[socalwrench]

The crankcase of any engine needs to be open- allowing it to breathe. If the crankcase was closed- than it would build pressure and blow up throw the piston rings (hurting the seal), and would push on the bottoms of the pistons (hurting power and mileage).

On earlier engines, a simply open element breather located on the valve cover works fine. You should also have a PCV (positive crankcase ventilation) valve hooked up to the bottom of your carb. Newer engines starting using more complex tubing to prevent any HCs from getting into the atmosphere. So, instead of open type breathers- it is usually connected to the intake or air filter housing. Many still use a PCV valve as well.