Topic: How Do you know?????

[ron72401]

Hey Folks,

bought a set of carbs off of ebay for my 78 CB750 and need to rebuild them. The deal is I don't know how to verify the year that these carbs belong to. I was told they are from 1974 but I would like to verify that before I buy some rebuild kits. Can anyone tell me how to identify as set of carbs to a year?

Thanks in advance for your help and reply.

[TwoTired]

Any numbers/stampings on them you'd care to relate?

[ron72401]

I haven't found any but I haven't searched either. Where would they be?

[TwoTired]

I don't know the specifics of the cb750 carbs prior to 77, apart from what is in the Shop manual.  My collection of CB550's has taught me that there are specific setups for each model change, or change in exhaust system.
For the Cb550 Keihins, The stamping is on each carb body mount flange, top right.  Perhaps someone else knows what the different set up specifics for the Cb750s, correspondings to the various exhaust differences, are.
Cheers,

[m goodwin]

The numbers on my 74 CB750 which is stock, are 657B they are written on the left side of the flange..  There are also a few marking on the right side of the flange but they do not appear to be numbers or letters.  Hope this helps.

[frostypuck]

I'll have to double check, but, I think the lifting mechanism on the '77 and '78 carbs are enclosed in the body of the carbs, whereas the earlier ones have the rubber boots over the chrome arms If you can see lifty-bits when you crank the throttle, it's a pre-77 setup. Also the choke is going to be operated via cable with the knob mounted up by the handlebars, instead of directly on the carbs. Any more specific questions, I'll photograph the differences between my '76 and my '78 for you.
Chris in Boston

[ron72401]

Per m goodwin's suggestion about finding the number on the flange there is one. It is 086A. Does that tell anyone anything? These carbs do not have a cable for the choke, but a lever on the left of the carb body. Two throttle cables, a pull and push. I was told that these old carbs are easier to adjust to engine modificaitons like modified intake and exhaust, that is why I am looking at using these older carbs instead of the 78 carbs that were original to the bike. Thanks again guys for your help.

[longtall]

http://hondachopper.com/carb.specs.html
 The older carbs are easier to find rebuild kits for. 

[scondon]

If your bike is an "F" model you'll need to do some mods to get the older carbs to fit. Diameters are larger on the stock 78 F carbs. If your bike is a "K" then, well, never mind

[longtall]

Oh yeah, you will have to use the older model intake boots also 69 to 76

[ron72401]

Yeah I've learned that I have an F engine in a K frame. I've put a MAC 4 into 1 exhaust on it and I understand that the mains on the stock carbs are a press fit so was thinking that an older set of carbs where I could change the main jet would be a good plan. I've never attempted installing carb kits, tuning carbs or adapting different carbs to an engine but I'll try anything once .... well almost

Any sage advice is always appreciated. I am at the point of having to decide whether to use the original 78 carbs that came on the engine or this older set of carbs that are supposedly easier to tune to a 4 into 1 exhaust. I have pods and the orginal intake. I keep reading that the pods are problematic when it comes to tuning, so I've not decided yet which direction to take. I am not after max performance but I do want a good running engine under most riding circumstances.

I look forward to hearing what you guys think is the best approach.

thanks

[scondon]

 You can change the mains in the 78 carbs, its just the idle jets that are pressed in. One of the advantages of the earlier carbs is that you can remove the float bowls to get at the jets without having to take the carbs off the bike. One of the advantages of the 78 carbs is they have an accellerator pump which helps getting off the line, and they have a better choke set-up.

  If you have the 78 F motor(easily identified because it's painted black) then it will not be easy to fit carbs/airbox from other models. The intake fittings on the engine are larger in diameter so the smaller diameter carbs/manifold rubbers from other models will not be an easy fit. If you are not the original owner of the bike then this may have already been addressed or sorted out. Main thing is to make sure of what you have so you'll know what fits. Pod filters for K models and 75-76 F will be too small to fit later F carbs, and visa-versa.

  Pods may take a bit more work to get the jetting/mixture right. They do add a more "throaty" sound to the engine. On the other hand I've found that I prefer the airbox on an unmodified engine as it tends to keep acceleration smoother. Adding a K&N filter that allows more air than the stock paper filter may be a good choice(just my opinion and not backed by any scientific data).

  I've only ran one Mac exhaust and found that it was about as restrictive as the stock one it replaced. I didn't have to re-jet to use it. I've never changed from a 4-4 to a 4-1 though so maybe someone else can give better input.