(Re) Building the better cb750 airbox....maybe...
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(Re) Building the better cb750 airbox....maybe...

This is a discussion on (Re) Building the better cb750 airbox....maybe... within the Technical forums, part of the Caferacer.net Forums category; There is a lot of talk on this site about pods, and air boxes and air flow and such, and since most of it seems ...

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Thread: (Re) Building the better cb750 airbox....maybe...

  1. #1
    Senior Member Geeto67's Avatar
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    (Re) Building the better cb750 airbox....maybe...

    There is a lot of talk on this site about pods, and air boxes and air flow and such, and since most of it seems to come from newbies who maybe haven't seen stuff like this I thought it might be nice to talk about something other than pods and carb tuning and blah, blah, blah.

    So this is a Henry Abe AirBox:
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    Actually it is a bit of a misnomer because Henry Abe wasn't the only company to make them, Santee, Ness, AEE, all offered similar products and designs. The basic premise was to have something cooler looking than an airbox but worked better than a generic pod filter.

    Over the years I have collected a few of these where I have found them and decided to rebuild one for use on the CB750F I have a thread about, and also another to sell. But I thought it might be good to go over what makes these tick.



    So here you see the basics. On the far right is an assembled heavily road worn cb750 unit. Next to that are the airbox specific rubbers, the carb side mounting plate, the inner filter mesh, the outer filter mesh, followed by the outward facing plate. Below you see the Z bracket that is used to keep the filter from collapsing under vacucum and one of the two bolts that holds the unit together. Above is a piece of Unifilter foam, to the far left is a stock cb750 intake rubber, and in the tray is all the hardware I am too lazy to spread out like the clamps and the other body bolt.

    Please forgive this pic as I didn't realize the focus was misadjusted until I uploaded it this morning, I'll fix it at some point:

    On the left is a stock cb750 intake rubber, on the right is the Henry Abe airbox rubber. A couple things to note:
    1) the cb750 one is much longer and has a short external mounting flange followed by a large amount of the velocity stack inside the airbox. Conversely the henry abe specific one has a long external mounting flange but is much shorter overall in height. Very little of the stack is in the airbox to allow for room to suck in air.
    2) both have a velocity stack shape but the HA's rubber has a more open bellmouth with a longer taper.
    3) the flange where it goes into the body of the filter is slightly bigger. I can from experience tell you that you can use a stock cb750 rubber in an HA airbox but you need to seal the flange with some silicone sealant to prevent air leaks, and you will have to cut down the velocity stack portion as it is almost up against the inside of the front plate.

    The original filter material was likely a foam of some type and sheet Uni Filter foam is easy and cheap to come by. Here I cut the filter sheet into strips for installation:

    This filter will have to be oiled. perioically and replaced when it becomes brittle.

    not all the rubbers are in good shape:

    these aren't reproduced so you have to use what you can. I find that black liquid electrical tape helps seal up any missing chunks and some regular black electrical tape around the outside helps seal it all together. Since these do not have to pass fuel and will mostly sit on the outside of the carb I am not concerned about any repairs. any raised dried liquid tape can be trimmed with a razor blade.

    Here is a close up of the actual breathable outside element:

    now I know what you are thinking - those holes look kinda small. I think it too. The reality is that this piece is structural to the airbox in that the two outer plates clamp firmly against it. You can drill these but I caution you that if you drill too big you can weaken the plate and the airbox will crush in on itself when you go to tighten it up. There are some other tricks you can do and we will talk about them in part 2: modifying.

    Installing the filter:
    you take the two strips of foam you cut and lay them inside the outer element. Make sure they are well oiled as per UNI's instructions.

    then you lay the inside reinforcement element so that your engine does not suck in a bunch of foam.

    To be continued.....
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

  2. #2
    Senior Member Diamondj's Avatar
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    Hey Geeto - send your best Abe rubber to these guys to reproduce:

    CSM Restoration Parts

    They repro'd the old T500 oil dams and the early T500 airbox boots that would be unobtainuim otherwise.

  3. #3
    Senior Member Geeto67's Avatar
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    Quote Originally Posted by Diamondj View Post
    Hey Geeto - send your best Abe rubber to these guys to reproduce:

    CSM Restoration Parts

    They repro'd the old T500 oil dams and the early T500 airbox boots that would be unobtainuim otherwise.
    Good to know, but I don't know if there is a market for it. Most of these Airboxes sell for between $60-200 depending on condition and there are quite a few out there. It's a sandwich design and anybody with the ability to build a birdhouse can make a similar one out of a K&N oval filter and the stock intake rubbers.
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

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  5. #4
    Senior Member knappyfeet's Avatar
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    Nice topic. Making one out of an K&N oval filter is a great idea. Now you got me thinking.

