Good question but the answer is sort of a challenge. Rochester made a carburetor for the 261, but we don't know if what is on there is that carburetor or some other Rochester B and we don't know what problems it may have because of what is called the rebuilder's mixmaster. If you want to know if you have the one made for the 261, remove it and turn it upside down. Measure across the throttle body opening (the round hole). If it measures 1.75" you at least have the right throttle body for the 261 and you might also have the right carburetor in total. If it measures 1.5" you have probably have a 235 carburetor. At least as I recall. Don't quote me on the measurements but the 261 Rochester had a larger opening than 1.5".
Carter never made a YF model for the 261, so many folks have used YF models made for other similar engines. Jon (carbking) would be the best person to ask about this as he knows the specifics about the various Carter carburetors and how they might work for the 261. And there are other (more expensive) options like Zenith. AMC used a Carter YF on a 258 engine, but that was as I recall in the early 70s. There may or may not have been smog get-up on it. If so, I would expect some adjustment might be needed. Offhand I can't remember the model number but the size of the engine is close to 261.
