Outboard Maintenance Guide - Tamar Marine: Boating, Fishing & Marine Gear

Outboard Maintenance Guide

Whether you use your boat year round or have to pull it out down south for the cold months, you’ve got a few things to do to keep your outboard running and ready when you want it. Like anything you need to care for, doing smaller amounts of preventative maintenance on a schedule pays off against letting everything slide until you have a problem or a big job to catch up.

Most of these tasks you can do in your driveway with the boat on the trailer, though you should do some on the ramp or in the parking lot when you’re putting the boat away. A few extra minutes tucking her away can save you hours down the road.

Outboard service

Your outboard manufacturer will have a maintenance and service schedule laid out in the owner’s manual. If you don’t have the owner’s manual, do yourself a favor and find one on-line, in and electronic or paper copy. It will spell out frequency for oil changes for four strokes, lower unit maintenance, spark plug care, and many other tasks. Your outboard is the most complex piece of machinery on most small boats, and will require the most service.

Maintenance periods on smaller outboards without hour meters are tougher to track. Estimate and keep some kind of log if you can. Most scheduled maintenance items are “once every 100 hours or once per year.” Keep in mind that these manuals assume seasonal usage for markets like North America or Europe. If you use your boat year-round like many Australians, you may exceed 100 hours of engine run time in twelve months. It’s better to change the oil a little too often than it is to let it slide too long.

If you do your own maintenance, a service manual (or “shop manual”) for your engine is an invaluable tool. Unlike an owner’s manual, it includes detailed information about disassembling the engine and troubleshooting a range of engine troubles. The best shop manual is the manufacturer’s for your specific model, but this is often hard to track down for older engines. Look for third party books from Seloc or other sources which cover your engine if you can’t find an original.

Flushing and Rinsing

Most manufacturers recommend flushing your engine with fresh water every time you use it in salt, brackish, or murky water. While this isn’t always practical right after you pull up the boat out on a trailer, if your outboard doesn’t have a flush port, keep a pair of “earmuffs” for your engine to give it a quick flush with a garden hose when you get it home.

If your boat is in a slip, rinsing and flushing can be tricky, but take the time to lift the engine out and run it with a hose connected periodically. Larger engines may have a flush port to help this, but small outboards will need the earmuffs.

For boats on a mooring, flushing after use is an even bigger problem since you can’t do it every time you use the boat, and you’re going to use the engine as soon as you rinse it on shore to get back to the mooring. Periodically bring the boat into a working or temporary dock somewhere, hook up a hose and give it a flush.

Accumulated salt in the engine can eventually block the cooling water intakes and lead to overheating. And salt built up on metal is never good for it. Even periodic flushing is a benefit even if you can’t do it every time, as the manufacturer suggests.

Oil and Lubrication

Four-stroke engines need periodic oil changes. Check the manual for the type and grade of motor oil, and be sure to change the oil filter with the correct replacements. Some engines below 15 HP don’t use replaceable filters, and other small engines have an internal filter screen which needs periodic servicing, though it may not be easy to get to without some disassembly.

Before you change the oil, run the engine to warm it up. Run it in a large bin of water or with a hose and “earmuffs.” Just don’t run it with no water or you’ll also have to learn how to replace an impeller.

Changing the oil with the engine on the boat at the dock with the boat in the water isn’t practical. Many engines have drain plugs, and no matter how you get the oil out of the engine, the risk of an expensive oil spill is too great.

Finally, if you are storing or laying up your engine for a long time, change the oil beforehand instead of waiting for recommissioning. Contaminants and dirt in the used oil will have the whole layup period to settle and clump up, and any acids or corrosive contaminants have time to work on the engine.


Your outboard will have at least one anode on it below the waterline. Depending on its size, it may have more than one. Smaller engines almost always have one on the underside of the cavitation plate, and larger ones may have additional anodes on the sides of the lower unit above the plate, or even above the waterline. Familiarise yourself with all of them.

Since outboards shouldn’t stay in the water when not in use, anodes can last a long time. But check them periodically, and replace any showing signs of excess wear, softness or sponginess. Most manufacturers recommend annual replacement of exterior anodes.

Lower unit

The lower unit gear oil is often ignored, but it has a recommended service schedule which is the same as the engine oil – every 100 hours of use it should be drained and replaced. You will need oil as specified in the manual, a set of o-rings or washers for the plugs, a pump with the proper fitting for your engine, and a pan to catch the oil. It’s easiest to change the lower unit oil at the same time you change the crankcase oil instead of separating the schedule.

Most lower units have two plugs for the lower unit, a lower drain plug and a fill plug higher on the leg. If this task isn’t done regularly, the plug may be hard to move, but a large flat-bladed screwdriver will usually do the job.

