To add to what Ken said, I haven't found a Rochester B yet that didn't have a hangup with the power piston somehow, either a warped casting making the piston sticky or carbon in the bore. Make sure it slides freely and that the ball seats when it retracts. A sticky power valve will make the engine run rich all the time. When the throttle is closed with the engine running vacuum draws it up leaning out the mixture.
A 261 should have a #57 or 58 main jet, if you have a B from a 250 it will be a #62 or 63 which will be too rich.
