Mercury 40-115 HP 3-4 Cylinder 2-Stroke Repair Manual 1965-1989

This repair manual by Seloc covers 1965 to 1989 Mercury Outboard motors, 40-115 HP, 3- and 4-cylinder 2-strokes. Step-by-step illustrated keyed procedures guide you, step-by-step, through every job. The manual includes all adjustments necessary making it the guide of choice for the do-it-yourselfer, professional boat mechanics or marine engine repair students.
Book Excerpt: 1965-1989 Mercury 40-115 HP 3-4 Cylinder 2-Stroke Repair Manual TROUBLESHOOTING TYPE III IGNITION SYSTEM

Always attempt to proceed with the troubleshooting in an orderly manner. The ?shot-in-the-dark? approach will only result in wasted time, incorrect diagnosis, replacement of unnecessary parts, and frustration. Begin the ignition system troubleshooting with the spark plugs and continue through the system until the source of the trouble is located.

SPARK PLUGS

1. Check the plug wires to be sure they are properly connected. Check the entire length of the wires from the plugs to the distributor. If the wire is to be removed from the spark plug, ALWAYS use a pulling and twisting motion as a precaution against damaging the connection.
2. Attempt to remove the spark plugs by hand. This is a rough test to determine if the plug is tightened properly. You should not be able to remove the plug without using the proper socket size tool. Remove the spark plugs and keep them in order. Examine each plug and evaluate its condition as described in Section 5-2.
3. Use a spark tester and check for spark at each cylinder. If a spark tester is not available, hold the plug wire about 3/8? to 1/2'? (9.53 to 12.7 mm) from the engine. Turn the flywheel with a pull starter or electrical starter and check for spark. A strong spark over a wide gap must be observed when testing in this manner, because under compression a strong spark is necessary in order to ignite the air/fuel mixture in the cylinder. This means it is possible to think you have a strong spark, when in reality the spark will be too weak when the plug is installed. If there is no spark, or if the spark is weak, the trouble is most likely in the distributor or in the switch box.

COMPRESSION

Before spending too much time and money attempting to trace a problem to the ignition system, a compression check of each cylinder should be made. If the cylinder does not have adequate compression, troubleshooting and attempted service of the ignition or fuel system will fail to give the desired results of satisfactory engine performance.

Subject: 1965, 1966, 1967, 1968, 1969, 1970, 1971, 1972, 1973, 1975, 1976, 1977, 1978, 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989 Mercury 40-115 HP 3-4 Cylinder 2-Stroke service, maintenance, repair. ISBN-10: 0893300136 | ISBN-13: 9780893300135 | Seloc 1406