Doing a compression test on an outboard motor?

Question

okay so I remove all the spark plug wires, then remove one spark plug and put the compression tester in that hole, and id put the boat throttle all the way down, and then turn the engine 4 rotations and read the results... is this correct? and do this for all four spark plugs? im very unfamiliar with engines and especially outboards, I do believe I could find the spark plugs however. So please let me know if im missing out and what not, and any pictures would greatly help as I am uncertain what a lot of terms mean.

thanks

Answer 1

To do the compression test: there are several types of compression testers on the market. If you buy one, read the directions with it as to how it goes in a spark plug hole. Some s***w in, some you just hold in tightly. If you borrow one, ask the owner to make sure. You must remove all of the spark plugs to do the test. The best thing to do is turn the engine over by getting a remote starter switch from the car parts place. It has 2 alligator clips that you connect to the starter solenoid and a button you push to crank the engine. Cost about $12. The reason for this is if you crank it with the key, you are allowing electricity to get to the plug wires and they can spark and ignite the gas mixture that will be blown from the plug holes. Leave the key in the off position, while doing this test.

Attach the compression tester in the proper manner and crank the engine at least five full revolutions. As others have said you should not have more than 5 lbs pressure difference between cylinders.

Answer 2

Once you've screwed the tester into a spark plug's hole, and advance the crankshaft, it'll pressure up and record the max pressure of that cylinder.

So you'll repeat the test on each cylinder, advancing the crank around 'till you register the high pressure of the compression stroke in each bore.

Bear in mind that this is a maximum compression test only. Some diagnostics require a 'leak-down' compression test which'll show any compression leakage over time.

Good Luck!

Answer 3

I have always removed all the spark plugs and advance the throttle to wide open. It really doesn't matter though, the biggest thing is to measure them all under the same conditions. I would roll it over for two to three seconds. The compression numbers are supposed to be all within about 10 psi of each other. It is also a good idea to do it when the engine is cold and when it is warm as tolerences can change when it's warm.  It's really alot easier to check compression on an outboard than other engines. Just make sure no one is near the spark plug wires when it's rolling over or pull the safety cord from the helm.

Answer 4

If you are that unfamiliar with outboards why would you attempt this.  Especially if you are not even sure you can find the spark plugs.  Take it to a dealer with a mechanic certified to work on your brand of outboard.  It will save you many headaches and probably a lot of money getting it done right the first time.