a 3310 is calibrated a bit on the rich side from the factory as it was intended to be a performance carb used with a bit more cam that a stock engine...on the other hand, it really tends on how rich it actually is and what signs you have to come to that conclusion. A smeel exhaust is not always a good sign...
But, I would guess the next most common factor would be a blown power valve, or a power valve with too high a vacuum rating so that (in either case) the PV is passing fuel when it should be closed.