Changing the lower unit before layup is convenient because it keeps you on the same maintenance schedule as the engine oil. But lower unit oil doesn’t get the same exhaust gases contaminating it, so the risk of leaving lower unit oil for several months is lower.

Fuel System

Your outboard will have a fuel tank, a pump, lines, filters, and a carburetor. The tiniest engines may not have a filter or an external tank, and use gravity to feed the fuel instead of a pump. Larger engines may have multiple carburetors, and many engines use fuel injection and eliminate carburetors completely. Trace your fuel system from the tank to the engine and make sure you know what the components are.

Fuel Lines

Inspect your fuel tank and fuel lines at least annually, and before and after any long lay-ups. Check the priming bulb to make sure it still pumps and holds pressure, and make there that any vents are clear and there are no leaks or signs of wear or excessive sun fading on the tanks.


Most fuel filters have a replaceable element which should be changed every 100 hours, or at least once a year or when you suspect clogging. Many small engines use a disposable in-line filter you can swap out. For external filters, check the bowl and drain any collected water periodically.


Clogged carburetors and carburetor problems could get their own article, but there are a few steps you can take to help keep them clean and running. Inside the carburetor, tiny jets spray fuel into the throat. They are calibrated for a precise spray, and if they get blocked or gummed, the engine may run poorly or not at all. Old fuel left inside a carburetor will go stale and may evaporate, leaving gummy deposits or varnish.

When laying up a carburetted engine, it’s critical to get all the fuel drained from the internal bowl. Many engines have a drain screw under the bowl. If there is none or it is hard to reach, you can start the engine and disconnect the fuel line, then let the engine run until it burns all the fuel in the bowl and stalls out.
If you know you won’t use your boat for a few weeks or longer, draining the petrol from the bowl can save you some headaches. Here, before you pull the boat out of the water, run the engine empty as described above.

Fuel Injectors

If your engine is fuel injected, you might leave servicing the fuel injectors to professional, certified outboard mechanics for your brand. Injectors are delicate and precisely calibrated, and getting even a tiny bit of dirt or contamination in them can damage the injector. It is quite difficult to remove an injector without contaminating the engine.

Inspect them for visible wear, but avoid taking them out or adjusting them unless you know what you’re doing and have a good shop manual and the right tools on hand.

Fuel stabilisation

Old fuel goes stale as different compounds in the fuel evaporate off and moisture and other contaminants can find their way into the tank. When laying up your boat, if you have too much petrol left to dispose of easily, use a fuel stabiliser to ensure easy starting and running when you re-commission and launch again. Follow the instructions on the bottle for the proper ratios.

When recommissioning, if you have a lot of old fuel left in the tank that wasn’t stabilised, consider disposing of it at an appropriate facility. You can mix it in with new petrol if you have a lot to use up, but be aware that trying to run your engine on fuel that has been sitting for months may not go smoothly.

Spark Plugs

You need three things to make a petrol engine run – fuel, air and spark. Periodically inspect your plugs and ignition wires; they may need occasional replacement. Even if there’s nothing wrong with the engine tune, producing millions of sparks over a couple of seasons will wear the plugs out.

Every 100 hours (or once a year), pull the plugs and check them for wear, corrosion, carbon buildup, proper gapping, or other problems. Replace the plugs if you see any problems. Also inspect the wires for corrosion under the cap, or any breaks or cracks in the insulation.

When replacing plugs, check the spark plug gap settings in the manual, and make sure the plugs are gapped correctly. Also, torque the plugs properly. You don’t want them too loose, and you don’t want to damage them with over tightening.

Preparing for Storage

To get your outboard ready for storage, you will need to:

  • Change the engine oil (on four-strokes).
  • Change the lower unit oil.
  • Flush the engine.
  • Drain the fuel from the carburetor.
  • Fog the engine with oil.
  • Wipe down and clean off the engine.
  • Stabilise any fuel left in tanks.

Working on Your Outboard

Small outboards – under 25 HP – are fairly portable and you can take them off the boat for service and repair. Larger engines aren’t removed for service. If your boat is trailerable, it’s often easiest to work on your trailer in your driveway or at the marina.

If you don’t have a trailer, you’ll want a sturdy sawhorse with a vertical crossbar to clamp the engine to, or an outboard engine trolley sized for your engine. Most tasks require your engine to sit upright, and you can’t work on or run an unstable engine propped up on something in your garage. It’s also unsafe if your engine can fall on you.

If you have a basic set of engine tools and the toolkit that came with your engine, you should be covered for most of what you need. For basic tasks and maintenance, you may need:

  • Screwdrivers
  • Pliers
  • Socket wrenches. 1/4″ drive wrenches are best, because they don’t apply as much torque to delicate small engine parts.
  • Torque wrench
  • Spark plug socket
  • Oil pan or tray
  • Lower unit fill kit
  • Spark plug gapper
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