A leaky inlet manifold will admit an extra amount of air that will completely alter the proportions of a mixture. Thus the mixture will be wrong, although the carburetor adjustment seems to be correct. Manifold leaks are usually at the joints, but occasionally a manifold is found with a hole in it due to poor casting or material, or a crack may develop. Difficulty in starting due to poor compression caused by stuck valves or rings will show its cause by the ease with which the engine can be cranked. If an engine is free enough to turn over, poor lubrication or cooling will not interfere with starting it. Faults in these systems show themselves only when an engine is running. ENGINE LOSES POWER An engine will lose power through a defect of compression, carburetion, ignition, cooling or lubrication, or because of a mechanical fault. If the trouble comes from cooling or lubrication, the engine will overheat and thus make the cause known. A bearing that binds will become very hot, while if the cooling system fails, the engine will be hot all over. When the engine is excessively hot, the pistons will expand, and much of the power of the engine will be used up in forcing them to move. An engine that is not hotter than usual, and is having regular and even explosions, probably loses power through a loss of compression. This is the most usual cause of this trouble, and it is located and remedied as explained in Chapter XII. If compression is good, the loss of power may be due to a clogged muffler or exhaust pipe, which will not permit the free escape of the burned gases. This condition will prevent full charges of fresh mixture from entering the cylinders, and the engine then cannot be expected to deliver full power. Another possible cause of a loss of power with the engine apparently in proper condition is the sticking or poor adjustment of the governor. The factory adjustment of the governor should not be changed, however, until it is definitely proved that that is where the trouble lies. If the engine misses fire, or runs irregularly, the loss of power will be due to faulty carburetion or ignition. The mixture may be too rich or too lean; in either case the trouble will be remedied by readjusting the carburetor. A mixture that is very much too lean will make itself known by _backfiring_; there will be little explosions at the carburetor. This should be remedied at once, for the danger of fire from it is very great. Black smoke at the exhaust is a sign of a mixture that is too rich. An engine will not deliver full power if it is run on a retarded spark. A loss of power from this cause will be accompanied by general overheating of the engine. ENGINE STOPS The manner in which an engine stops will indicate the reason for it.