[Kenth]

Running the HIC w/o the gasket a vacuum leak occurs at the missing gasket, like the HIC never closes.
The stack provides fresh air from inside the air filter thru the HIC when HIC opens due to higher than normal engine temperature.
Setting the idle mixture screws requires to plug the stack with a pencil or rubber plug or whatever so additional air from HIC wont disturb the setting.
Some HIC covers has a plunger that needs to be pushed against the HIC valve too keep it closed when setting idle mixture screws.

The only way to permanently disable the HIC is to plug the hole with an aluminium plug where the valve sits and replace the cover so road dirt wont enter the engine thru the stack.