Insufficient flexability of clunk line if it is clearing the bottom of the tank or otherwise unstuck. The smaller the fuel tank the more this becomes a problem.

-OR-

If you want the most reliable fuel delivery possible with a muffler pressure system then try Tetra or Jett Bubbless tanks. These have no clunk at all with none of the associated problems.

http://www.centralhobbies.com/Fuel/fueltnk2.html

John