If an engine backfires when first started and does so continuously it is best to check up on the wiring and timing of the ignition system. If the engine is running smoothly and suddenly starts to backfire through the carburetor it is possible that the magneto coupling has slipped. If there is water in the carburetor it may suddenly shut off the supply of gasoline and cause so lean a mixture that backfiring results. Dirt in the line or running out of gasoline would have the same effect. If backfiring in the carburetor is experienced in addition to the missing of the engine it is probably due to too lean a mixture. Backfiring also results from the inlet valve sticking or not seating properly. If the engine suddenly develops a knock while in operation it may be due to the ignition being too far advanced for the condition for which the car is operating and the spark lever should be retarded. This will be noticed mostly when the car is under a hard pull such as on hard hills or going through sandy roads. If the engine develops a knock, after having been run for a short while, which can not be overcome by retarding the spark it may be due to carbon in the cylinders or an overheated engine, both of which would cause pre-ignition of the charge. By pre-ignition is meant that the incoming charge when under compression is ignited due to the heat in the cylinder regardless of when the ignition spark takes place. Loose bearings and loose pistons will knock but these should be easily distinguished from ignition knocks as they are present at all times. If the engine shows a lack of power it may be that the ignition system is too far retarded due to the coupling driving of the magneto having slipped. If too rich a mixture is used it will cause a loss of power but can easily be distinguished by the black smoke which is given off at the exhaust pipe. Every precaution should be taken to locate the trouble when an engine shows a lack of power as it may be caused from the valve not seating properly, carbon in the cylinders, overheated engine, lack of lubrication, or the governor connection sticking. If lack of lubrication is causing the trouble it will soon lead to mechanical trouble such as scoring the cylinder walls or burning out the bearings. An engine will often give an apparent indication of a lack of power due to the brakes dragging or the clutch slipping.