Den99,
If the hiss sounds like a vacuum leak, you might want to try a shade-tree technique for finding a vacuum leak: open very slightly a hand-held propane torch and move it around the base/body of the carburetor (and/or near where you hear the hiss). If the RPM increases, you have a vacuum leak causing the hisssss.
If you have a stethoscope, you could use it to find the location of the hissss - then, open the propane near the hisssss.
Good luck.
