Topic: Difference 064A and 657A CB750 carbs

[AshimotoK0]

A member on the UK site needs a replacement outer RHS carb for his Cb750. His are 657A model
I have a set of carbs which look very similar but they are 064A and I will donate one carb body to him if it's suitable. Does anyone know what the differences are between 064A and 657A please?

Also does anyone know if one of the  inner carbs in a rack  can be modified to an outer one, as the only visible difference I can see is that the vacuum take off point is on the opposite side, which is not the end of the world.

Ash (UK)

[AshimotoK0]

Anyone know the answer?

Cheers  Ash (UK)

[el__burro]

I guess not.

Thanks
El__burro

[greenjeans]

I have a set of untouched 064s that I'm breaking apart today to clean.  I'll take a look and see what I think.

[Brantley]

657a and 064a bodies are interchangeable. Stamp only denotes which internals were installed at the factory. Unless you get real creative you cant swap positions due to the crossovers and vent tubes.

[HondaMan]

The 657A carbs are not emission-controlled, the 064 versions are. The difference was/is in the taper of the idle air screws, which control the mixture to 2500 RPM, which [at the time] was the upper RPM limit for the EPA tests. In practice, the 7/8 - 1-1/8 turn mixture change of the 657 carbs is about 15% richer/leaner than the 64 series, due to more accurate tolerances in the later carb.

But, they will work fine together: you may find the 64 air screw must be set a bit further out than the other carbs, like 1/16 turn more, to make the plugs all the same color after 500 miles or so.