These bikes need the airbox
properly installed to run correctly. Over the years I have
experimented on the dyno by running them with no airbox and pod
filters, versus heavily modified air boxes (swiss cheese lids)
and basically nothing works as good as a mostly stock airbox with
no more than about eight 1/2" sized holes drilled towards to
the top of the airbox.
I haven't personally tried to do the K&N air filter hack mod which is very popular on the VTR250 forums and the reason I haven't is because in all my years of testing air filters, jetting carbs and tuning Fi I have found that the K&N filters more times than not actually hurt the performance of the bike rather than help it.
I wrote an article about my findings here: