1942 - 1947 Chevrolet Shop Manual



	6-31

	ENGINE TUNE-UP One of the most important duties to perform on modern high compression engines is proper engine tune-up. This operation more than any other determines whether or not the car or truck will produce the maximum amount of performance with the greatest amount of economy. Only by performing these operations and staying within limits, clearances and specifications is it possible to get the performance and economy built into the Chevrolet engine. Before proceeding with an engine tune-up, first determine whether or not the engine can be successfully tuned. Therefore, the first operation is a careful check of the compression—An engine without fairly even compression cannot be tuned! Compression Before making any checks on an engine it should be run for several minutes and allowed to warm up and lubricate the valve mechanism. The compression of each cylinder should be checked first because an engine with uneven compression cannot be tuned successfully. 1. Remove all spark plugs from the engine. The ignition should be turned off, and the hand throttle control pulled out to the open position. 2. Insert the compression gauge in a spark plug hole and hold it tightly. Crank the engine with the starting motor until the gauge reaches its highest reading, which requires only a few turns of the engine. Repeat the same test on all cylinders and make a note of the compression on each cylinder. The compression on all cylinders should be 110 pounds or better, and all cylinders should read alike within 5 to 10 pounds for satisfactory engine performance. Should a low compression reading be obtained on two adjoining cylinders, it indicates the possibility of a leak from one cylinder to another, usually caused by a leak at the cylinder head gasket. If the compression readings are low, or vary widely, the cause of the trouble may be determined by injecting a liberal supply of engine oil on top of the pistons of the low reading cylinders. Crank the engine over several times, and then take a second compression test. If there is practically no difference in the readings when compared with the first test, it indicates sticky or poorly seating valves. However, if the compression on the low reading cylinders is higher now and about	uniform with the other cylinders it indicates compression loss past the pistons and rings. The cause of low or uneven compression must be corrected before proceeding with an engine tune-up job. Spark Plugs Clean the spark plugs thoroughly, using an abrasive type cleaner. If the porcelains are badly glazed or blistered, the spark plugs should be replaced. All spark plugs must be of the same make and number or heat range. Adjust the spark plug gaps to .040", using a round feeler gauge. CAUTION—In adjusting the spark plug gap never bend the center electrode which extends through the porcelain center; always make adjustment by bending the side electrode. Reassemble the spark plugs in the engine, using new gaskets wherever necessary. The spark plugs used in the Chevrolet engine are of 10-millimeter size. The heat range of the 10-milllimeter plug is about that of a number "47" during the warming up period, but on heavy service such as in trucks and at high speeds, it about equals a number "42" plug in the 14-millimeter size. Care should be used when installing the 10-milli-meter spark plug or the setting of the gap may be upset. If a tension wrench is used when installing the plugs, the proper tension is 15 foot pounds maximum. If a tension wrench is not available, use the following procedure—when using old gaskets screw each spark plug in "finger tight," then with a wrench tighten plug 1/8 to 1/4 turn beyond this. When new spark plug gaskets are used, screw each plug in "finger tight" and then with a wrench tighten each plug 1/2 to 3/4 turn beyond this. Battery Test Connect the positive terminal of a voltmeter to the starting switch terminal and the negative terminal of the voltmeter to a good ground. Close the starting motor switch and crank the engine for 15 seconds. If the starting motor cranks the engine at a good rate of speed during this period with the voltmeter reading 5 volts or better, it indicates a satisfactory starting circuit, which includes the condition of the battery terminals and cables. However, if the cranking speed is slow, or the voltmeter reading is under 5 volts, the starting motor, battery, and battery cable terminals should be checked individually to locate the source of the trouble.