Proper way to compression test

[swick]

What is the proper way to do a compression test on an outboard?

I have read different views: some people say take all plugs out, some say take one out at a time. And does it matter if the boat is in the water or not?

How long should it really take to do?

 

[Silvertip]

Re: Proper way to compression test

Takes two minutes per cylinder unless you have five thumbs on each hand. it can be done in the water. It doesn't matter if the plugs are in or out. On a two stroke it also doesn't matter if the throttle is wide open or not but on a 4-stroke it does. The process is to remove the plug, screw in the compression tester and spin the engine over 3-5 times with the starter.

 

[swick]

Re: Proper way to compression test

so I should screw in the tester, turn the boat on and off about 5 times? Check the reading each time I imagine?

 

[JB]

Re: Proper way to compression test

NO!!

http://forums.iboats.com/showthread.php?t=313884

 

[swick]

Re: Proper way to compression test

gotcha, I think:

1. Fully charge battery. (no prob)

2. Start engine and warm her up. (why warm it up? Won't it make plugs hot?)

3. Remove all spark plugs and disconnect fuel line. (from gas tank?)

4. Rig a jumper to engage starter solenoid without turning ignition on. Pull the emergency kill switch on rope start engines. (is this necessary? can i just have someone crank it over with key?)

5. Screw guage into #1.

6. Open throttle all the way. Turn engine over until reading on guage stops increasing or 5 times, whichever is more.

7. Repeat #5 & #6 for all cylinders

 

[Silvertip]

Re: Proper way to compression test

On a two stroke the throttle does not have to be wide open. Spinning the engine over 3-5 times turn the key so the engine flywheel rotates 3-5 revolutions (or until the gauge stops rising as was also suggested). If you have the throttle wide open the starter may not engage. Leave it in neutral.

 

[Silvertip]

Re: Proper way to compression test

Yes the plugs will be warm. That has nothing to do with compression. Use the plug socket to remove the plugs and you won't have to touch them with your fingers. Engine compression will be different when warm than when cold. Since you run the engine warm, why not check compression warm!

Disconnect the fuel line from the engine or the tank whichever is most convenient. If its a lower HP engine it probably has quick disconnects on either end.

This is again related to the wide open throttle issue. You can't energize the starter with the key with the throttle wide open. (Think about that and the reason why!)

 

[swick]

Re: Proper way to compression test

Thanks again Silvertip. Yes I see what you are saying about the starter with the throttle being wide open. So in essence there is no need for jumper wires or jumping starters right? Just don't open the throttle up all the way right?

 

[swick]

Re: Proper way to compression test

Oh yeah, and I guess we will water test it first and then check the compression. Since it should be warm this only makes sense. And if everything runs fine during the water test we shouldn't have much to worry about when comes time to compression test it.