Cold weather
It needs to be leaned out a bit in winter..... Make sure that you also lean out the PAJ to match or you will be unbalanced. Stock jetting is way to rich for temps under 30-40 degrees.
I have friends in the great north (Alaska) who cut a piece of window screen to fit the size of the Radiators then use duct tape on the lower half of each radiator (On the Screen) when the temps are under 10 degrees or so. On the other hand some of them don't use anything at all and say it's fine.
Bonzai 
too lean for cold weather with stock jetting ?????
You should richen up in the winter and lean it out in the summer! the 450 likes the fuel.
You should richen up in the winter and lean it out in the summer! the 450 likes the fuel.
Yea richer in the winter for sure. Cold air is Dense air. More air = more fuel.
Check out Motoman383's site here on TT. He has a great excell spread sheet jetting chart that should get you in the ball park. And dont forget to up the starter jet also. Not the pilot jet (although that needs to be bigger also) but the starter jet. The Starter jet only has an effect on Jetting when the choke is on. Bigger starter jet = easier starting in cold weather. If your bike starts and stalls like a dozen times before finially staying running in super cold weather this is the starter jet causing that. In the summer my yz400 runs a 65 starter jet. In the winter I run a 72. Starts much better this way.
A 4 stroke running lean in cold temps won't melt down like a 2 smoke but it'll run crappy and be hard to start. You've gotta richen up as the air gets colder. As for the rads, I've heard some folks block 'em and some don't. I don't block mine and run regularly at -25 and as cold as -40 with no problems. Make sure your antifreeze is up to snuff and use lighter engine oil.
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Do four strokes need to be jetted richer for cold weather like 2 strokes & if below zero should I unhook or block air flow to at least 1 rad to help oil stay hot?