98 670X thermostat?

Hey guys I have a 98 670 Summit X bored twice over (+1mm) and it seems to have too much engine cooling. When it is really cold out or I get into some deep powder I foul plugs although the jetting is excellent on the trails. To combat this last year I would increase the heat range from BR9ES to BR8ES on the cold powder days both plugs are specified in the manual. Increasing the heat range helped but it still has trouble keeping the plug hot enough. I am fairly confident in the jetting since it runs great anywhere but in the powder and the insulator color is nice butterscotch.

This summer I put some CPI twins a and Polaris series 4 159" on. So I believe I am getting more even more snow to my coolers.

Question is, is there a thermo stat and do they fail open or fail close? Any other insight? Dare I go BR7ES on the cold fluff days?

I checked my service manual and it says there is one but babbits doesn't show one for my sled.

How often do they fail closed. What would be a good "preventative" milelage to replace so I don't have one fail closed on me.

Thanks guys I ordered a thermostate Tuesday before I left for Thanksgiving. I'll put it in when I return home. Appreciate the number Repair Boy I'll give you a shout if I need.