The engine itself might be sensitive due to the aluminum expanding or contracting if it's too hot or cold.. since the fasteners are steel, they'll change at a different rate.
The other factor is the air at that temperature. I was thinging about it some time back, and ended up posting this thread
. Basically, I was wondering why my motor ran differently first thing in the morning (really peppy and ready to go) vs. on the way home from work in the afternoon (happy to go, pretty predicatable output).
Basically, when it's close to freezing you're pulling in more air.. It may not sound like much, 70 or so CC's instead of 66, but if you're trying to tune your carb it has the potential to throw you off a bit. Likewise if you're trying to do it when it's a staggeringly hot day, you're actually drawing less air so you end up running rich.