The thermostatic air cleaner (Thermac) system uses a damper assembly in the air cleaner inlet, controlled by a vacuum motor to mix preheated and cold air entering the air cleaner. This is necessary to maintain a controlled air temperature into the carburetor. The vacuum motor is controlled by a temperature sensor in the air cleaner. The preheating of the air cleaner inlet air allows leaner carburetor and choke settings, while maintaining good driveability.
If you live in an extremely cold climate you need this system. Where I live it doesn't get all that cold so I removed the air door from the snorkel all together. I wanted all the air flow that I could get and still use the factory air cleaner assembly. I also converted mine into a dual snorkel, but that is another story. Removing the Thermac system will cause your engine to take longer to warm up, so be prepared for this if you decide to do this.