5 Yamaha 40 Outboard Problems You Should Know (Explained)

Yamaha makes some of the most popular boat engines globally, and their 40 horsepower outboard is very common.

While they are widely known for their quality, there are still a few problems that may arise from time to time that you should be aware of if you own one:


Is Your Engine 2-Stroke or 4-Stroke?

While most of the problems you may encounter occur in either type of engine, it still matters which kind you have when getting parts.

The basic functions that happen in an engine occur in a single “cycle.” In a 2-stroke motor, this cycle happens in 2 strokes of the piston; this is 1 revolution of the crankshaft. In a 4-stroke, the cycle happens in 4 strokes of the piston or 2 revolutions of the crankshaft.

The basic difference here is that 2-strokes are designed to run at higher RPMs faster, but 4-stroke engines are more durable in the long run.

Yamaha makes 40 horsepower motors in both 2-stroke and 4-stroke varieties.

In 2-stroke engines, the oil must be mixed with the gasoline; oil changes are not required as the oil is continuously injected. In 4-strokes, there is an oil filter and cavity, and it must be changed every 100 hours or so.

Read your manual to determine which style of engine your Yamaha 40 outboard is, though if you can identify an oil filter, you obviously have a 4-stroke.

If you bought your new engine in 2010 and live in the USA, you almost certainly have a 4-stroke. Almost all 2-strokes have been illegal to sell in the USA since then, as they do not meet the new emission requirements implemented that year.

Note, however, that 2-stroke engines themselves are not really illegal. It is simply that most of them do not meet the legal emission requirements.

It is best to consult your manual to be sure of which engine you have.

1. Failure of the Oil to Lubricate the Cylinders

It is critical to mix oil with fuel in a 2-stroke engine.

Some 2-stroke Yamaha 40 outboards, particularly older models from the 1980s and 1990s, had a mechanism that injected oil into the fuel rather than requiring it to be pre-mixed. These would occasionally fail.

Regardless of the reason for the failure, this causes the pistons to run dry, and they will expand and seize up in the cylinder. This leads to scoring in the cylinders and the pistons getting stuck.

This is also possible in 4-strokes when the oil filter is clogged, or the oil injection system has gone bad (like a cracked or clogged hose).

This is an expensive repair, as the cylinders will need to be rebored and the piston and rings replaced.

2. Fuel System Failure

Modern fuel systems on outboard engines are complex. In the old days, fuel was delivered to the engine in a gravity-feed system, but that is no longer the case.

The modern fuel-injected Yamaha 40 outboard has 2 fuel filters, 2 fuel pumps, a vapor separator tank (VST), a VST filter, and a high-pressure fuel rail. Any of these can be causing a problem if your fuel is not reaching your tank.

Because of this complexity, it is best to let your dealer handle any diagnosis and repairs.

The failure in a fuel system may also be outside of the engine, in the tank hose, connectors, or even the priming bulb. This is easy to identify if the failure is here. Visual inspection or working with the priming bulb will reveal any failure here.

Replacing the fuel pump or other internal components can be expensive, but the other mentioned components are relatively cheap and easy to replace.

3. Troubles with the Carburetor

Outboard motors, like all engines, are either fuel-injected or use a carburetor. Yamaha 40 outboards have been built both ways over the years, though most are now fuel-injected.

The carburetor mixes the right amount of fuel with air in an engine, which gets sucked into the engine. This combustible mix is what allows your engine to run.

Carburetors in lower horse-powered engines like the 40 specifically need to be run often during the summer. This is because microbial growth in the fuel sitting in the carburetor can gum up the workings, causing the valves to fail and let in an improper amount of fuel or air.

Dirt also causes this problem.

This can cause problems ranging from simple rough idling, misfiring at higher speeds to the engine cutting out constantly.

Several models of the 4-stroke 40 using carburetors from before 2010 had particular difficulty gumming up, and the design made it difficult to clean them. Replacing them was the only option, and the difficulty in this design-led to most of the 4-stroke 40s going to fuel injection.

4. Failure of the CDI

The capacitor discharge ignition (CDI) can fail. This ignition system is common to not just outboards but also lawnmowers, chainsaws, and other small engines.

When it fails, this can lead to misfires and rough running, and even stalls.

The CDI is the most recent Yamaha 40 engine that can store information to be connected to a dealer’s computer and diagnosed.

In some models and older 40s, this is more difficult to diagnose, and replacing the part is the only way to be certain.

This part is relatively easy to get; there are some OEM models available on Amazon.com. An experienced powerboater may be confident enough to replace it independently, but getting a mechanic to do it may be wiser.

5. Issues With the Starter

Several things can cause problems with the electric starter.

The most obvious is damage to the wiring, which you may be able to tell by visual inspection.

Two other common problems with the starter are a blown a fuse or problems with the battery. Remove the electrical cover and remove the fuse holder with pliers. If the connection at the fuse center is broken, then you have a blown a fuse.

If the fuse is OK, check the battery by carefully disconnecting it and cleaning the terminals. Please charge the battery and reconnect it.

If these fail, the problem in the electric starter is more complex and will require your dealer to inspect it.

General Pros and Cons of the Yamaha 40 Outboard

Yamahas are known to be a reliable brand, with over 10 million motors sold worldwide.

So while we have been examining the problems with the 40 horsepower outboard, there are many good things in their favor.

You do not have to search very far online to find how much people like them and why:


One of the biggest pros of the Yamaha 40s (or any Yamaha) is the reliability of the motors.

The company’s reputation for quality has been long established since it began making outboards in 1960. They are among the most low-maintenance brands available.

Yamaha 40s are also widespread, with many dealers worldwide that supply them and many mechanics beyond them that are authorized to do repairs. Almost anywhere you find yourself to be, there is an excellent chance you will have access to service, should any problems arise.

The Yamaha 40 is a little more compact than most other outboards in the same power range, particularly in its chief rival, the Honda 40.

This makes it a little bit easier to deal with maintenance.

Yamaha outboards are also quieter than most other brands, again in particular comparison to Honda. Outboards are loud, period, but Yamaha’s 40 is will be easier to hold a conversation over.

These are typical reasons owners like their Yamaha 40s. The most important reason, though, and the biggest reason for any outboard owner, is their reliability.


  • Failure of the oil to lubricate the cylinders
  • Fuel system failure
  • Troubles with the carburetor
  • Failure of the CDI
  • Issues with the starter

Final Thoughts

Yamaha 40 horsepower outboards are among the most reliable on the market. While this article focused on their potential problems, that is not an indictment on the brand.

In truth, these potential problems can strike almost any motor and are not truly endemic to the brand or the model.

Specific problems with the Yamaha 40, like the carburetor arrangement in certain models before the mid-2000s, have been corrected, and the engines improved.


UTI: 2-stroke or 4-stroke

Carburetor Trouble

2-Stroke Engines

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