Prior to purchasing a used outboard, One of the many important tests a individual could perform on a outboard motor is a compression test. It is very simple in my opinion and could make the difference between a reliable, dependable, smooth running engine, and a shinny expensive anchor. The thing to remember is after you complete a test with acceptable results, is that it doesn't guarantee your engine is going to run smoothly (or run at all). other tests and maintenance should be conducted. Carburator tune-up and cleaning, Ignition test and inspection ect...

In my own opinion this test will come down to two things,

1-"MONEY" you just eliminated one of the most expensive repairs or problems in a outboard motor. It could vary from simply changing a head gasket to Cylinder re boring & honing, and re sleeving oversizing piston rings ect...not a easy task for the novice and it's almost guaranteed to cost you more then you paid for the motor. Yes your engine will be operating like it's new after this process but why go through the trouble.

2- "TIME" The time it takes for you (or a mechanic! $$$) to find the exact problem could potentially be substantial. A simple thing as changing piston rings  (on most older and some newer models) means to practically dismantle the whole engine, that doesn't happen in 5 minutes. And trust me, you always run into problems, snapping rusty head bolts is common and nerve racking ordeal that I don't wish upon no one.

Here's how I do a compression test

The first thing you'll need to get is a compression tester. You could get them at any major auto parts store. or Ebay but I recommend spending a little money here and getting quality.(you'll be surprised how often you use it)

-If the engine is working, start it and let it warm up then shut it down (preffered but not necessary)

-Disable ignition system,(varies from models) this could be done by pulling your lanyard for the stop button, unplugging the power pack, unplugging CDI or simply removing all plug wires, keep them far away from each other and the motor. you don't want the wires to jump spark.

-Remove all spark plugs and retain their order for trouble-shooting (appearance)

--Shut off fuel or disconnect fuel lines

-Place throttle wide open and leave the choke off. You want the maximum amount of air to enter the crankcase.

-Install your pressure tester to the top cylinder. remember all plugs are out! Make sure that the tester has been cleared or purged from your last application. "Sounds funny but I do it all the time"

-And crank the motor over. Honestly it's better to crank it over too many times, then not enough but generally 4 to 8 times. Better yet let the cylinder build it's peak pressure.

- Repeat to other cylinder(s).

-Each model will have an OEM parameter for pressure and variation

-15% variation from the highest to lowest reading is generically acceptable.
*different gauges may give different readings, but they all will reflect variation. Thats what we're most interested in...

For example; #1 cylinder has 100psi and #2 has 95psi That's a 5% difference and is acceptable.

Hope this helps out, good luck and happy boating!!

Thanks

Andre

Andre's used outboard parts