what is the flaper in the air box?

[budda]

I was changing the air filter and saw a vacume valve looking thing in there and saw it was hooked on a door or the flaper as i called it

so i started it up and gave it some gas and the valve closed the door.blocking off the air coming in to the filter. i said **** that cant be right and looked for other openings and found none. now i'm not the sharp tool in the shed, but thats just not right.

 

[Mr. Anderson]

below a certain rpm, i think 5k, it is suppose to be closed to increase the air velocity and smooth out lower rpm, it opens up above 5k rpm as more airflow is needed. on a stock bike this is fine and doesn't need to be modified.

 

[mountainmotor]

FWIW Yoshimura even recommends leaving the flapper in with use of their stage II cam set .

A few years back a UK tuner made +185 HP with those cams , all short stacks , stock bore and compression with the flapper intact .