This regulation represents the current recommendations of the Vehicle Emissions Inspection Panel of the Motor Vehicle Manufacturers Association, Inc., regarding engine maintenance inspection required to minimize vehicle emissions. This section is advisory and is primarily related to 1968 and later models. The inspection of emission control systems applies only to spark ignition engines, not to diesels. This regulation is applicable to school vehicles only. Crankcase Ventilation. The ventilation system of the crankcase is common to engines using either an “air injection” system of control or an “engine modification” system. The engine should be warm and running at idle before inspection of vent system. Air Injection Type—Idle Speed and Timing. An air injection-type exhaust emission control system can be identified by an air pump and a distribution system to the exhaust ports. The air pump is normally driven by a belt off the crankshaft or water pump pulleys. Only the plumbing, idle speed and ignition timing need be inspected. Reference is made to the Idle Setting Limits Table I. Engine Modification Type—Idle Speed and Timing. An “engine modification” control system may be most easily identified as one without an air pump. The emissions are primarily controlled by engine design. The items to be inspected are vacuum, idle speed, ignition timing, and idle air-fuel mixture by measuring the carbon monoxide (CO) content of the exhaust. Refer to Table II, Idle Setting Limits. Engine Modification-Type Fuel Mixture (Carbon Monoxide). The carbon monoxide (CO) concentration in the exhaust of most vehicles may not be more than 3 percent. Be sure that the testing is done by an experienced operator and that the exhaust analyzer is properly calibrated.