Carburetor Specifications
U.S. CB750 Models
1969-1983
Year Model Casting # Type Float Main Slow JN Screw
69-76 CB750K B750A & 7A 1 26 120 40 3 1
    657A 1 26 110 40 4 1
    657B & 086A 1 26 105 40 4 1
77 CB750K 41A 2 12.5 115 35 1 11/2
78 CB750K PD42B 2 14.5 110 35 NA  
79-81 CB750K/C VB42A 3 15.5 68/102 35 NA 11/2
82 CB750K VB42AG 3 15.5 68/102 35 NA 11/2
81 CB750C VB42A 3 15.5 68/102 35 NA 11/2
82 CB750C VB42G 3 15.5 68/102 35 NA 11/2
82-83 CB750SC VB42AG 3 15.5 68/102 35 NA 11/2
75 CB750F B750A & 7A 1 26 105 40 3 1
    657A & 657B 1 26 105 40 4 1
76 CB750F 069A 1 26 105 38 2 1
77 CB750F PD41B 2 14.5 105 35 2 1
78 CB750F PD42A 2 14.5 105 35 NA  
79 CB750F VB42B 3 15.5 68/100 35 NA 11/2
80 CB750F VB42B 3 15.5 68/98 35 NA 11/2
81 CB750F VB42B 3 15.5 68/102 35 NA 11/2
82 CB750F VB42AG 3 15.5 68/102 35 NA 11/2
75-77 CB750A PD44A 2 14.5 102 38 3 11/2
78 CB750A PD43A 2 12.5 108 38 NA 11/8
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Casting numbers on the early style carbs is found on the top of the carb just behind the stay plate.
It's stamped on a flat oval spot as shown above in the closeup.
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Casting numbers on the later 77/78 style carbs is found on the castings just above the float bowl.
It's stamped on a flat rectangular spot as shown above in the closeup.

Casting numbers on the quad cable pull 69/70 Sandcast style carbs is found on the stayplate, not the castings.
Thanks to Oldhemi for the info!
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Source: http://members.tripod.com/~Wrenchbender/specs.html (now defunct)