Saving Old Seagulls

My ever reliable Century 100 (November 1973), was prepared for the boat clubs annual river cruise. Yesterday I spent time checking that all was well with the mechanicals - decant the gearbox sludge and replace with new 140 oil, check all nuts were tight and/or had wire restrainers, generally clean the crankcase and block to identify potential oil leaks. Remove the bowl to check for cleanliness inside. Make sure the 20:1 needle is fitted and that the banjo and fuel tap filter are clean. I even cleaned up the leading edge of the propellor and smoothed the surfaces (from casting dags).
I decided to swap the carburettor with an identical model from another (again ever reliable) motor, because it looked cleaner !!!!
Launched this morning (after checking that it would start on the transom on the trailer) ... no matter what I tried I could not get it past idle. It was running very rich, and I could observe fuel vapour spitting out from the carbie mouth.
I raised (then lowered) the needle - no change. I rechecked the spark plug gap - no change.
I desperation I changed the carbie (yes, I always carry spares!) - and it leaped into action! Easy start, high speed, idle, water pumping -- no problems!!!
Now why would it do that?

Stuck needle valve?

Hi Charles, one of the first things I checked ... seemed to rise and fall as I worked the float lever.
I replaced the (dud?) carbie (without any alterations) to the origonal motor this morning ... started first pull, from idle to high rev's - no problem!
So I decided to sell the first motor that would not start (C100, 40+, and several 102's) ... some had not run for several years ... EVERY BLOODY MOTOR started first pull!!!!