F2.5A carburetor seems always clogged?

[mischa.mole]

Two years ago we purchased a used F2.5A. Great little engine! Until a few weeks ago, everything worked great, we used it on a daily basis. Then suddenly while driving, it started to ran not smooth anymore, and stalled if I didn't reduce to (nearly) idle. Everything seemed (enough fuel, spark plug etc) fine, so I guessed the fuel line is blocked somewhere. I removed and cleaned the tank, removed and cleaned the carburetor and it worked again like a charm. Two days later the same thing. I again drained and removed the carburetor, cleaned it, blew it through with a bike pump (only thing I had), checked the tiny valve and the float and the pin, and everything works again. I clearly saw that the carburetor has been opened already by the previous owner, but everything seems fine. After another two to four days the same thing. Sometimes it also happens when a small wave shakes our dinghy or something. I really think there is just not enough fuel reaching the nozzle for some reason, but how can this get clogged so fast? We are ALWAYS closing the air intake of the tank, using inly the freshest and cleanest gasoline, but the same thing keeps happening. I already figured out a way to drain the carburetor really fast, attach my bike pump and and blow the carburetor without removing it, but this can't be a permanent solution? If i just open the drain screw of the carburetor, there is enough gasoline coming.
I'm THIS close to simply buying a new carburetor, or is there anything else I could try?

Thanks and best regards from Key West
Mischa