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Airbox question

Old 06-06-2010, 10:23 AM
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Airbox question

Last night I took out my airbox and noticed that the sponges that cover the top left and rear right holes (don't have a clue what to call this) have totally disintegrated. So I tore what was left of the sponges out of each one because I didn't want it sucking into my carbs. I also only have one cap that was on top of the front one (I'm assuming there should be two caps, one on front and one on rear).

So my question is what are these two areas for? I know that there are hoses going to each carb from there, but I don't know what the purpose is or why there are the other little holes under where the sponge used to sit. I think my bike is running ok without the sponges and rubber caps in that area, but not sure since it's been that way since I bought the bike (I have nothing to compare it to). Do I need to get replacement sponges and caps? And what's the purpose of all this crap?

P.S.: I have a K&N filter. I think the BMC filter has caps built into it from the pictures I've seen, so I've been thinking about switching to a BMC filter. Would I have to rejet going from K&N to BMC?

Last edited by evines; 06-06-2010 at 10:27 AM.
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Old 06-06-2010, 11:40 AM
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those are filters for the slides. K&N filters have seperate triangular "plugs" both front and rear that go over them. BMC has them built in. Both filters should have red filter elements from the filter oil. I have not seen a stock filter so I cannot say. I switched to a BMC from K&N when I installed a Factory 1.1 jet kit. My guess is that you do not have to rejet BUT, see how it runs. I have read that many K&Ns tricked their owners into jetting too rich. Mine definately was too rich, runs great now. JB
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Old 06-06-2010, 12:25 PM
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Originally Posted by residentg View Post
those are filters for the slides. K&N filters have seperate triangular "plugs" both front and rear that go over them. BMC has them built in. Both filters should have red filter elements from the filter oil. I have not seen a stock filter so I cannot say. I switched to a BMC from K&N when I installed a Factory 1.1 jet kit. My guess is that you do not have to rejet BUT, see how it runs. I have read that many K&Ns tricked their owners into jetting too rich. Mine definately was too rich, runs great now. JB
I have some stupid questions. Are the "slides" the plate that I can see over each carb inlet (you can slide them back with your finger)? If so, I'm not sure I understand how this works, but I guess it has something to do with pressure differentials pulling the slide open at higher RPM?

So do I need to get replacement filters (sponges) since mine were destroyed or is it OK without it? And do I need to find another plug since I only have one? I would guess that not having the little filters would allow unfiltered air into the carbs, especially since I'm missing the plugs over it.

I'd really like to understand how this works, and my google searches haven't helped so far. I just don't see why the little holes and filters are necessary. It seems redundant since you already have filtered air in the airbox. Why can't I just plug those little holes, remove the rubber plugs that go over the whole thing, and let that tube connected to each one get air from the airbox? Thanks.

Last edited by evines; 06-06-2010 at 12:29 PM.
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Old 06-06-2010, 12:31 PM
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Yes, you want those little filters. They are pretty cheap (17254-MBB-000, $1.55 at ronayers.com) and prevent foreign particles from getting to the carb slide diaphragms and the associated small orifices that feed them.
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Old 06-06-2010, 12:38 PM
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I just thought about this some more, and if I understand correctly, the slide needs a pressure differential to open, and it gets that differential from the air in the carb vs the ambient air. The reason there are holes in this area is to allow ambient air in, and the filters filter that ambient air coming through the holes (duh). The plugs are over the whole thing because the pressure in the airbox probably isn't the same as ambient air pressure. Am I thinking about this correctly?

Thanks for the replies. I basically knew nothing about bikes before I started reading this forum, and now I feel like I know at least a little bit.
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Old 06-06-2010, 02:24 PM
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your theory is correct now.

It always gives me a calm feeling to actually understand how something works.
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Old 06-06-2010, 05:10 PM
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Originally Posted by cornandp View Post
your theory is correct now.

It always gives me a calm feeling to actually understand how something works.
I'm the same way. I gotta understand the whole system, the big picture, then I can usually figure out the details and troubleshoot from there.

I ordered replacement filters (thanks Hawkrider), and I think I'm going to also try the BMC filter, as I've read good things about it and I like the fact that the plugs are built into it.

I'm a little confused as to why my bike is running ok without the plugs. Seems to me the slide wouldn't open without the plugs, or at least wouldn't open normally.
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Old 06-06-2010, 05:27 PM
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Nah, it'll run fine without 'em. I had mine fall apart last year and I ran for several months without. Just make sure and get all the little broken bits out so they don't get sucked into the carbs and plug something.
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Old 06-06-2010, 05:59 PM
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x2. Clean out all the mess. I found pieces of those little filters all up in my carbs when I cleaned them.
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Old 06-06-2010, 06:14 PM
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I got my bike used and didn't even realize they were supposed to be there for a thousand miles or more. Was worried I might need to tweak my tuning after installing them but I didn't.
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Old 06-07-2010, 12:47 PM
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Originally Posted by Hawkrider View Post
Nah, it'll run fine without 'em. I had mine fall apart last year and I ran for several months without. Just make sure and get all the little broken bits out so they don't get sucked into the carbs and plug something.
Are you talking about the small filters or the rubber cap that goes over the filters? I probably wasn't clear, but when I said "plugs", I was talking about the rubber caps that go over the filters. I have a K&N filter, but am missing the rubber caps that go over the small filters. Do I need to get replacement rubber caps for the slide to function properly? And if so, where would I find them? They come with the K&N filter (they're not on the stock bike).

The BMC filter has the caps attached to the bottom side of the filter, so switching to BMC would solve the rubber cap problem. I'm just wondering if it's necessary since I like my K&N just fine, and would rather not spend money if I don't have to.
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Old 06-08-2010, 09:52 AM
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Yes you need those plugs as without them you are letting unfiltered air into the airbox. So even though the slide will function fine, the whole that is left open when there are missing wiill cause a problem.

Try contacting K&N and they might send you a pair of the block offs.
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Old 06-08-2010, 12:02 PM
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Originally Posted by 8541Hawk View Post
Yes you need those plugs as without them you are letting unfiltered air into the airbox. So even though the slide will function fine, the whole that is left open when there are missing wiill cause a problem.

Try contacting K&N and they might send you a pair of the block offs.
Um, sometimes I'm really dumb. I never thought about contacting K&N directly, so I'm glad you said something. K&N said I could get the block-offs for less than 4 dollars, so I said "yeah, sure, let's do it." Then he took my mailing address and said they were on their way. When I asked how I pay for them, he said "Merry Christmas." So I guess I'm a K&N fan now.

I was going to get a BMC filter before this, but they are out of stock stateside right now.
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