When fuel boils in the carburetor it is said to percolate. The boiling fuel, of course, comes under the head of Vapor Lock. Percolation results in a very lean or rich mixture which may cause power loss if lean or hard starting if rich.
On some carburetors the possibility of this trouble is avoided by adding an anti-percolating valve to the carburetor design. The valve is sometimes linked to the throttle and opens when the throttle is closed allowing gasified fuel to escape to the atmosphere.
If the anti-percolating linkage is so adjusted that the valve opens early, air may be drawn in and this may cause a "flat spot" the next time the throttle is opened.
If the valve fails to open when the throttle is closed and the engine shut off, the intake manifold may be flooded, causing hard starting.