Flapper valve question - Honda RC51 Forum : RC51 Motorcycle Forums
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post #1 of 20 (permalink) Old 08-25-2011, 02:58 PM Thread Starter
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Flapper valve question

So my newly bought 03 had the flapper mod (the guy stuck a piece of wood in the vacuum tube ). I can see the flapper still in the air intake. I was wondering if it is stuck closed? The flapper is down, e.i, covering the hole. Is this the open or closed position? I just took then front end apart and I want to wait to remove the flapper next time I open it back up. However, I am concerned that the flapper is stuck closed. Thanks for any information.
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post #2 of 20 (permalink) Old 08-25-2011, 05:34 PM
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gotta take out the flap

2004 nicky hayden RC51
to many mods to list
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post #3 of 20 (permalink) Old 08-25-2011, 07:12 PM
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It should be in the stuck open position. Not closed. That means your bike is starving for air. Is it running rich?
Anyway, here's a guide if you don't already have it.

John, 2000 RC51 #000100

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post #4 of 20 (permalink) Old 09-06-2011, 07:41 PM
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Is there anyway to tell if the bike's been modded already? I've bought mine used (SP2), and would rather avoid taking off the tank etc... if I don't have to. Thanks!
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post #5 of 20 (permalink) Old 09-06-2011, 11:48 PM Thread Starter
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Smile

Dakota- My bike had the Dan Kyle cap where the diaphragm used to be. Also the hose to that diaphragm is blocked off. However, he didn't take the flapper out-which is closed by the diaphragm. I had to remove it since it was starving my bike of air. So check to see if the diaphragm is there. If it is, then check the air hose connected to it. Finally check the flapper in your air intake. Follow this guide http://www.rc51.org/valve.htm
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post #6 of 20 (permalink) Old 09-07-2011, 09:59 AM
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I might be wrong on this, so someone please correct me if I am, but I thought the default flapper position was open, (ie no vacuum feed to the flapper). I thought it took a signal to close the flapper. Therefore I assumed the flapper would stay wide open once the vacuum line was disconnected.
Right or Wrong?

John, 2000 RC51 #000100

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post #7 of 20 (permalink) Old 09-07-2011, 12:29 PM
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The default flapper valve position is open.
The flapper valve closes when:
SP1 and SP2 RPM > 4900 RPM
SP1 throttle open > 40 deg.
SP2 throttle open > 10 deg.
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post #8 of 20 (permalink) Old 09-07-2011, 02:13 PM
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Thank you Sub,
And furthermore, if the vacuum is unhooked (and the line properly blocked), the flapper will remain wide open, 1) cause there's no vacuum to move the flapper, and 2) I thought the linkage to the flapper would effectivly hold it wide open when riding.
Right or Wrong?

John, 2000 RC51 #000100

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post #9 of 20 (permalink) Old 09-07-2011, 05:48 PM
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Quote:
Originally Posted by jondog9 View Post
Thank you Sub,
And furthermore, if the vacuum is unhooked (and the line properly blocked), the flapper will remain wide open, 1) cause there's no vacuum to move the flapper, and 2) I thought the linkage to the flapper would effectivly hold it wide open when riding.
Right or Wrong?

my plan was also to unhook the hose and seal it ....this is what Rogue has on there website.

By disconnecting the vacuum line to the flapper valve in the air intake duct you can completely eliminate the hiccup at 6800 rpm & reduce the power dip we have all seen on the dyno runs between 5000 & 7000 rpm.

As the flapper valve is an air intake noise control device which is not used on the basic racer, but rather only on the street legal bikes where noise levels are monitored by the EPA. It is advised that you only do this mod to "race only" bikes.

Having gotten the legal disclaimer out of the way let's get back to the flapper valves function. The valve is partially responsible for the power dip between 5000/7000 RPM. The flap is normally open. If you disconnect the vacuum line it will always stay open. You can remove the flap if you like. although it shouldn't make much difference with the stock air box. The HRC and Moriwaki air boxes have a larger duct with no flap for the race motors. The flap closes at about 5500 rpm which reduces the intake duct about 65%. It opens back up about 6700 rpm. This is done to reduce intake noise on the road bikes. The mod is done by simply disconnecting the hose at the diaphragm switch & plugging the hose with a screw,bolt or whatever you find handy or aesthetically pleasing.
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post #10 of 20 (permalink) Old 09-07-2011, 06:49 PM
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Quote:
Originally Posted by jondog9 View Post
Thank you Sub,
And furthermore, if the vacuum is unhooked (and the line properly blocked), the flapper will remain wide open, 1) cause there's no vacuum to move the flapper, and 2) I thought the linkage to the flapper would effectivly hold it wide open when riding.
Right or Wrong?
The flapper stays open unless actuated by the flapper actuator.
You can disable the flapper actuator by two means:
1) Disconnect the vacuum line to the flapper actuator diaphragm and plug the vacuum line.
2) Disconnect the flapper actuator solenoid connector mounted on the left radiator on SP2s.
The ECU actuates the flapper solenoid which in turn, allows a vacuum to pull on the flapper actuator diaphragm.
If disconnected, the solenoid never opens to allow a vacuum to actuate the flapper diaphragm.

Last edited by SubSailor; 09-07-2011 at 06:56 PM.
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