I'm not at all familiar with Banshee's but if it has a thermostat check to make sure it's not stuck open if they stick open the coollant never gets a chance to cool in the radiator when your sitting at idle then when you start riding ram air cools the radiator and drops the temp
Seen this happen alot on cars

 

I don't think it does but i have not tore into this quad much yet.. That will change soon though..lol

Outlaw, I hear ya man been there done that a thousand times with them thermostats[img]i/expressions/face-icon-small-mad.gif[/img]

 

Trailrider500, I just tore my banshee apart last week for a transmission problem... If it is the water perpeller they are not that had to replace...You have to take the right crank case cover off (I think I already had it off) then 5 screws and and a C clip...should be under a $50 fix...and a couple hours

Hope that helps!

 

Sounds like your definitely running lean,especially with the colder weather. Turn the air screws in 1 turn to richen up the idle circuit at the least. You said you did a plug chop and it was fine but I highly dout it since Banshee's jetting is tremendisly affected by change in temperature so check it again, I can almost garuantee you need to go up two main jet sizes.