Like Jim said, check that the clunk is doing it's thing in a normal fashion (not folding back on itself) and restricting fuel.
Bubbles USUALLY occur when there is a air leak ANYWHERE between the fuel line connections from your tank to the carb. Afterall, if an engine is pulling air and the fuel tank is 1/3 rd. filled or over, it has to be coming from somewhere.
Things to check:
Hardened or loose fuel lines close to every connection.
Clunk inside tank folding back and restricting fuel as Jim suggested.
See if the needle valve will shut off the air completely by blowing thru the fuel line while the valve is in the closed position, if not, it could be a bad O ring on the needle.
Don't know what kind of engine you're running either, some are known for serious carburetor and needle valve leaks like my Tower .46 but I have mine under control. New carb. O ring and needle seat cured it.
Also ... I noticed some folks at our flying field over the weekend pumping the unused fuel inside there planes tank back in to there fuel jug .... That is a BIG NO NO ... it will seriously contaminate the fuel and can cause it to foam way more than normal. The fuel will also lose it's punch (degrade it).
I would try going thru the tweaking stage first, a couple of clicks rich or lean might cure it.
Good Luck
Mike