Tuning and adjusting an outboard engine is similar to tuning and adjusting other two stroke engines. The procedures outlined here are applicable to almost any two stroke outboard motor, regardless of manufacturer. Before you proceed with an outboard tune up, you should determine if a specific engine has special problems or unique characteristics that require consideration. You can deciper potential problems by test running an engine, either on a boat or in a testing tank. Sometimes a simple visual inspection is informative. For example, blistered paint on an engine cover may well indicate repeated overheating during operation.

Working on an outboard does generally require a special mounting table or bench that can support the engine as it undergoes a tune up. However most of the time a solid raised bench to which the engine stern bracket can be clamped is sufficient. You would mount the engine on the bench just about as it would be mounted on a boat, making sure that the engine is securely fastened, and that the bench is sturdy enough to withstand engine weight without tipping over. Be sure that the shop where you will work is adequately ventilated. In addition, before you conduct engine performance tests, remove the propeller and replace it with test wheel to prevent injury.

An outboard engine tune up will usually include the following:

1)Compression check
2)Inspection of the powerhead
3)A check of the ignition system
4)A check of the fuel system
5)A throttle synchronization
6)Lubrication of the engine
7)A tank test


The initial step in an outboard engine tune up is the compression check. Before beginning this test, the ignition system should be disconnected to prevent the engine from accidentally starting. If the engine possesses an electronic ignition system, separate the connector plug between the magneto and power pack. If the engine has points, disconnect the plug wires from the spark plugs of each cylinder, and ground them.

To perform a compression test, the engine should be turned over by hand and a gauge used to record the amount of compression in each cylinder. If the engine you are testing has an electric start, instead use the recoil backup to turn the engine over as you checkthe compression. There should be a lively bounce as each piston passes its top dead center (TDC) position. Generally speaking, compression is probably too low if you cannot feel this bounce as its pistons pass TDC, or if you find that the recoil start is difficult to pull as a piston reaches and then passes TDC. It is best not to rely on an electric start when performing a compression check; the cranking speeds of an electric start system can vary depending on battery strength and overall starting system condition, and will sometimes return inaccurate compression test results.

To obtain the highest gauge reading for a cylinder during the compression test, the engine should be cranked through four or more compression strokes of a piston with the throttle wide open. If your compression gauge returns a difference of more than 10 psi between any two engine cylinders, it indicates a problem. If your compression reading in any cylinder is between 10 and 15 psi less than the optimum compression number provided in the engine service manual, then a problem exists.

If you observe either of these scenarios during the compression test, you will need to remove the cylinder head or heads, and the engine exhaust cover, to search for the cause of the problem. If the compression in one or more cylinders is low, you should check the condition of the cylinder walls, and the piston and its rings. If the compression is high in one or more cylinders, the condition may be caused by a carbon buildup on either the cylinder head or piston head. Inspect the engine exhaust ports, the cylinder head, and the pistons and their rings. If you discover an excess amount of carbon, the powerhead will need to be disassembled and cleaned.


Some outboard manufacturers recommend that the engine cylinder head and exhaust cover be removed for a tune up. Removal of the head will allow a close inspection of the cylinder head, the cylinder walls, and the pistons and their rings, while removal of the exhaust port permits you to look for excessive wear, and carbon accumulation. After you have removed the cylinder head and exhaust cover, try rotating the flywheel slowly in a clockwise direction, and visually inspect the pistons and rings, and the cylinder walls. The flywheel should turn smoothly and evenly. Be aware that you should never rotate a flywheel in the counterclockwise direction, as this can damage the blades of the water pump impeller.

Next, check the pistons and rings, and the cylinder walls for signs of excessive wear or damage. Check to see whether the rings are sticking as a result of carbon accumulation. If one of these conditions is found, you will need to remove, disassemble, and repair the powerhead. If the pistons, rings, and cylinder walls otherwise appear to be in good condition, carefully scrape away any carbon deposits from the engine exhaust ports, and clean any petroleum gum and varnish deposits you have found on the pistons and rings with cleaning solvent. Carefully clean the carbon deposits from the top of each piston. If the deflector on top of a piston is scored or marred, it will disrupt the flow of fuel air mixture into, and exhaust gas emissions out of, the combustion chamber area.

