Is it a reed valve engine or a tee dee with a venturi in front of the cylinder. Most of the reedies will have a built in tank with the needle valve protruding from the rear cover of the tank. Is this what you have?

If so you need to ascertain how the tank pickup has been arrainged. Those tanks do not have a clunk and the pickup line is bent almost ninety degrees with a spring inside. This tube can be orriented in four directions and it must be positioned properly depending on intended use.

For Freeflight or RC with the cylinder upright the tube must go to the bottom of the tank For the same situation but a sideways mount the tube must be positioned to go to the bottom of the tank. If the cylinder is inverted then that tube must be pointing to the bottom of the tank.

For controlline use the that pickup must be always arrainged to the outside of the circle whichever direction the cylinder is mounted.

Its possible you are trying to start it with the pickup line at the top of the tank. You need to remove the four tank screws and open the venturi/back plate up to check the pickup orientation.

John