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Strange slapping sound after opening airbox?

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squiers007

Well-known member
Joined
Oct 7, 2007
Messages
180
Hey all, I recently performed the open airbox mod to my 03' XB9R and now have a strange vibration that sounds like slapping plastic from about 3200 rpm until 3500rpm. I only notice the sound when I am riding and cant hear it at all if i rev the engine while in neutral. I checked all of my plastics and the baseplate of the airbox to make sure it wasnt that and have even sealed the new filter cover to the top of the air filter, but the sound is still there. I know others have complained about more noise overall and a sucking sound after performing this mod, but this is a completely different sound from that. Anyone have any suggestions or other places to check? Thanks.
 
I also get a vibration after doing the open airbox mod. I have always thought it was just normal due to the increased airflow. Like you, I have checked every part I can think of to make sure nothing was loose, but the vibration persists. I just got used to it!

Maybe some others can enlighten both of us. I have not gotten a opportunity to hear someone else's bike with the mod.
 
b/c it's open your hearing the vacuum pulses of air being sucked into the velocity stack/throttle body I got worried when I heard it the first time to. All that plastic "insulates" the sounds and when ya open it tada hehe
 
Yeah, think of it this way, each time one of the pistons drops, the cylinder fills with air (about 600 cc's on a 1203) Thats a lot of air that has to be sucked in in a very short period of time. I'm not sure, but i think at about 3000 rpms thats at least 3000 "breaths" a minute. That converts into 50 breathing sounds per second which can easily sound a heck of a lot like a vibration. Which for all intents and pourposes it is.

Being as there are 2 cylinders, each may breath in one revolution so, potentially, you are hearing 100 breaths per second.

I'm not mechanically inclinde at all so this information may be very unaccurate, just thought i would try to aliviate my Short timers syndrome at work. and to boot it's friday.
 
b/c it's open your hearing the vacuum pulses of air being sucked into the velocity stack/throttle body I got worried when I heard it the first time to. All that plastic "insulates" the sounds and when ya open it tada hehe

X2. Mines open all the way, to the out side and its so loud I have to wear ear plugs. low rpm's its quite but when I get on it it sounds like a Harley with drag pipes in my helmet.
 
I have noticed that too. Depends on my helmet. With my old HJC I wear in cold weather, the resonance is deafening without plugs. But with my Gmax its fine. I assume its because the HJC is a little bigger and has a lot less airflow. The Gmax has like 100 vents! I cant wear that thing under 50 degrees, but its awesome in the summer.
 
So what you are telling me is that the "vacuum pulses" should sound like plastic hitting metal? This doesnt make sense to me. I can definetaly hear air rushing in, which I assumed was the "pulses" but I dont see how air movement, unless it was causing some other part to move, would cause a sound like what I am hearing. I will try and see if I can figure out a way of making a recording, but it might be rather difficult since it only does it while I am riding.
 
If your phone can record sound, you can put it in your front pocket or something similar.
Just a idea.
If it wasnt going to snow tomorrow I would try it with mine!
 
it's the combination of alot of things to make the sound what I said in my post above your also hearing the intake valves open and close it's normal bud weird and scary takes awhile to get used to but nothing gonna happen to the bike
 
Ok, so I took the bike apart again today and took a closer listen. What I dicoverd is that a)the sound is louder with my helmet on (yes I was wearing my helmet in my garage just standing around) and b)i believe now that it is the vacuum that is causing the sound. It was hard to believe but the sound was definately coming from inside the stack so I think I may experiement with some foam inside the outer airbox cover and see if that reduces the noise since I would rather not hear it. If it works I will let everyone know what I did. Thanks for the help.
 
So the noise reduction idea failed so I guess I am just going to not worry about it and hopefully get used to it. Oh, and for all those out there living in the freezing cold just thought I would get some punches in while I can and let everyone know that it was 80 today in Corpus and the riding was superb!
 
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