When the parts are clean, reinstall the exhaust cover. Always use a fresh gasket, and before the new gasket is applied, be sure to thoroughly clean any old gasket material or cement from the gasket surfaces. Unless otherwise specified by the gasket manufacturer, lightly coat both sides of the gasket with gasket sealing compound.

Before the cylinder head is reinstalled, it should be resurfaced to remove any high spots from its gasket face. Then the head can be reinstalled using a fresh gasket. Lightly coat both sides of the gasket with gasket sealing compound unless the manufacturer specifies otherwise. Remember that cylinder head bolts should be tightened in their recommended sequence, and to the torque values specified in the engine service manual.


To obtain reliable service and prolong the life of an outboard, you must be regularly lubricate it to service manual specifications. It is critical to use the appropriate lubricants. Most outboard manufacturers will recommend using their own lubricants on their products. Below is a chart that lists the lubrication points of a typical outboard motor, plus the type of lubricant that should generally be used at each:


1)Clamp screws, steering handle, tilt run lever
LUBRICANT TO USE: triple guard grease
FREQUENCY OF LUBRICATION: 60 days fresh water; 30 days salt water

2)Fuel shut off linkage, choke, throttle linkage
LUBRICANT TO USE: triple guard grease
FREQUENCY OF LUBRICATION: 60 days fresh water; 30 days salt water

3)Shift lever shaft, upper and lower swivel brackets
LUBRICANT TO USE: triple guard grease
FREQUENCY OF LUBRICATION: 60 days fresh water; 30 days salt water

4)Throttle cam, carburetor linkage
LUBRICANT TO USE: triple guard grease
FREQUENCY OF LUBRICATION: 60 days fresh water; 30 days salt water

5)Rear motor cover latch
LUBRICANT TO USE: triple guard grease
FREQUENCY OF LUBRICATION: 60 days fresh water; 30 days salt water

6)Gearcase (capacity of 14.7 oz. or 435 ml)
LUBRICANT TO USE: HI-VIS gearcase lube
FREQUENCY OF LUBRICATION: change after first 20 hours of operation; check again after 50 hours of operation, and add lubricant if necessary. Drain and refill after every 100 hours of operation or once each season, whichever occurs first

Note that engines used in salt water require more frequent lubrication than engines used only in fresh water. Salt water is far more corrosive, and it breaks down oil based lubricants more rapidly. When you are lubricating a gearcase, the ignition system should be disconnected to prevent accidental starting. If you suspect leakage of lubricant from a gearcase, the gearcase should be pressure tested.

To pressure test a gearcase, remove the propeller, and drain the gearcase of lubricant. Install a gearcase pressure tester in the gearcase drain hole. Pump air into the gearcase until the tester gauge reads between 3 psi and 6 psi, then immerse the gearcase in water. If you observe bubbles escaping from the gearcase, it means that the gearcase seals are faulty and must be replaced. After the seals have been replaced, be sure to pressure test the gearcase again. If no bubbles seen at between 3 and 6 psi, increase gearcase pressure to between 16 and 18 psi. If still no bubbles appear, then the gearcase seals are good. Remove the pressure tester, reinstall the drain plug, and slowly refill the gearcase with lubricant; refilling slowly better encourages trapped air to escape.


When you test run an outboard, it is important to remember that it should never be operated out of water. Without water resistance on the propeller, an outboard will overrev to a point where it likely sustains significant and irreversible damage. Since many outboards employ water cooling systems that take in water from their surroundings as they are being operated, they will quickly overheat if ever run outside of the water. Again this can result in severe damage to the powerhead.

To test run an outboard in a shop setting, a testing tank should be used. A testing tank is a large tank filled with water. If you do not own a tank, sometimes an oil drum filled with water can be used to test small outboard motors. As mentioned above, an outboard should never be run out water, even momentarily. If a test tank is not available, and an oil drum is too small or cannot be found, then the engine must be installed on a boat for its final test run.

