, so when it was cleaned, the thing leaked and had to be replaced!
If you clean the rads, flush everything, and it still runs hot, it may be worth checking the water pump. I'm not sure if there's much evidence of circulation visible in the top rad, so you may need to take it off and take it apart to check it. A worst case scenario might be that either the impeller has come adrift from the drive shaft, or the fins have corroded (a trait of the pump fitted to my missus' car :s ). Ignore the Haynes where it says 'some residual oil' might come out when you take it off and drop it out before you take the pump off. I took it off expecting a slight dribble, and about half a litre pissed out onto my friend's drive!
If you've got a working pump, clean rads, new coolant and a decent stat, and you STILL get overheating... maybe the gauge is playing up? The Haynes has instructions on how to test the temp sender to make sure it's sending the right story to the gauge.
Beyond that... gawd knows!
