Join Date: Dec 2008
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foam filter's such as twin air and uni have to research data on their website about the performance and filtration of their filters unlike k&n. I have never had a probley with k&n there good filters. they only way to getter better airflow now is to take off the airbox and install a clamp on air filters, but if you ride in water than this wouldnt be good. if your 400ex is barley lean then its not going to hurt it, they are one of the most reliable engines (you get more power if your a little lean than rich). try to find a dynojet needle, if your bike is back firing when you let of the gas then this needle will clear that up.