That can be tough. To check the tank and fuel filter (you have one I hope) Attach a piece of fuel line to the inlet and outlet of the gas tank, then submerge it under water, block one end of the fuel line and blow in the other, if there is a leak it will bubble, this also works for the filter. If the leak is in the engine itself it can be much harder to find, I find it easier to just go through and seal the entire engine. Check the o-ring at the base of the carb make sure it is not torn or in crappy condition, also check to make sure it is fully seated and the nut is tight. I seal mine with racers edge carb sealer but you can also use ultra copper gasket sealer too. Make sure none gets down into the engine. Another common place for air leaks is the back plate which is slightly more difficult to seal. You will have to remove it and put a small amount of gasket sealer on the gasket for the backplate then reinstall it. Start with the easy stuff first like the tank filter and seal the carb.
Ok, I didnt think to ask if your tank had a primer, that HAS to go! You can remove it and seal it with a screw and some silicone or just buy a new tank for $12. That is the most common place for an air leak. The newer models no longer have a primer due to the amount of problems with them. Again that HAS to go!!!