Hang idle or a slow return to idle after a rev is commonly caused by air leaks. The air leaks can be caused by improperly seated carb mounting boots, intake funnels that are not sealing the carb body to the airbox base, or air filter housings that are leaking under the air filter.

To test for leaks, you can use a spray liquid like brake parts cleaner or a gas, like propane, around all of the connection areas to determine if there is a change in RPM. (Be very careful when using flammable liquids or developing pockets of flammable gas near the engine).

If for example, there is a change in RPM while testing the carb mounting boots, this would indicate a leak.  You can try tightening the clamps a little to seal it, but in most cases the boots will be hard or cracked from age and it wont help. As you find the air leaks, use the corrective action needed to seal it. Silicon Seal not allowed.