  6. #5
    Senior Member Geeto67's Avatar
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    Quote Originally Posted by knappyfeet View Post
    Nice topic. Making one out of an K&N oval filter is a great idea. Now you got me thinking.
    Wouldnt be the first:
    http://forums.sohc4.net/index.php/topic,108560.0.html
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

  7. #6
    Senior Member Cyorg's Avatar
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    I know you know your way around these things, but I feel I need to pontificate a little on the subject…..I can't remember if there is any extra space on those once they are installed on the M/C, but if there is any extra room (probably not much), I would be inclined to make up a wider section of mesh to allow for use of the stock Honda stacks if possible. Like I say, I don't recall if it's possible on those, but I do remember some chrome air boxes (and they may have only been imported into Canada from Taiwan) where you could toss the stacks that came with it and there was sufficient room to use the Honda ones and they would fit quite well. Most times when the air box was installed they were also doing pipes. I do recall the first one that landed in my stall back in 1977. The pipes and air box had been installed by the customer and it was running like a sack of shit. It was a nightmare to jet the thing and at first it felt like I was chasing moonbeams. It was only after sitting and staring at the air boxes that I decided to switch the stacks and give it a try. At the time, I had some understanding of the theory surrounding intake tract length , but I was stunned by the difference it made. All of the aerodynamic fungus (as I prefer to cal it) caused by the aftermarket air box was fucking up the flow past the jets which made jet changes a waste of time.

    I don't recall the brand of air box , but it looked more like a mailbox with louvers stamped in it.
    Last edited by Cyorg; 05-19-2015 at 07:20 AM.
    "Non urinat in ventum"

  8. #7
    Senior Member Geeto67's Avatar
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    It's funny you mention it, because the assembled one I have in my pics actually has the stock cb750 intake rubbers in it. When assembled there is maybe 1/4" between the bellmouth and the back of the plate which isn't that great for flow.

    There were a lot of sandwich style ones made, and of varying quality. I have a box for a kz900 also and the v-stacks for that one are vastly different than the honda ones - they are almost flush on the inside.

    To be honest, construction of a sandwich style box using the honda parts is so simple I might tackle it for part III of this thread. I have wanted to make one out of lexan and a K&N filter for years just because I think a clear one would be cool looking. If you have access to a drill press, a hacksaw, and a tape measure and the K&N catalog you can make your own.

    Keep in mind, airfilters of this type are not a performance enhancement. They do not have plenums full of free air and are not Helmholtz resonators. what they are is a fashion accessory for a stock bike that takes some of the difficulty in tuning a stock engine for pod filters by keeping the velocity stacks and providing slightly less turbulent air behind the filter. I know we can't stop these moron newbies from thinking they need to throw away their stock airboxes, but maybe we can provide an alternative so we don't have to see so many idiots buying emgo pods and then wondering why their bike runs like shit.
    Parasuco likes this.
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

  9. #8
    Junior Member Parasuco's Avatar
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    hey geeto, how much do sell them for, i wouldn't mind replacing my bulky stock airbox with this
    1982 Honda CB750k
    O quam bonum et jucundum habitare fratres in unum - psalm 133

  10. #9
    Senior Member Geeto67's Avatar
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    Lets pick up where we left off:

    The UNI filter needs oil in order to work properly. I bought this stuff because it was the right oil and cheap ($15), but it is very thick and hard to work with due to how slow it moves and how difficult it is to get off your hands once on. I recommend gloves for handling this stuff:



    I don't have pics of me oiling it because I didn't want to touch the camera but here is the assembled oiled filter and a pic of when it was dry showing how stuff goes together:




    It is imperative that you squeeze all the excess oil out of this foam for two reasons: 1) it will breathe better, 2) it makes the process less messy going forward.


    So let's talk about the rubbers. while all still pliable many of the carb side ones were starting to show cracks. I chose to wrap the parts that attach to the carbs with high temp electrical tape so as to prevent air leaks and also hold the rubbers together when the clamps tighten on them. I could have used liquid electrical tape as well but I thought that was overkill.







    and here we are with 4 rubbers ready to go into the airbox backing plate:
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

  11. #10
    Senior Member Geeto67's Avatar
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    Fitting the rubbers....

    You might think that because the engine side is so narrow and the V stack side is so wide that these rubbers are best suited to press in from behind the plate, engine side first. Well the rubber there is thick and not so pliable where as the stack itself is very thin and can be folded easily.



    it is really easy to get the stack portion in, but harder to get it to sit inside the flange. I was tempted to use a metal tool to work it in but resisted and with a few swear words managed to get it to sit flush:







    This is one area where the stock cb750 airbox rubbers struggle with since the hole they were made for is smaller. Once in they tend to not seal and shift about causing air leaks. Were I to use a stock cb750 rubber I would have to use some sort of sealant.

    Here is the seated rubber inside the partially assembled box for height reference:




    and here is a stock cb750 airbox rubber partially seated:


    Truth is it still has another 1/2 inch to go but I didn't feel like seating it only to remove it again (that's it on the right):


    You can see how much less room there is as compared to the stock abe box v-stack:


    You could use the stock rubber but you would have to widen the unit so it could have room to pull air:


    Anyway, I repeated fitting the HA rubbers untill all installed:




    If you are wondering why these work at all it is the shape. The Stock cb750 rubber has a more gradual taper and is longer overall, but the HA rubber has a more aggressive taper and transfers the length to outside the box to move it further away from the engine into an area of cooler air. You do loose the helmholtz effect the plenum affords you but not to such a degree as pod filters would. Still, these stacks are a better shape than the K&N pods and don't have a restriction ring like the emgos. Although I have no proof I am less inclined to believe you would lose HP on a stock cb750 like you would with a pod filter, but you are probably sacrificing some part throttle smoothness. From past experience these are less hard to tune than pods.

    test fitting the unit:
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

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