The tank test is usually the final stage of an outboard engine tune up. During the tank test, the engine is mounted on the tank, and allowed to run in the tank water. The purpose of a tank test is to:

1)Check and adjust timing, if necessary
2)Adjust the carburetor jets
3)Check cooling system efficiency
4)Evaluate overall engine performance

Before beginning a tank test, all nuts, bolts, and screws on the engine should be tightened to specified torque requirements. Below is a sample torque chart from an outboard engine service manual:



1)Flywheel nut 30 to 40 ft. lbs. 40 to 50 ft. lbs.
2)Connecting rod screws 60 to 66 in. lbs. 60 to 70 in. lbs.
3)Cylinder head screws 60 to 80 in. lbs. 12 to 14 ft. lbs.
4)Crankcase to cylinder head screws 60 to 80 in. lbs. 60 to 80 in. lbs.
5)Spark plugs 17.5 to 20.5 ft. lbs. 17.5 to 20.5 ft. lbs.
6)Pull at propeller shaft to tilt up lower unit 31 to 34 ft. lbs. 31 to 34 ft. lbs.
7)Power pack mounting screws 4 to 5 ft. lbs. 4 to 5 ft. lbs.
8)Ignition coil mounting screws 5 to 7 ft. lbs. 5 to 7 ft. lbs.



1)No. 6 7 TO 10 – 0.8 TO 1.2
2)No. 8 15 TO 22 – 1.7 TO 2.5
3)No. 10 24 TO 36 2 TO 3 2.7 TO 4.0
4)No. 12 36 TO 48 3 TO 4 4.0 TO 5.4
5)1/4 inches 60 TO 84 5 TO 7 6.9 TO 9.5
6)5/16 inches 120 TO 144 10 TO 12 13.55 TO 16.27
7)3/8 inches 216 TO 240 18 TO 20 24.2 TO 27.1

Remember that failure to comply with recommended torque values can result in operator injury or motor damage or both.
When you are tightening two or more screws on the same part, it is better not to tighten any of them completely, one at a time. To avoid distortion of the part, you should tighten each of the screws to one third of its specified torque, then to two thirds of its specified torque, before finally torquing it down completely.

When a motor has been run long enough to reach operating temperature, allow it to cool and then recheck the torque on all cylinder head screws and spark plugs.

The propeller must be removed from an engine, and replaced with a test wheel for safety during a test. An outboard should never be run, even in water, without a propeller or a test wheel attached. The engine service manual will indicate the proper width and diameter of the test wheel that should be used for testing purposes. The best results will be obtained from a tank test, particularly with respect to carburetor adjustments, if a tachometer is used in conjunction with the test wheel. A tachometer permits more accurate rpm readings.


The effectiveness of an outboard cooling system can be checked by testing the temperature of the powerhead. To test powerhead temperature, you will need to obtain special testing sticks which resemble large crayons; they are comprised of temperature sensitive wax like compounds, each formulated to melt at a specific temperature. The normal temperature range for most outboards is between 125 and 165 degrees F. Therefore two different sticks should be used to test powerhead temperatures, one that melts at approximately 125 degrees F, the other that melts at approximately 165 degrees F. Try to test powerhead temperatures while the engine is attached to the boat; this will more accurately reflect the conditions under which the engine will be operated. If a boat is unavailable, a temperature test can also be performed just fine in a test tank.

The testing procedure is simple. You run the engine for five minutes at half throttle; then with the engine running at between 900 and 1,000 rpm, make a mark on the cylinder head with both testing sticks. In its unmelted state, the wax marks will appear dull and chalky. Sometimes the testing sticks will not be able to mark a painted surface. In such an instance, you will need to hold the sticks against the powerhead surface during the test.

Continue to run engine, and observe both wax marks. When the powerhead temperature reaches 125 degrees F, the mark made by that stick will begin to melt, appearing glossy. If the powerhead reaches 165 degrees F during operation, the mark made by that stick will also turn glossy, indicating that it has begun to melt. If the 125 degree mark melts, and the 165 degree mark remains intact, you know that the engine was running within its specified temperature range; from this you can assume that its cooling system is functioning properly.

If the 165 degree F mark does in fact melt during the test, the engine is running hotter than it should. A number of problems could account for this; a clogged water intake for example, or a faulty thermostat or water pump, or there could be blockage in the powerhead cooling passages. If neither wax mark melts within a reasonable time, then the engine is running cold, normally the result of a faulty thermostat.

To check the powerhead temperature of a smaller outboard engine that does not contain a thermostat, you will need to use only a 165 degree F testing stick. You would hold the stick against the side of the cylinder block, but not against the cylinder head. A temperature reading should not be taken from the cylinder head of a small engine because their heads generally operate at hotter temperatures than the maximum engine operating temperature found in a service manual. You would evaluating the results of this test as described above.