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Discussion Starter #1 (Edited)
Hey there - I recently purchased a 2002 919 from a dealer across country, so I don't know the history on this bike. One of the previous owners had completely cut off the air duct and removed the diaphram from the valve filter assembly. This diaphram controls an air flow damper in the air duct. I could get the idea of wanting maximum air flow all the times, but I also trust the engineers. Whats the good and bad about this mod?

I purchased a new air duct to protect the air filter from sucking in all the rain here in portland, and am wondering if I should purchase a new diaphram and vacum hose to return it totally to factory condition?
 

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Hmmm. I'm not at all sure about the 919, but if it's anything like the RC51, that flap that restricts air flow can be removed for better air flow yeilding better performance. It was put in place to reduce noise from the induction in the high RPM ranges.
Hopefully someone will pipe in with previous knowledge of this model.
 

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Discussion Starter #3 (Edited)
When you say yielding better performance - I'm curious what that means. More Low-end vs. High-end power, increased throttle response or fuel economy? What are your thoughts on those things? I'm also curious about thoughts on the engines operational performance itself - like is it harder on the engine at low speeds vs. open throttle. I know this must mess with the bike computers expected fuel mixture.
 

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I dont think that the flapper mod does a thing. Zero performance gain, nor loss.

On the 929/954 the flapper is to reduce noise in the low rpm range, not up top. Its movement is controlled by a servo and it opens the flap at 8k rpm.
The tiny difference the flapper could make at low rpm is nowhere near being able to alter the a/f ratio, i wouldnt worry another second about it.
 

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Air is like a low-viscosity liquid. Anytime you make it turn corners, it slows it down and restricts its flow, especially at higher velocities (read higher r.p.m.s). The stock 9er makes the air turn at 90 degree angles 3 times (if you count the airflow from the rear wheelwell intake area, which is a low pressure area to begin with). I cut a 1 1/4" X 2 1/2" window, (about the size of the air intake on the snorkel tip) in the front of the airbox intake housing, opposite the 90 degree bend to the intake snout where the vacuum-controlled diaphragm valve is located. I put screen over the opening I cut and epoxied it in place. Now the air has an almost direct straight shot into the air filter at the front. Try it!
 
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