Chainsaws

I bought a Poulan Pro model pp5020av last week. When I tried to use it, it started and idled ok, but even after warmup, it would quit each time I opened the throttle. After a few more tries, I got it to run long enough to make one cut in a 8 inch log. When I released the throttle, it quit and would not restart. I then found gas leaking out through the center of the gas cap. I did find the cap was missing the retainer. Does the unit require the fuel tank to remain sealed while operating, or is the leaking cap and the run problems seperate problems.