The boat must be kept in a good condition for the sailing enthusiast to take a smooth and trouble-free boat ride. One of the most important part of the vessel is the outboard motor, of course. We all know that while on the run, the outboard motor can go wrong in many different ways. In this case, learning all about the common problems as well as the procedure of testing compression in outboard motors is essential, to save on the time and money needed to tow the boat to a dock in case of a mishap. Read on to know about outboard motors compression testing.
In general, there are two simple tests used to separate good running outboard motors from the worn out ones in engine compression. The results from these tests are a good indicator of the condition as well as the deciding factor when it comes to purchasing an outboard. This might save you a lot in the long run, in terms of money and repair time. Be sure to check the specific features of each model, since they come in varying designs and will give out different ideal compression reading.
Many small horsepower 2-cycle outboards (of up to 25 hp) that were built before the 1960s came with a compression ratio of about 6:1 and 88 psi. These numbers are good for most of the low horsepower models, though there are variations and exceptions to the general rule. In our case, the compression ratio means that the outboard compresses the fuel/air mixture at 6 times atmospheric pressure. Because most of us donít live at the sea level, the precise theoretical cylinder pressure is ideally less than that, since atmospheric pressure rises at higher elevations.
Low Compression
In most cases, good working outboards will have a compression ratio between 85 to 75 psi. Readings below 70 psi should be considered low compression. Along the obvious thing such as blown head gaskets, etc. one of the most common cause of low compressions in outboards is a misfit or leak due to the wear and tear of the cylinder walls or the piston rings.
To maintain a good compression, the piston rings should be free to expand along the cylinder walls. Bindings or any other form of tightness of the ring against the wall restricts the expansion activity of the ring, causing a substantial loss of compression and outboard power as well. Escape or leakage of compression through the piston rings is called ìblow-byî.
The piston rings can seize up in the piston grooves due to insufficient ring-gap clearance or piston groove, or due to carbon accumulation. This causes hard starting performance, loss of power, and unsatisfactory slow speed.
Testing Compression In Outboard Motors
1. Bounce/Leakdown Method
This test can be performed without a compression gauge. First, turn the outboard motor over by hand, using the recoil start ideally at the normal starting pull speed. Measure or gauge the amount of bounce you feel on your hand as each of those pistons pass the top dead center (TDC) stroke position.
If the engine is healthy, the bounce is easily detectable. This means that at a high compression, you should feel some resistance and a tendency of the starter handle to recoil back as it reaches the TDC. If the engine bounce is weak, it might be a sign of a low compression as the piston pass the TDC position, as it just oozes out.
Keep in mind that this test is more subjective, and not very repeatable. However, it can come in handy especially at the time of purchasing an outboard. Be sure to perform this test only when the outboard is at cold temperatures and free of oil and other additives.
2. Pressure Gauge Method
This is perhaps a better reliable and repeatable method of testing compression in outboard motors. It involves using a pressure gauge, which can be obtained at a relatively low price from your local tool warehouses and hardware supply stores. Compression gauges often come in testing kits, with several fittings to adapt the pressure gage to an individual outboard spark plug hole.
To get accurate readings while you test each of the cylinders, the outboard should be cold and dry. This means that the outboard should not have been used previously, and no oil or any other additives have been added to the cylinders before the test is done.
Procedure:
– Make sure fuel is not present in the cylinder by turning off the fuel valve
– Disconnect the spark plug wire and the spark plug one at a time, making sure you donít mix up the wires. Ensure you ground the spark plug wire as you test the compression. You can easily ruin the coil by cranking the motor with the free hanging plug wire
– Screw the tester in with its fitting adapter into the spark plug hole
– With a pull start of about 4 to 8 times the normal starting speed, crank the motor over. Pay careful attention to notice (on the gauge) a stabilized peak pressure. This value is the compression of the cylinder
– Now release the pressure on the gauge
– Repeat this with other cylinders. Each should read within 3 to 5 pounds of each cylinder.
As a piece of advice, to avoid problems, be sure to check the fuel level as well as fuel connections t avoid stagnations. Ensure that all the fuel line are fully operational, and are not curly, damaged, drowned, or broken.