Compression checks can be used to let you know about how well the outboard engine is working. When it comes to How to test outboard motor compression it is important to know about the four phases that are distinct in a 4-stroke cycle. This will include the exhaust, power, compression and induction. Much can be learned when checking the ability of the engine to compress the mixture of gas and air.
When considering outboard gas engines, they require compression, an unobstructed exhaust, spark, fuel and air in the correct amount at the correct times. When the piston goes up and then down inside the cylinder, the rings in the piston will seal against a cylinder wall in order for the fuel and air mixture to compress at the top part of the cylinder. If the rings and the piston are unable to compress this mixture in order for the spark to ignite, the engine will either run poorly or not at all. When the rings are unable to seal properly, they are unable to transfer the heat generated from pistons onto the walls of the cylinder. When this transfer does not occur, the piston will increase in size due to the heat and then become stuck in the cylinder which results in damage.
Learning How to test outboard motor compression will provide an indication on whether the piston-rings are in an operational order and whether the cylinder is in good working order. These tests are used to measure the pressure that is created associated with the motions of the piston in the cylinder, provided in PSI (pounds per square inch). Typically a compression test is fairly straightforward, but various outboard types can result in various important complexities. The steps to follow are basic guidance for this procedure. Factors that can affect this test will relate to whether the engine is a 2-stroke or a 4-stroke, has a computer onboard, has a fuel-injection, has a starter-motor or is hand cranked as well as how the ignition is disabled.
Professionals will generally tell you that the better compression gauges come from companies such Mac or Snap-On are best. However, reasonably priced compression gauges can be purchased from nearly any type of auto-parts store and will be okay to use occasionally. It is advisable to obtain the correct adapters in order to fit your spark-plug holes.
There are a few considerations to keep in mind when checking the compression on a cold outboard engine, and most advise that the test should be performed when the engine is warm. This is due to the fact that the reading will be more accurate as well as relevant to how the engine is running.
Below is a 6 step process on how to test outboard motor compression:
Step 1
Be careful of the spinning propeller if you happen to shift or start the engine while you are setting up. The engine should not fire and you can achieve this by pulling on the switch that kills the engine. If you need to take out the battery you will need to reconnect in order to crank up an electric-start. Take out all your spark plugs and number each lead so you will know where to replace them.
Step 2
Various experts will recommend that you should move your throttle well-forward in order to allow air to enter into the cylinder before cranking the engine. When it comes to the electric start engines, it may be impossible to perform this task unless your engine features a bypass-button, typically located on the throttle-hub. If you are unsure use the owners manual to assist you further. It is also important to realize that there are engines that are able to take in enough air without having to perform this task and will still be able to provide a suitable compression test.
Step 3
Now screw the compression gauge in the 1st cylinder. It is advisable at this point to make sure that all your plug leads are far away from any moving-parts or the flywheel to avoid an unwanted accident.
Step 4
Now crank your engine using the pull cord or the starter through a minimum of 5 flywheel revolutions. The other method is to wait until the needle on the compression gauge has stopped moving forward. The outboard engine does not have the ability to start up, but a bit of fuel will be inside the cylinder, so remember that sparks or smoking from the grounded-connected spark plugs have the ability to ignite gases. When dealing with fuel-injection engines, these injectors spray gas vapor that is extremely dangerous into your cylinders. This can be avoided if the system offers you a way to avoid this.
Step 5
Take note of the reading for each cylinder.
Step 6
Now repeat steps 1 to 5 for every cylinder and record each reading.
Interpreting The Results
The next step involved on how to test outboard motor compression will be to determine your readings. Typically a reading that comes up below 90 PSI on the modern 2-stroke outboard engines indicates an issue. Generally, most engines should produce readings from cranking compression’s between 90 and 105 PSI, dependent on the model, make and year of the engine. The 4-stroke outboard engines should produce higher compression readings that are typically between 110 and 140 PSI. If in doubt it is always advisable to contact a professional technician for the job.