Definitely remove the check valve.

My first thoughts reading through this is that there may be issues with the tank. You'll need to figure out if the tank is set up properly. Look at the diagrams attached and double check that your tank is setup in the same manner. Also, you will need to check the tank position in relation to the carb of the engine. The neck of the tank needs to be approximately at the same level as the carburetor of the engine.

Note on the diagram. The stiff tubing shown in the tank isn't necessary, but in my experience it helps to keep the clunk from going forward and getting stuck in the tank.

Ken