Hey im trying to fix a 2000 gtx. It started with hesitation and stall when coming off idle. I just bought this machine so I did a carb rebuild checked popoff pressure and accelerator pump. I have the manual. I put it back together and now its not getting fuel. Accelerator pumps air. I am getting back pressure into the tank from the top fitting on the carb. When i open the fuel cap it hisses. Is it supposed to do that. Any ideas or experience before i tear it back apart.