The way to prime the engine is to put your finger over the exhaust hole and pull the starter. The engine pushes air out of the engine which usually goes out of the exhaust but as you have your finger over the exhaust the air is forced down the tube attached to the exhaust and into the fuel tank which then pushes the fuel into the engine which is the cycle.

If the engine does not do this it means the air is leaking somewhere. If so then first of all check all the screws around the engine are tight, especially the ones that go through the engine and hold the manifold on. Also check that the exhaust is pushed firmly into the silicone tube so there is an airtight link between the exhaust and manifold. Also make sure that the fuel tank lid is down and that there is no holes in the fuel line.