How to maintain
and repair a
boat's outboard motor.

    Outboard-motor mailbox in front of a house     Contact me.

This page updated: March 2008

Mastertech's "Outboard Troubleshooting Guide"
SailNet - Doreen Gounard's "Caring for the Cruising Outboard"
Article by Tom Zylder in 1/2000 issue of Ocean Navigator magazine.
Outboard maintenance article by Jan Mundy in issue 2000 #3 of DIY Boat Owner magazine.
"Outboard Care" article by Peter Caplen in Ocean Voyager 2001 from Ocean Navigator magazine

Decent book: "Keep Your Outboard Motor Running" by Richard Thiel.
Theory, operation, maintenance, troubleshooting of 2-strokes in a short, common-sense way.

If your outboard motor gets dunked in salt water,
summarized from "The Compleat Book of Yacht Care" by Michael Verney, and from Clymer "Powerboat Maintenance Manual":
If dunked while running:
  1. Wash off with fresh water ASAP.
  2. Drain all fuel.
  3. Remove spark plugs.
  4. If any sand/dirt ingested, don't crank it; get professionally serviced.
  5. Hand-crank while holding upside-down and at other angles, to drain water. If cranking is difficult, stop cranking; get professionally serviced.
  6. Flush crankcase and cylinder with alcohol to absorb water.
  7. Wash disassembled parts in hot soapy water, air dry, then coat with oil.
  8. Start motor.
  9. Run motor under load for at least 30 minutes, to heat it up and clear out water.
If dunked while not running:
  1. Wash off with fresh water ASAP.
  2. Start motor.
  3. Run motor under load for at least 30 minutes, to heat it up and clear out water.

See "Dinghy's outboard motor won't start" item in the Equipment Problems section of my Operating Lists page.

What an outboard motor needs to run:

Runs rough:

Condition of spark plugs:

More condition of spark plugs,
from article by Jan Mundy in issue 2000 #3 of DIY Boat Owner magazine:

More condition of spark plugs,
from article by Steve Auger in issue 2003 #2 of DIY Boat Owner magazine:

If you see black "oil" dripping out of the propeller hub:
that's not oil, it's carbon coming out of the exhaust. Use a de-carbonizer (AKA engine cleaner).

4-stroke maintenance tasks (in addition to 2-stroke tasks):

From The Marine Doctor:
When changing the outboard gear oil:

Inspect gear oil for metal particles (metal flakes). Presence of fine metal particles (powder) on the fill screw magnet is normal. The presence of chips requires the gear housing to be disassembled and components repaired or replaced.

Note the color of the gear oil. White or creamy indicates a presence of water; seals should be replaced. A yellowish color due to the agitation/aeration of gear oil is normal and should not be confused with the presence of water.

Testing cylinder compression,
from The Marine Doctor's Forum:
  1. Remove all spark plugs.
  2. Install compression gauge in spark plug hole.
  3. Ground ignition components.
  4. [Some say: open throttle all the way.]
  5. Crank engine over until the compression reading peaks on the gauge.
  6. Check and record compression of each cylinder. The highest and lowest of readings should not vary by more than 15%. A reading below or around 100 PSI (or whatever is specified for your motor) could indicate an engine wear problem.
  7. Cylinder scoring: If the powerhead shows any sign of overheating, such as a discoloration of the paint ... visually inspect the cylinders for scoring or other damage.

Compression tests are important because an engine with low or uneven compression cannot be tuned properly for peak performance.

From Richard Kollmann on Cruising World message board:
Re: 3.5 Nissan

These little 3.5 Nissan motors can provide good service if properly cared for. The three problem maintenance areas are:

Overheating, caused by not flushing the cooling system properly and storing unit horizontally. The water cooling jacket around the cylinder is unprotected aluminum and it will corrode, clogging the complete water chamber. The first indication that there is aluminum oxide building up in the cylinder head is low or no water from small water discharge line.

Contaminated fuel. There is only one fuel strainer for the engine and it is in the fuel tank and it will not keep dirt or water out of carburetor. Straining all fuel through a good filter like the Mister Filter and installing a small seethrough inline filter is a good idea.

Top Bearing Failure. The top crankshaft main needle bearing receives lubrication only from the 50 to 1 fuel mixture which is not adequate when engine is stored. The crankcase and cylinder must have additional oil added to them and the engine stored in a dry place.

From Roger L. on The Live-Aboard List:
I just did a lot of work on my 9.9 Yamaha 4-stroke in 2005, and in the process found out about their history of fuel pump woes and the most recent fix. If you have fuel starvation problems the new water-cooled fuel pump is the fix. In the past I've mistakenly thought I was dealing with dirty carb problems when the motor mysteriously quit, took a while to restart, or didn't do well at low RPM. I was aware of the repeated design changes in the fuel pump, but somehow I didn't get it. This time Yamaha got it right, and those problems turned out to be the fuel pump all along. Big difference in the way it runs.

From Bob Brookes on Cruiser Log Forums:

A wonderfully reliable engine with a few faults easily fixed.

Comes with a stainless shear pin guaranteed to chew the prop hub and sell lots of replacement props. I use a piece of 1/8" brazing rod. Of course they break easily when hitting rocks and fish nets but that's what a shear pin is for, to save your prop. Just carry a lot of them in your dinghy tool kit.

The fuel filter is located inside the fuel tank and removed for cleaning by loosening the clamp holding the fuel shut off valve. Problem: the clamp is stainless steel but screw is iron and impossible to remove without a hacksaw. Replace with a stainless screw even on new engine.

Also on that new engine pull the 4 big stainless screws on the sides of the engine cover and coat the threads with anti-seize compound. After a year or so they are impossible to remove from the aluminum block.

Ever try to remove the carburetor drain screw without removing engine from the transom? Then, after you have the engine up-side-down in your lap, try getting that screw back in. Solve this by cutting off the lower section of the plastic case, that covers the lower carburetor, and there's that screw easily removed with your tool kit pliers, and reinstalled with your fingers, without lifting from the transom. If you groove the threaded end of the screw with a small triangular file you need not fully remove the screw to drain.

Be aware that Yamaha, and others, are sold with the fuel mixture set rich for break-in. Performance and fuel economy are improved by moving the mixture 'C' ring up one notch.

From Steve on "Nonesuch": small Tohatsu 4-strokes (4, 5, 6 HP) have problem with being hard to start. Avoid the problem by leaving the motor tilted up whenever leaving it overnight. Must have something to do with gas in the carburetor.

Also see the Outboard Motor section of my Boat Engine page.

Calvin asking his Dad how a carburetor works