The major difference from the 966s to the 2100s was Carter found the idle circuit on the 966s to be slightly rich for the 235; and leaned it for the 2100s.

Both have a 1 5/16 inch (33.3 mm) venturi, which is slightly smaller than the venturi GMC used on the Zenith carb for the 270 (34 mm).

I would guess the 966s would be a somewhat better choice for the 261 because of the richer idle circuit.

Please understand the above is a GUESS based on statistical data; I have not personally tried this conversion.

Jon

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