I had the EXACT problem. Try seeing if you can press down on the fuel tank lid while priming it (the finger-over-exhaust tip-way). The spring on the lid may be worn out and not making a good enough seal. I had to get a new lid, spring, and seal (the only way to get the spring is to get all that stuff in a pack) and it primed. However, a little while later, I had the same problem. This time it was that my stock plastic pipe had a leak between the pipe and the coupler.

So, try pushing down on the fuel tank lid while priming (you may need a third helping hand), and, if that doesn't work, try checking that the exhaust coupler is all sealed well and that the 360 exhaust manifold is tight against the engine. Also try checking the fuel line, like "nitroguy123" said. Good luck.