It sounds like you have a blockage or air leak in the fuel system.

1. Check your fuel lines VERY CAREFULLY with the engine running. Do you have any air bubbles ? If you do, pull the tank and replace ALL the fuel lines INCLUDING the ones in the tank. Double check the fuel inlet nipple(s) to the carb and needle valve to make sure they are tight. If they are loose, tighten them and put some locktite around the outside to prevent them loosening up and to seal any gasket leaks. Run the engine. If the problem persists go to step 2.

2. Check the spray bar in the carb. Disconnect the fuel line from the needle valve, and with the throttle wide open, blow through the line. Now do the same thing with the throttle closed. If there is no difference in the amount of force needed, the blockage is in the spray bar. Remove the spray bar and clean it in some fresh fuel or acetone. MAKE SURE YOU DO AN EXACT COUNT OF THE NUMBER OF TURNS ON THE LOW SPEED NEEDLE, it will minimize the retuning of the low end.

3. If the spray bar is OK (or it is still doing it after cleaning it ), move to the needle valve next. Disconnect the fuel line from the tank to the needle valve and put some fuel through the needle valve to flush it out. Make sure the connections are tight for the needle valve and fuel nipples.