My thoughts:
A. Dirt in the needle and seat as stated above. Remove needle and seat and clean and reinstall.
B. Possible mislocated/misinstalled gasket. Could a gasket have not sealed properly or put on backwards (i've done that) sometimes the gaskets only work one way, but are capable of being installed backward or upside down and in some cases a passage may be blocked that should be open. (A good reason to keep the old gaskets until you get it back together - for reference).
C. Probably not likely, buy worth mentioning. How hot is the engine when this happens, could the fuel be boiling over?