Page last updated July 17, 2008



Outboard Engine Compression Check

The compression check is probably one of the first things you would want to check before you purchase a used outboard motor. The procedure may seem daunting to a beginner, but is fairly quite simple and it is crucial that an engine have good compression before you purchase.

The compression between each cylinder should not differ by more the 10 - 15 PSI. If it is more, it could require quite a bit of work to fix and some may not be salvagable (scored cylinder bores). If you catch the compression problem early, you can simply use "engine tuner" or something that will remove the carbon buildup. Doing the compression test when you first get an engine will get you the compression reading in case you have problems down the road. you at least have something to reference.

If you find an engine that has bad compression, you are taking a risk when buying such an engine, the amount of work required to bring back to life may surpass the value that it is worth. Compression is one component required to ignite the fuel, if it is lacking, you may not get ignition or will have problems. It is probably better to pass it up and wait for another "to good to believe" offer or "steal" to appear. The fix could be simple (not likely), but what are the chances, you decide.

Tools required:

  1. Compression kit with adapter for your spark plug hole. The standard fitting that comes with a compression tester should suffice but before you purchase, take a plug with you to make sure the kit comes with an adapter that will fit your bore.
  2. If you have a "kill switch", simply remove the lanyard from the switch and your engine will not start. (it kills the engine in case you are thrown off the boat, remove it from the switch and it won't start no matter how much you crank it). If you DON'T have a kill switch, you need to remove the juice going to your ignition. Unplugging the gang plug to your ignition system will do the trick. Also, you need to devise a rig to ground the plug wires that are loose when you remove them. When you test the engine and the ignition system has not been disabled, you risk igniting fuel that is being injected into bore with plug wire dangling freely (possibly sparking). Wire secured inside spark plug wire boots and then to engine block should suffice. It would be foolish of you to ignore the grounding procedure as you risk serious injury.
  3. A partner to to key ignition while you observe dial.

REMEMBER: When testing, if you do not have a kill switch, you need to disable the ignition system by unplugging the gang plug leading to your ignition system. If you have a distributor, look at what is feeding it the distributor (wire).

You can also simply disable the ignition by activating the safefty kill switch or removing the lanyard. Again, test it out. There are other methods for testing the wire to make sure no current is flowing, but either way, make sure you ground plugs wires to engine block.

Here are the steps for testing compression:

You basically want the engine to turn over a specific amount of times without actually turning it on (with the compression gauge attached to a plug socket). So, the electrical charge to the plugs needs to be eliminated. Fuel MUST be in place; it supplies the lubricant on a 2-stroke, never turn the engine over on a 2-stroke if you have no fuel running to it.

  1. Disable ignition system and ground spark plug wires to engine block.
  2. If wires are not labeled, make sure you label plug wires before disconnecting, (use white masking tape to write TOP, MIDDLE, BOTTOM or 1,2,3, etc) You would hate to not know what order to reconnect them wires. Some engines have wires marked or distributor has spark numbers, check and make sure.
  3. Next, remove ALL spark plugs and keep them in order so that they can be replaced into their original sockets in the same order.
  4. Place the compression hose into spark hole #1 and zero the guage.
  5. Check your choke and make sure it is in the open position. Make sure the throttle is open all the way. If your system is equipped with a neutral safety switch, you may need to manually engage the throttle from the engine block while the remote is in neutral. You want to allow as much air to enter the bore as possible to get an accurate reading and a neutral safety switch will prevent the battery from turning the engine unless it is in neutral. Be careful, flywheel and starter will move so ensure your hand is not in the way of any moving parts.
  6. Crank the engine and observe the reading. Each cylinder should be cranked an equal number of times and / or seconds in order to get an accurate reading for comparison. If you have pull to start, pull it 4-5 times for each cylinder as long as you do the same for each cylinder. Make sure you zero out the dial each time.

If you observe major differences in compression, it isn't completely hopeless yet. You may do further testing to determine if the faulty readings are from a cold or dry engine...meaning engine has not been run for some time.

Some people suggest covering the bores with some lubricant such as WD-40 and such. You can try fogging oil. Spray it into the bad chamber and make sure you aim at the outer edges. Try the test again and see if the compression improves. If it does, you may need only to remove carbon buildup with an engine tuner mixture. I use Sea Foam, some swear by "Mercury Power Tune". Every 50 hours of use will do the trick. Engine carbon buildup is one thing that will destroy your engine so de-carbonization process is a must. It will help your compression and help maintain a healthy engine.

There may be other reasons for lower compression. Worn, blown gaskets could be a culprit. Look for rust on spark plugs or water in cylinder bore. Blown gaskets can be replaced by you, but if done improperly, you risk blowing a new gasket due to overtorqueing or damaging exhaust covers (they also break easily, especially if the engine is old and pitting has occurred inside the cover).

It might be that the cylinder rings are worn and need replacement or worst case scenario, the engine bores have scarring and needs a complete rebuild..if even possible. You be the judge.






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