Compression test...best place to ground it.

Hello all. Need an quick answer as I'm in need of compression testing an outboard this week. I'm testing a 1985 Johnson 9.9. I have some 16 gauge wire attached to alligator clips. I was attaching one end to the plug wire but I'm not sure where the best place to ground it to the outboard would be? Is there a safer place to attach it than another?

Also, I've read that this test should be done to a cold engine, not a warm one. Right? Last, the outboard should have the tank unattached and run until it stalls so all the gas is removed from the system. Is that right?

Hoping to come home with a motor this week and just don't want to miss anything. Thanks in advance!

Bob,
If you want to test the compression of the powerhead,
you use an air pressure gauge like this (from HF).

if you want to just test the coils to see if they have spark,
Remove the spark plug, attach the plug wire cap to the plug, and,
holding the spark plug in something that is insulated, glove, paper, etc,
ground the base of the plug to the powerhead. Turn the engine over.
There should be a strong blue spark at the electrode.
But, a spark plug tester would be better.


And it is good that you are doing your homework . . .
nothing more aggravating than to get a new motor home
only to find it has some serious issues.

pymybob said:

Hello all. Need an quick answer as I'm in need of compression testing an outboard this week. I'm testing a 1985 Johnson 9.9. I have some 16 gauge wire attached to alligator clips. I was attaching one end to the plug wire but I'm not sure where the best place to ground it to the outboard would be? Is there a safer place to attach it than another?
Thanks in advance!

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If doing the spark test, you don't ground the plug wire, you ground the plug body to the engine. Using a spark tester is easiest though.
I assume you want a compression test though, johnny filled you in on that. There are youtube videos to show you how to do both.
Tim

Actually the spark test should not be across the spark plug.
A valid spark test should be done at cranking speed with no spark plugs in the engine. A set-up where the spark has to jump at least a 1/4" gap is needed.
Make up something or buy something to accomplish this.
Jumping the plug gap requires very little secondary voltage and will not uncover weak spark issues.

So, do I need to ground the spark plug wires when I do a compression test? What I've seen online seems that I do and I just need to know where I should ground it to the motor, if it does matter. I've already picked Up the compression kit from HF. Good deal! I was only going to do a compression test. Thanks everyone for the replies!

Ground the plug wires anywhere there is an unpainted fastener would be the easiest.

pymybob said:

Also, I've read that this test should be done to a cold engine, not a warm one.

Click to expand...

Specs given in manuals are cold compression.

Remove all plugs and make sure it's WOT

Pappy said:

Ground the plug wires anywhere there is an unpainted fastener would be the easiest.

Click to expand...

That's what I thought. Thanks for confirming!

Skiffing said:

pymybob said:

Also, I've read that this test should be done to a cold engine, not a warm one.

Click to expand...

Specs given in manuals are cold compression.

Remove all plugs and make sure it's WOT

Click to expand...

What do you mean by "WOT"?

Wide Open Throttle

WOT

There is nothing in an OMC manual on a two stroke that says anything about having to test compression at WOT. Throttles closed.
Some 4-strokes yes....not this engine.

Think about it.....the engine RUNS at idle with the throttles fully closed. It obviously pulls enough air to create compression to do that.

You don't have to - but you need fewer un-lubricated cycles of the engine to get the reading on your compression gauge if you remove the air restriction, so you've got nothing to lose by doing it WOT and it's likely a bit better for the motor...

for 35 years, i have built/raced/tuned 2 stroke race engines in motorcycles and atv's .. when a compression check is done, it is done at WOT and cold .... remove all the plugs and spin it over to get your readings .. check both cylinders and they should only be a couple pounds difference, they in my opinion should be with in 1 1/2 pounds of the control cylinder.

Why does the test need to be done at WOT? I cannot picture in my mind why throttle position would matter. Thx!

pymybob, Did you check the motor ? What was the price ? Did you get it ?
I also have a mid '80s 9.9 Johnson and don't know what to do with it.
has not been run in many years.

SumDumGuy said:

Why does the test need to be done at WOT? I cannot picture in my mind why throttle position would matter. Thx!

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At idle, for each cycle of the engine, you get a little bit of air and a little bit of gas into the cylinder for a small explosion and low rpm. At full throttle, for each cycle of the engine, you get a lot of air and gas for each cycle. If you can't get air into the cylinder, what is there to compress? So having the throttle butterfly open (WOT), allows more air to get into the cylinder faster. Yes some air can get in even if the throttle is at idle position, but you'll have to crank the engine over more times to get your reading and even then it may not be accurate. I don't know for sure that it's the same on two strokes, but I've seen times when testing compression on a four stroke wouldn't register max compression even after turning the engine over several times with the electric starter.

SumDumGuy said:

Why does the test need to be done at WOT? I cannot picture in my mind why throttle position would matter. Thx!

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What you are testing in a compression test is how tight the motor is, specifically it's top end. The tool used in this case is air. A pressure gauge installed in the motor measures how much of the air is holding in the top end. All a motor really is at is most basic is a big vacuum pump. You want the throttle body to be open in most cases when performing this test.

Four stroke outboards are tested with the throttle blades open from what I have read in the manuals. Do not know if that applies to all the brands.
Johnson and Evinrude outboards have never specified anything other than throttle blades closed on their 2-cycle engines.

In my experience testing at WOT creates a little more compression due to more fuel being in the cylinder sealing the the rings better. That is fine as long as you have good compression and a uniform compression on all cylinders. Testing with the throttle closed will have less compression but it still should read high enough to be acceptable to start the motor. If your rings are worn and or cylinder liners scratched you may have a low compression with throttle closed and good compression at WOT due to fuel sealing the rings/scratches.

KMixson said:

In my experience testing at WOT creates a little more compression due to more fuel being in the cylinder sealing the the rings better.

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Actually, fuel is pulled into the cylinders by vacuum created by Bernoulli's principle.
Vacuum is produced in this case by trying to pull a high volume of air rapidly past a venturi or small opening. This low pressure on the back side of the venturi and throttle blade will readily pull fuel through the idle circuit at an idle or closed throttle setting.
As the throttle plate opens and passes over the calibration holes that make up the idle circuit the flow stops due to the rise toward atmospheric pressure.
At a WOT setting at cranking speed Bernoilli's principle pretty much goes out the window since the velocity of the air passing over the venturi will not be enough to effectively pull fuel through the idle circuit (since it is now no longer behind the blade) nor the larger column of fuel located in the fuel bowl at the bottom of the high speed nozzle.