DonCherry

难道真是只换AIR FILTER这么简单
This is often ignored parts, but pretty important for good mixture. You should change it as they recommended. Bad mixture when engine is cold is normal because when engine is cold, the control system is under "Closed Loop". You should know it if you have electronic course in university.
若是后者怎么查，怎么修，贵吗？
So forget it, it's normal.

perego

thx DonCherry.
u'r very helpful.