When you are in the market to purchase an outboard, a simple test of engine compression can be the difference between making a good investment, or paying for something that really isn’t up to scratch. In addition, the test results can save you, the potential owner, a whole lot of money, time and effort if a costly repair job can be avoided. So let’s take a look at what causes low compression and the types of outboard motor compression tests that are easy to do.
What Are They Typical Causes Of Low Compression?
The main reasons for low compression are the obvious things, such as a head gasket that has blown. However, other common possibilities also exist. For example, too much wear and tear on the piston rings or cylinder walls can cause leaking, hence contributing to low compression.
In addition, an accumulation of carbon in the piston grooves, or an inadequate piston groove can cause the piston rings themselves to seize.
Low compression is evident in a noted loss of power, a difficulty starting and a performance speed that is slower than normal.
So What Is Normal Compression For An Outboard?
Knowledge in this area is crucial if you are to truly understand a low compression problem. Each model has its own specifics, design and variations, meaning it’s not a case of one size fitting all. That being said, we can provide you with a rough guide.
For small horsepower outboards that were produced before the 1960s (not in excess of 25 hp), the compression rate was in and around the region of 6 to 1. Hence for many outboards in this category a comp ratio of 6:1 and 88 psi are normally deemed good values.
In general, you are hoping to see a psi of between 75 and 85. If the reading comes in below 70 then you may be looking at low compression.
What Are The Common Outboard Motor Compression Tests?
We’re going to look at two common outboard motor compression tests, as well as a few tips to make sure the test runs smoothly.
The Pressure Gage Test
You can normally pick up a compression gauge from your local hardware store, they don’t tend to cost too much. The kit will have the compression gauge, as well a variety of fittings to use to connect it to the spark plug hole of the outboard.
It is important to note that the outboard should not have been in use before the test is carried out. For the testing of each cylinder to be accurate the outboard needs to be both dry and cold. In addition additives, oil and such like should not have been added to the cylinder before the test is carried out.
The Leakdown Test
Also known as the Bounce method, this is a test that can be carried out in the absence of a pressure gauge. To carry out the test you need to use your hands to turn the outboard over, as you use the recoil start you want to sense how much bounce you are getting when each of the pistons passes TDC (top dead center).
So what kind of bounce should you expect from a healthy engine with good compression? Certainly it should be obvious to feel. Watch out for the starter handle beginning to recoil back at the TCD point, this is a sign of good high compression.
If, on the other hand, passing TDC was a rather boring and weak affair, then an indication of low pressure certainly exists. Just as for the previously described pressure gauge test, the outboard needs to be cold and dry and without the addition of any oils or additives in the cylinder.
Of course, this test is rather subjective, however it can still serve its purpose at the time of purchase. Depending on the level of accuracy you require, as well as the size of the investment, you may be happy to utilize this test, or you may prefer to head down to your local hardware store and pick up an inexpensive pressure gauge.
What Are The Best Tips For Ensuring That Outboard Motor Compression Tests Run Smoothly?
The Cylinder
Ensure there is no fuel in the cylinder or turn off the valve.
The Spark Plugs
Work with the spark plugs one at a time to avoid any confusion. Before you begin any compression test make sure that you have grounded the spark wire.
If The Outboard Has Low Compression Is It Still Worth An Investment?
This one is really an individual choice. When you are going at a high speed, the low compression won’t really have any negative impact. However, low speeds are going to be the issue. At a low speed the performance will likely be poor. In addition you may find yourself with a continuous problem when it comes to starting.
Remember, low compression can be a sign of problems elsewhere on the outboard. There could be a leakage which will ultimately get worse as time goes on. There may also be damage or considerable wear to the engine which could translate into a hefty repair bill in the not too distant future.
Certainly carrying out outboard motor compression tests takes a little time and effort, it may also take a little money if you plan to purchase a pressure gage. However understanding the pressure readings of an outboard before you buy it can be the difference between making a great purchase, or buying something you later regret.