what is the flaper in the air box?


#1
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
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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.
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Mr. Anderson

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#2
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

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#3
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 .
 

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