Ryobi 790R

Hi, two years ago my string trimmer became hard to start. I reran the fuel lines and it would run if I used starting fluid. This year it wouldn't start. I replaced the fuel filter and line, rebuilt the carburetor, changed the spark plug twice and changed the fuel. It still wouldn't start. I had some extra fuel line, so I replaced the return line from the primer bulb to the tank. I pulled the return line from the top of the carburetor to the primer bulb off with the intention of buying some more fuel line. Meanwhile, I read on the internet that it might be the fuel cap, so I went back to the garage and took the cap off forgetting the line from the top of the carburetor to the primer bulb was on the garage floor. For the first time in two years, it fired right up. I put the line back on and it wouldn't start. I put the cap back on and with the line off and it runs. I can blow through the line and it is only a year old. Why would it run only with the fuel line removed?

Thanks for any help!