This Clymer DIY repair manual covers 1964-1971 Mercury 3.9-135 HP, 1-cylinder to 6-cylinder outboard motors.
Book Excerpt: 1964-1971 Mercury Outboard 3.9-135 HP Repair and Service Manual
Engine problems are generally symptoms of something wrong in another system, such as ignition, fuel or starting. If properly maintained and serviced, the engine should experience no problems other than those caused by age and wear.
OVERHEATING AND LACK OF LUBRICATION
Overheating and lack of lubrication cause the majority of engine mechanical problems. Outboard motors create a great deal of heat and are not designed to operate at a standstill for any length of time.
Using a spark plug of the wrong heat range can burn a piston.
Incorrect ignition timing, a defective water pump or thermostat, a propeller that is too large (over-propping) or an excessively lean fuel mixture can also cause the engine to overheat.
Preignition is the premature burning of fuel and is caused by hot spots in the combustion chamber (Figure 16). The fuel actually ignites before it is supposed to. Glowing deposits in the combustion chamber, inadequate cooling or overheated spark plugs can all cause preignition.
This is first noticed in the form of a power loss but will eventually result in extensive damage to the internal parts of the engine because of higher combustion chamber temperatures.
Several factors can cause a lack of power and speed. Look for air leaks in the fuel line or fuel pump, a clogged fuel filter or a chock/throttle valve that does not operate properly. Check ignition timing.
A piston or cylinder that is galling, incorrect piston clearance or a worn/sticky piston ring may be responsible.
Look for loose bolts, defective gaskets or leaking machined mating surfaces on the cylinder head, cylinder or crankcase. Also check the crankcase oil seal; if worn, it can allow gas to leak between cylinders.
This is caused by one or more pistons with incorrect bore clearances, piston rings with an improper end gap, the use of an oil-fuel mixture containing less than 1 part oil to 50 parts of gasoline or al oil of poor quality, a spark plug of the wrong heat range or incorrect ignition timing. Overheating from any cause may result in piston seizure.
Subject: 1964, 1965, 1966, 1967, 1968, 1969, 1970, 1971 Mercury Outboard 3.9-135 HP service, maintenance, repair. ISBN-10: 0892874414 | ISBN-13: 9780892874415