David,

my "'82 westy-with-1800 8v-gas engine" also has a K&N conical air filter and
also has a noticeable "roaring" noise under wide open throttle. My friend
with an identical swap has  used a replaceable K&N filter in a golf air
filter box (located where the battery was, battery now moved up under the
pass. seat.), and doesn't have this noise problem.

alistair