compression check

[Karla45]

Can I do compression check without the motor on the muffs?

 

[Bifflefan]

Re: compression check

I would hope so, the motor is not running, just cranking for a couple seconds.

 

[NYBo]

Re: compression check

I don't like the idea of spinning the impeller dry, even for the amount of time a compression test takes.

 

[jerryjerry05]

Re: compression check

Won't hurt it a bit.There is probably a small amount of water left in the pump.J

 

[trendsetter240]

Re: compression check

Can I do compression check without the motor on the muffs?

Yes you can. In fact you don't need them at all for a compression check.

1. Pull the kill switch lanyard out to disable the ignition.

2. Place the throttle in wide open position. (use the idle lever and keep the engine in neutral)

3. Remove ALL spark plugs.

4. Screw compression gauge into first spark plug hole.

5. Crank motor over at least 5 complete revolutions.

6. Record compression reading.

7. Repeat procedure for each remaining cylinder.

8. Replace spark plugs, throttle back to idle and replace kill switch lanyard.

 

[Karla45]

Re: compression check

I have read that i should warm the engine first. Is this important at all to compressoin test? I did one today and mine came out very differently from the mechanics. His test 135,135, 100. Mine 120,120, 100. I know that test will be different since not using same tester but should my numbers be closer than to his. When i first got the boat the compression was 138,135,135 done by a different mechanic. It seems to me like maybe his tester is better.

 

[trendsetter240]

Re: compression check

I have read that i should warm the engine first. Is this important at all to compressoin test? I did one today and mine came out very differently from the mechanics. His test 135,135, 100. Mine 120,120, 100. I know that test will be different since not using same tester but should my numbers be closer than to his. When i first got the boat the compression was 138,135,135 done by a different mechanic. It seems to me like maybe his tester is better.

Yes, you can warm the engine first and that is a good practice. However a cold compression test that shows > 15% difference between cylinders is some cause for concern.

If you run the engine to warm it up you must absulutely use water muffs or have the leg of the engine submerged in water to about half way up the leg.


Different compression gauges will show different readings. They are not the most accurate tool unless you spend big bucks. The most important thing is that the readings are over 100psi and within 15% on each cylinder.

You might want to try a decarb procedure to see if the one cylinder will come up to a higher compression. The rings might be sticking a little due to carbon build up.

 

[Karla45]

Re: compression check

how long would i need to run it on muffs to warm up?

 

[trendsetter240]

Re: compression check

how long would i need to run it on muffs to warm up?

Maybe 3 to 5 minutes at idle. Feel the temp of the tell tale water stream while the motor is running. When it's warm the engine is warm.

 

[jerryjerry05]

Re: compression check

It's not necessary to warm the engine to do a comp test.
Remove all three plugs.Use a screw in tester.Turn it over till the guage stops going up.You won't hirt the impeller.
Different guages will give different readings.Cheap guages usually give poor results.
The important thing is the readings be close.
After the test spray some oil in the bad cylinder.Then re-test that cylinder.
If it does nothing then the headgasket might be bad.If it improved it,then the rings are stuck and a decarb is necessary.
There is a reason it went bad.The rings carbon up,could be the carb set wrong or bad reeds.
The headgasket could go bad if run in salt water(you never did fill out the profile)if you live near the ocean.
If the tests don't tell you anything and the de-carb doesn't help then you'll need to remove the head and check the cylinder for damage.J

 

[Karla45]

Re: compression check

I live in North TExas no saltwater exposure. I did fill out my profile the other day so not sure why it is not showing up. I am sure you are right on the compression tester. I bougt at Auto Zone it is Actron for around $30 or more. I would buy a better one but have no idea which one to buy. My compression readings as of today after I decarbed are 120, 120, 120. My manual says they should be 135 but my thoughts are that my compression tester is not very accurate. 1989 FOrce 85hp.

 

[jerryjerry05]

Re: compression check

The results are the important thing.That they all are about the same is the important thing.
Different result from different guages.Actually the results should be(with a GOOD guage)around 150# or better.
Like I said the difference is the important thing.
120 on all three is good.
Fix the diaphram(other thread) and go get it wet.
Put in a tach.Or get a Tiny Tach for diagnosing the motor.J