When it comes to checking compression on outboard, this particular check is able to relate a lot about the health of your engine. Below are the reasons why as well as how this test is performed.
There are 4 distinct phases involved in the 4-stroke cycle, this includes:
•Induction
•Compression
•Power
•Exhaust
Much can be learned when you check on the abilities of the engine to compress the air or gas mixture.
When checking compression on outboard gas engines it requires compression, an unobstructed exhaust, compression, spark, fuel and air. This needs to happen in the correct amounts and at the correct time. When the piston travels in an upward and downward direction inside the cylinder, piston rings will seal against the walls of the cylinder so that the fuel or the air mixture are able to be compressed. This occurs in the area near the top part of a cylinder as the upstroke occurs.
If the rings and the piston are unable to compress this fuel-air mixture inside the cylinder in order for the spark to ignite, the engine is unable to run well. Additionally, when the rings are unable to seal in the correct manner against the walls of the cylinder, they are unable to perform the role of the transference of heat from pistons onto the walls of cylinder. Without this transfer, the pistons begin to increase in size caused from this heat and then begin to “stick” in the cylinder that can lead to extensive damages.
Checking compression on outboard is able to indicate whether the piston-rings are operating correctly as well as whether the cylinder is in a good condition. This test will measure the amount of pressure that is accumulated by motions of a piston in the cylinder. This is measured in PSI (pounds per square inch).
Basically, compression tests are extremely simple, but various outboard types can add various important complexities. Below is some information of the general steps to take. However, you need to keep in mind that the engine you own can necessitate different types of steps. This can depend on aspects such whether the engine is a 2 or 4 stroke or has a computer onboard or fuel injection, it features a starter-motor or is hand cranked. Other factors may also include how you disable your ignition. These types of variations along with any other problems may be deciding factor on whether you are able to conduct this task on your own. Below are the steps to follow:
Step One
•Remove the spark plugs from the engine, check them for any damages. Electrodes must be gapped correctly and the color must be a brown or medium tan after high-speed runs and black or dark brown after they idle at a slow speed. If the spark plugs are a light tan or have turned white, the engine may be running without enough fuel and must be checked well before any damages occur.
Step Two
•You need to make sure you take the necessary steps to stop the ignition from starting up or firing. This may involve pulling on the engine-kill switch or disconnecting your battery. However, you will have to reconnect the battery in order to crank up an electric-start.
Step Three
Many professionals suggest moving your throttle into the forward position to assist air in entering into the cylinder while cranking your engine. However, the electric-start engines rules out this option unless the engine features a bypass-button. If you are unsure use the owner’s manual supplied with the engine.
Step Four
Screw the compression gauge inside the 1st cylinder and make sure your plug-leads are clear from any moving parts or the flywheel.
Step Five
Crank your engine with a pull cord or using the starter through a minimum of 4 to 5 flywheel revolutions. Alternatively until the needle on the compression gauge has stopped advancing. Keep in mind that a bit of fuel will land up inside the cylinder which means sparks or smoking from the grounded and connected spark plugs can ignite gases. In the case of fuel-injection engines, your injectors can spray gas vapors that are dangerous into your cylinders unless the system offers a method to safely prohibit this.
Step Six
Take a reading for this cylinder.
Step Seven
Repeat this process on each cylinder recording the results.
Interpreting The Results
When checking compression on outboard it is a suggested time to buy new spark-plugs and gaps. Make sure you have purchased the correct types and then gap these according to your engine’s specifications. Torque the plugs using a torque wrench, this will ensure they are tight without having to strip the threads.
Not all engine manufacturers will provide cranking-compression standards on their engines. This is due to the various variables. What will appear in the majority of service-manuals will be that readings need to be in 10 PSI from one another. Typically, the readings that are under 90 PSI on modern 2 stroke outboard engines are an indication of an issue.
The majority produce a cranking compression reading of 90 to 105 PSI range. When it comes to the older models along with high-performance engines, readings should typically be from 115 to 140 PSI. This will also be dependent on the engine’s model, year and make as well as the manufacturer. The 4 stroke engines produce far higher compressions.