Yeah, this is a common problem on the cheapest Go engines. Had the same deal with my .21 3 port. I replaced my carb with an Alpha carb and haven't had any issues. Three things to try.

You can get a new carb:
RB carb on clearance at Amain
New carb from X-Act (basically yours with better o-rings and parts)

Or you could just get new parts such as these o-rings or this HSN rebuild set. Just make sure your carb looks the same as the X-Act one. X-Act sells parts for the Go .21 3 port.

OR you can try cutting pieces of fuel line to use as o-rings. Sometimes that works.