I have diffiiculty understanding how a distorted gasket could possibly touch the float as many have claimed... it's simply too far above the float. With the carburetor top half clamped down inverted, the weight of the float should have the needle shut off tightly. Very low pressure air and soapy water will make bubbles to show where the gas is really getting through.
The one place that's hard to test is the main nozzle in the venturi. It's gasket in below the fuel level in the bowl and will leak gas if it isn't sealed well