Topic: cr specials

[Medyo Bastos]

Anyone running these on a cb750?  Can you tell me what is missing from this threaded hole on one side of this carb?



Notice the carb beside it is not threaded on the same spot.

Here is what the other side of this carb looks like.




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[FJ40toy]

That is the idle air screw. The carbs are cast with the ability to have it on either side, depending on how it is going to be configured.

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[Medyo Bastos]

Do i need one on both sides? That is the other 3 are only threaded on 1 side.  ‘Preciate your help!


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[FJ40toy]

Do i need one on both sides? That is the other 3 are only threaded on 1 side.  ‘Preciate your help!


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You don't need one on each side. Cylinders 1 and 2 have the air screws on the left side of the carb. Cylinders 3 and 4 have the air screws on the right side of the carb. Makes it easier to adjust them.

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[Medyo Bastos]

So it ok for that one to be ooen to the atmos, right?


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[FJ40toy]

So it ok for that one to be ooen to the atmos, right?


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No, it needs an adjustment screw like the rest. JetsRus has parts if you need them.
https://www.jetsrus.com/carburetor/carb_keihin_CR_special_carb_exploded_view_parts.html

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[Medyo Bastos]

Hmmm, this carb has 1 on each side. Do i cap it off, or run 2 adjustment screws?


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[FJ40toy]

Hmmm, this carb has 1 on each side. Do i cap it off, or run 2 adjustment screws?


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That's odd. Typically they only have one. Maybe the previous owner drilled and taped the other hole?
The CRs I have on my 750 only have one per carb. You'll have to investigate if that second air screw hole goes into the carb body. You can spray some carb cleaner or WD40 to see if it comes out the bypass port on the back side, the engine side of the carb.
The photo is carbs 3 and 4 on mine. The air screws are towards the outside of the carb. If the second screw hole on yours is on the inside, I would put a screw in it to just plug it.

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[Medyo Bastos]

Yeah, that was the reason for my post. That carb has one on both sides, and i did shoot carb cleaner into it.

Here is a top view of carb 3. You can visualize what i am talking about.



Here is the questionable adjustment screw hole on the same carb.





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[FJ40toy]

Ok. I think a previous owner drilled and taped it. When you sprayed carb cleaner did it go any where? If not, you can leave like it is or put a screw in it just to keep and dirt out.

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[MRieck]

Yeah, that was the reason for my post. That carb has one on both sides, and i did shoot carb cleaner into it.

Here is a top view of carb 3. You can visualize what i am talking about.



Here is the questionable adjustment screw hole on the same carb.





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Correct....the bodies are mirror imaged. If it makes you feel better plug it but it won;'t make a difference.

[azuredesign]

I had the same issue, and believe that both sides are tapped before one side received a screw, and the other side a plug. I found a suitably threaded screw and closed off the open side that should have had a plug.

[todd.s.holloway]

Does anyone who uses these on a cb750 see an improvement in power? Or longevity/reliability of tuning?

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[bear]

I have used CR33's on my CB750 sohc based historic road racing sidecar for many years.
These carbs are 70's era competition technology and they work very well for that purpose.
But they have a rudimentary low speed circuit and can sometimes cause a small hesitation at low rpms.

I would be very surprised if there are not better alternatives these days.       

[todd.s.holloway]

I have used CR33's on my CB750 sohc based historic road racing sidecar for many years.
These carbs are 70's era competition technology and they work very well for that purpose.
But they have a rudimentary low speed circuit and can sometimes cause a small hesitation at low rpms.

I would be very surprised if there are not better alternatives these days.     

So it sounds like theyre a bit gurgly/stally at low revs? Other than that, you see a performance enhancement?

How do you fit them on to a 1975 cb750 sohc?

These were made stock for the dohc bikes later right? I like upgrades that are also period-ish correct and by the manufacturer!

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[bear]

I have used CR33's on my CB750 sohc based historic road racing sidecar for many years.
These carbs are 70's era competition technology and they work very well for that purpose.
But they have a rudimentary low speed circuit and can sometimes cause a small hesitation at low rpms.

I would be very surprised if there are not better alternatives these days.     

So it sounds like theyre a bit gurgly/stally at low revs? Other than that, you see a performance enhancement?

How do you fit them on to a 1975 cb750 sohc?

These were made stock for the dohc bikes later right? I like upgrades that are also period-ish correct and by the manufacturer!

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They come on a bank (aka rack). Their spacing can be adjusted to suit the intake your fitting them to.

Appropriate spigots are available to allow fitment to the Honda's std manifolds for the CR 29s and 31s.

No. Keihin CR carbs are an aftermarket competition carburetor.

I do not believe these carburetors are a worthwhile "performance enhancement" on a sohc road bike.
They are not a plug and play option in my opinion.

Cheers,
Brian

[cb550fcafe]

that screw hole is to retain a bracket for different spacing. On certain spacing requirements the two center carbs are far apart an the fuel "T" could slide out. There is a bracket that holds the "T" and uses the screw hole.