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Discussion Starter #1
i was looking around for a rack of used CR carbs but i didnt realize that there were so many types. there are round slide, flat slide and smooth bore.
what i dont know is whats the difference? what does one do better then the other. from what i see the CB750 guys use the round or smooth type. but why not the flat side?
dirt bikes use both?


There is the right way, the wrong way and the british way to do anything!
 

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The Flatslides are FCRs from Kehin and are excellent carberators. There are three reasons people don't use them:

1) they are hella expensive. Expect to pay $300 more over the Kehin CR smoothbore carbs.
2) They are kinda overkill. These are the carbs most sport bike guys replace their aging carbs with. YOu need a really radical motor to take advantage of them
3) They rattle which somepeople find annoying. I personally love the sound of flatslides but it is not for everyone.

The most common "CR" carbs used on cb750s are the Kehin smoothbore CR carbs (not to be confused with the Honda HRC carbs that were part of the 970 CR750 kit). I believe the size is 28mm or 26mm, but don't quote me on it. They are most easily identified by the red CR painted on the side of the carbs. These are far and away excellent carbs for the street as well as track. At one point they were made in magnesium and I have a set like that without idle circuits.

Sometimes referred to as "CR" carbs but are not actually are the KZ and GS 28 and 29mm smoothbores that came stock on GS750s and KZ900 -1000s. These look a lot like the Kehin CRs but are not (They are actually made by Mukuni). Early GS's (77-79) use a variation of the VM carb which is a roundslide carb and an excellent carb. smoothbore and roundslide are used interchangibly here in the description so becareful with that as well. Dynoman has the manifolds to adapt these carbs to the cb750. Ironically the KZ and GS share carb spacing (as well as piston size) so the manifolds should work for either.

What other "CR" carbs are you encountering?
 

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you know I think I should clarify something.

Roundslide and Flatslide refer to the shape of the carb slide. A round slide has just that a cylindircal slide, a Flat slide has a rectangular slide.

smoothbore refers to the shape of the inlet on the carbs. It is possible to have smoothbore flatslide and a smoothbore roundslide.

The FCR and CR carbs are both smoothbores (the CR are roundslides, FCR flatslides). The GS carbs may or may not be smoothbore (some are VM and some are VM33) but are roundslide.
 

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Let me clarify something else.

A smooth bore means that the airflow on the carb goes straight through and is un-interrupted by the slide:

http://www.thegsresources.com/_forum/attachment.php?attachmentid=648&d=1150361328

There are no throttle plates or remnants of the slide hanging out in the airflow.

Suzuki GS carbs, the non smoothbore kind, are still an improvement over the stock cb750 carbs and they are a lot cheaper than a new set of CRs or FCRs. However the 1977-78 CB750F carbs are of a similar design plus they have an accellerator pump (albeit a small one) that adds to throttle response. These are much much cheaper carbs and I have to say if I were building a budget hot rod or a sleeper this would be the set I want (you have to use the 77-78 rubbers). Plus you can use a stock airbox where as the GS carbs you are forced to use pods.

Another significant difference between the CR and FCR carbs is that the FCRs have an accelerator pump, which is good for throttle response in the lower rpm range.





Edited by - geeto67 on Sep 27 2007 2:16:30 PM
 

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Discussion Starter #5
so 31 or 33mm flatslide is over kill?...lol
saw a cheap set for sale. thought that they would be cool. i see CB750 guys using 29mm round slides for stock and mild motors and then steping up to 31-33 of wild stuff

There is the right way, the wrong way and the british way to do anything!
 

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They are overkill for the price, as in for the money you spend they are more than you really need. How cheap is cheap? and were they setup for a CB750 as a banked set of carbs?

Never turn down a cheap set of flatsides. YOu don't need the accelerator pump, and at a $300-$400 premium it isn't worth it but if have a wrangle on a good set jump on it.
 

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Speaking of carbs. I have a very nice matched set of 305 Superhawk (CB77) "Round Float Bowl" carbs.
Is there any demand for them? Someone once told me they are relatively rare, used only the first year or so on the CB77. Not exotic, just not that common.
JohnnyB
 

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I would imagine so johnny, since there seems to be a steady increase in the price of restored superhawks. You would have to find the right buyer (someone restoring a first year superhawk) as I don't think the HD crowd is falling over themselves trying to get these and also first year superhawks are themselves rare bikes.

you can always put it on ebay with a rediculious reserve and see where they go - always a good test of what the market is willing to pay.

Personally I would give you $50 for 'em to use on my dual carb Dream 305 project.
 

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Geeto,
I might do an ebay test. If they don't sell...they are yours for $50...plus I have rebuild kits for them. Including the kits I probably have $100 in them.
They really are a nice set, probably 8 or 9 out of ten on the used part scale. Had a hell of a time getting kits to fit the round bowl. Now the kits are more common place I think.
JohnnyB
 

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Heres the 3 different styles of the Keihin CR carbs for the CB 750.

The first set is often called the "sandcast CR carbs" as they have a rougher casting to them
The second set is very simular to the Sandcast set but has a finer finish (less rough).
Both of these carbs have Tickler choke systems (a brass rod that pushed down the float for a second to richen up the cold start).

The third set is the CR Special set that is still available new. It has a "normal" choke system with a choke lever on the side of the carbs.


Honda CR 750 CR Carbs


OLD Style CR carbs


CR Special carbs (New Style)


The flat slide carbs are more for the newer sport bikes, but they can be used on a CB 750 with the proper spacing or manifold adaptors (and of course, jetting). The FCRs are available in either side draft or down draft. The down drafts cant be converted to side draft and can be told apart by the angle of the float bowl (it should sit level when installed on the bike)

The Keihin CR are one of the best carbs for the CB 750.
DG
 
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