If you pinch both the other lines then your engine is trying to suck against a vacuum in the tank -- ie: there's nowhere for air to get in and replace the fuel that's being drawn out.

However, as others have said, chances are that you've hooked up one of the vent lines (instead of the clunk line) to your engine.

To check for this, see if your fuel pump will suck fuel out of the tank from the line that's on your engine carby. If it sucks air then you've got the lines around the wrong way.

You don't mention whether your engine has a remote needle valve or not. Some engines with the remote needle can develop leaks which show up as bubbles in the line between the needle-valve and the carby.