What should compression be on an outboard motor

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. 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. 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. The engine could be worn out, i. If this is the case, it's more likely that all cylinder readings will be low, not just one or two.

There could be internal damage, perhaps a broken piston ring, or scored cylinder, or broken or detonated piston, a bad valve seating, even a broken connecting rod.

In this case, the suspect cylinder's compression would be dramatically lower than the rest, or even zero. It's at this point, however, that you might want to defer to your mechanic, if you haven't already. If you did a compression check, at least you can relay your findings to the dealer. If your engine produces healthy, even readings, record this information for future reference as a baseline.

Also, these results will be helpful if you're selling your engine. While this seems like a fairly basic test, the specifics can vary greatly with the many variations of outboards and systems. Failure to follow steps appropriate to your outboard may cause damage and injury. Unless you're familiar with your motor's requirements and have a good understanding of how to do this, you may want to leave it to a trained outboard mechanic.

TIP:Having a second person on hand to crank the engine for you will save time and prevent you from having to climb in and out of the boat in many cases. John Tiger, an outboard master technician, got his first outboard at age seven, and has owned more than 60 boats and outboards since. We use cookies to enhance your visit to our website and to improve your experience. Membership Search. Service Locator. Get a Quote.

Become a Member. Renew Membership. Boat Insurance Membership Boat Towing. Boat Insurance. Boat Towing. Membership Plans Savings. Boat Show Tickets. Boat Lettering. Boat Loans. Vessel Documentation FAQ. Maintenance Techniques Tow Vehicles. How-To DIY. Cleaning Exterior Systems. Electronics Equipment Maintenance Techniques. Call For a Tow Checking Compression on an Outboard Engine. Note the reading for that cylinder. Repeat the process for each cylinder and note the results. Interpreting Your Results Not many engine manufacturers will give a cranking compression reference standard for their engines.

On average, an outboard motor will run about 1, hours or 7 to 8 years. Each outboard motor will be different but if you keep up maintenance and take care of the motor it can last much longer.

As a general rule a compression of PSI or better is excellent. Similarly, A compression of 85 PSI or lower is extremely bad. The most desirable situation is that all cylinders give the same or close to the same reading. Furthermore, That reading should be above PSI.

You should start to worry if one of your cylinders is getting close to psi. Most new engines in good condition compress at around psi. Suppose that 4 cylinder were at psi , the difference ratio would be An engine compression test will tell you if your cylinders have good compression. Low compression in one cylinder usually indicates a bad exhaust valve. Low compression in two adjacent cylinders typically means you have a bad head gasket.

Here is a list of the most common symptoms for bad piston rings: White or gray exhaust smoke. Excessive oil consumption. Low power for acceleration. Overall loss of power or poor performance.

Cylinder compression tests are performed to identify any cylinders that have poor compression. If a cylinder has low compression , perform a wet compression test to indicate whether it's a bad valve, head gasket, or worn piston rings causing the problem.

Cold testing The oil will provide a supplementary seal to the rings between the piston and the bore, and a compression reading that is around 10 per cent or higher than the cold test indicates wear in either the piston rings or the cylinder bore.

Carbon could wreak havoc in more ways than one. It could have built up on your valves, making them not seat right anymore, thus causing low compression. By reducing the cylinder diameter to increase the compression ratio. By increasing the cylinder diameter to decrease the compression ratio.