Testing outboard motors requires a little patience and preparation. Outboards that have sat for winter, or older outboard engines, have a tendency to be very temperamental when started and might have performance problems during a trial test run. A DIY repair person should use a process of elimination and observation when diagnosing a motor that acts contrary to normal operation. Outboard motors can almost tell you what is wrong with them if you listen and observe them carefully. Some of the most profound maladies have the simplest solutions.
Instructions
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Trailer the boat to a suitable work location, if possible. You can leave the boat moored to a dock for test purposes, but be careful of losing tools overboard. If you trailer the boat home, park it next to a water source and hook up a muff-flush device to the lower unit and connect it to a garden hose. Run the hose at medium pressure. Use a 55-gallon drum to submerge the lower unit in water, if a muff-flush is not available. Release the cowl snaps on the top engine case.
2Use a multimeter to check the battery voltage, if so equipped with an auxiliary battery. Place the red and black leads of the multimeter on the red and black leads of the battery and adjust the meter for volts. You should have a standing voltage of 12.5 volts or more. Charge the battery if it reads lower.
3Examine the plug wire -- or wires -- for cracks or disconnection, then remove the plug wire. Use a socket and wrench to remove the spark plug. Clean the plug electrode with a wire brush and check the gap with a feeler gauge. Refer to your outboard motor's service manual for the correct gap. Insert the proper feeler gauge gap blade between the electrode contacts and open or close them, so the blade fits snug as you move it back and forth.
4Screw a compression gauge into the spark plug threads and depress the gauge's Shrader valve. Pull the rope starter one full, firm yank until the engine turns over several times. If you have an electric starter, activate it and let the engine crank over at least six times. Take a reading on the gauge and compare the pounds per square inch (PSI) against the number that appears in your service manual. Generally, a reading below 30 psi of specifications denotes a compression problem. If above this limit, it means the compression will be adequate for starting and running.
5Unscrew the compression gauge. Replace the plug and tighten it with the socket and wrench. Reconnect the plug wire. Check your lower unit gear oil; find the top vent plug on the upper side of the motor lower unit. It will have an Allen fitting or a flathead screwdriver slot. Use the appropriate tool to remove it. If gear oil escapes the vent hole, immediately replace the plug and tighten it with the appropriate tool. If no oil exits the hole, bend your pinky finger inside and see if oil shows. If it shows, it has enough gear oil -- re-tighten the plug with the Allen wrench or screwdriver.
6Examine the propeller for nicks, cuts or bends. If you see any obvious deformation, you will have to replace the propeller or have it repaired. Pull side to side and back and forth on the propeller. There should be no slack or give, side to side or forward and backward. If excess slack appears, it means the thrust bearing, shaft key or propeller shaft is defective. Do not run the engine upon finding this problem -- replace the faulty parts. If the boat is in the water, pull the engine up into the upright lock position and lean over to manipulate the propeller.
7Turn the engine back and forth on its pivot mount; make sure it extends to its full arc to the left and right and that the steering cables do not bind, but travel smoothly and easily. Use a screwdriver to loosen and remove the air cleaner housing. Check the air cleaner element for cleanliness, by holding it up against the sunlight; you should see light appearing through the pores of the element. Replace it if it appears oil-soaked or clogged with debris and dust. Replace the element and housing, and tighten the mounting screws with a screwdriver.
8Check the gas tank level; you need enough fuel for a test run, at least a few gallons. Pull the cap and smell the gas; it should smell fresh and not have a stale varnish or bitter odor. Replace old gas. Give the fuel primer bulb a few firm squeezes and look for any leaks in the gas tank line that routes to the carburetor. Tighten fuel hose connections with a screwdriver. Pull the choke out for a cold start.
9Close the top engine case and secure the snaps. Start the engine. While the engine warms up to normal operating temperature, put your hand down over the exhaust outlet on the lower unit and feel for a warm discharge of exhaust water, with good pressure. Also wipe away any debris or silt from the fresh water intake vents on the lower unit. A good warm discharge tells you that the water pump impeller works and that no obstructions in the tubing and passages exists. Push the choke in when the engine warms up.
10Listen to the idle. It should be consistent and smooth, with no hesitation or stumbling. Cast off and run the boat out into open water at medium speed. Listen to the engine response; again there should be no hesitation or stumbling. Raise the engine to full throttle and listen for an engine miss, popping or fuel starvation. Slow the boat and place it in "Neutral." Select "Reverse" and slowly back up, then place it in "Neutral." Observe how the shifting cable responds; the transmission should shift smoothly, with no distinct clangs or thumps.
11Place the transmission in forward, from an idle. Give it full throttle. Listen for backfiring, or hesitation. It should pick up speed, with the engine winding out with a strong howl. Run it at full throttle for several minutes and watch your temperature and oil gauge, if so equipped. The temperature should remain with manufacture's specifications, as well as the oil pressure. Check the steering response, with both hard right and left turning maneuvers. Look for any slack or "slapping" in the steering cables and connections. Perform this same test-run if you have a muff-flush or barrel.
