covering the air intake restricts air. This forces the air:fuel mixture ratio to be more rich .. something the carb should be doing without your help. So, it could be assumed the carb is having trouble delivering enough fuel... you'd trace fuel flow and find the retriction.
but since you've other issues, the fault could be elsewhere.
A 2-stroke's crankcase takes part in the engine's pumping action.. Mixture is pulled into the crankcase and is then forced up above the piston.
If the cylinder base (or intake manifold) has a leak, fresh air could be sucked into the crankcase through the leak. This fresh air would upset the air:fuel mixture that came in through the carburetor. This extra air would lean the mixture. (too much air for the amount of fuel)
Choking off some air at the carb may artificially correct the problem.
Consider yourself fortunate just to be aware of a problem like a torn gasket .. don't ignore it or try to work around it. Fix the things that you discover to be wrong as you go along.. Replace or repair that gasket. They are easy to cut from gasket paper.
I wouldn't be suprised if the hissing noise along with the carb metering problem were both cured.
To test for leaks, get a can of Starter Fluid and spray the area.. if this affects engine speed, there' a leak.