Weird JW compression test.

[stanley]

Why would rope pulling give significantly higher numbers than spinning with drill?I did the same routine with an identical engine and same gauge,spinning with drill gave slightly higher readings.

[chinewalker]

I avoid spinning any motor by the flywheel nut – good way to split the hub by over tightening the nut. I always pull over by hand, or if electric start, then I use that.

As for your numbers, I suspect you can actually pull the motor over harder than the drill can spin it, on a per thump basis.

[stanley]

So,given a 6 lb difference in the two,which is the most accurate representation of engine condition?

[Pete]

Certainly weird results but expected with these tests. If you do hand pull you will probably do 5 pulls to get a max gage reading and there is time between pulls for built compression to leak that the next pull needs to make some up to further advance the gage needle, maybe there is a slight drop in the gage reading to hold pressure? maybe you have a faulty gage and hose ❓ How long is the gage hose ❓ With a drill it will be 12 or more revs and a better chance in less time to get the max pressure in the gage. I think pull or drill shouldn’t be that different if good gage, short hose and no leaks .

I see you get 6 psi different. Neither is absolute accurate just a relative comparison is being done. We do not know the complete detail conditions of your test ❓

Note for me hand pull gives more info. For ex. I pay attention to the 1st reading that is very indicative of the lower compression ratio (CR) based on a CR using the cylinder volume above the top of the exhaust port to TDC which is less than the full volume CR from TDC to BDC. Most 2 cycle outboards I have checked have about 9:1 based on full volume (TDC to BDC) and about 5.5:1 based on TDC to the top of the exhaust port. So 1st pull tells me how that builds with the lower CR. Several pulls thereafter give info on the compression build to max. I stop at 5 pulls more than enough. For future testing I keep track of the compression readings for each pull and compare to previous readings to see the pressure build. That gives me a better indication of the cylinder and rings wear condition rather than just one max psi reading that you would get just recording the max reading. In my experience as engines wear they take more pulls to reach max pressure in the gage.

I calculate expected compression psi from a known CR using the thermo gas laws to compare to gage test pressure.

I don’t like rotating the engine a lot (like a drill) with no fuel+ oil ingesting. I always test dead cold so temp is easier to control and one less variable out of the picture + throttle wide open. I do another test with a several oil squirts of oil in cylinder pull over to distribute then test to see what about 100% ring seal will give, difference without oil indicates how much leakage there is . Leak down test can also be done for better idea of cylinder and rings condition.

I always use the same gage this most important for consistent results. Carefully store the gage.

[The Boat House]

• This reply was modified 5 years, 5 months ago by The Boat House.
• This reply was modified 3 years, 2 months ago by The Boat House.

[stanley]

I’m using a gauge with a short hose and threaded plug hole end.The numbers I get are consistently 80 lbs by hand and barely 70 with drill.I can use the drill,lay it down,rope it over 5 times and get it up to 80.For comparison I have a good runner of the same model that is steady at 85lbs spinning it with drill or pulling it over by hand.This motor absolutely will not Idle or start without a shot of gas into intake.I have taken carb and mag plate off the runner and put them on it with no change.It has to be an issue with the power head.The higher number says it should idle but the lower would explain why it doesn’t.I’m stumped.

[PugetSoundBoater]

Zul8tr- I like your thorough and detailed description of your compression test procedure.

"Some people want to know how a watch works, others just want to know what time it is"
Robbie Robertson

[Pete]

quote PugetSoundBoater:

Zul8tr- I like your thorough and detailed description of your compression test procedure.

Thank you
Consistency with tools and process is key with any testing 😉

[cajuncook1]

Some years back, a member by the name of Lloyd Lautner, explain a way by which compression testing should be performed in a certain manner that made a lot of sense. This explanation is not word for word but basically a summary of his explanation.

Compression should be measure when the motor is cold and dry. "Dry," meaning no extra oil or addition of oil during testing. This is to maintain testing conditions as much as possible without adding variables to influence readings. If a motor is a rope start motor then compression testing should be done using the rope start method. If a motor is a electric starter motor then compression testing should be done using the electric starter method. Electric drills should not be used in compression testing.

By using the rope start method or electric starter method helps keep evaluation of compression consistent. Different makes and types of compression gauges can influence compression readings, but that is another discussion.

Some outboard motor companies did not release acceptable compression values in their repair literature. OMC did not release compression values, but Chrysler and Mercury did…..If I am mistaken please correct me.

Since some companies did not release compression values, mechanics in the industry recorded and established what acceptable compression values were when the motors were relatively new. We now use those compression numbers to determine what is acceptable compression for these vintage and antique outboard motors.

Again, it is about maintaining consistent measuring methods for rope start motors and electric starter motors to get compression readings and comparing those to established compression readings to determine if the motor has acceptable compression values.

Cheers,

Cajun

[cajuncook1]

quote Stanley:

I’m using a gauge with a short hose and threaded plug hole end.The numbers I get are consistently 80 lbs by hand and barely 70 with drill.I can use the drill,lay it down,rope it over 5 times and get it up to 80.For comparison I have a good runner of the same model that is steady at 85lbs spinning it with drill or pulling it over by hand.This motor absolutely will not Idle or start without a shot of gas into intake.I have taken carb and mag plate off the runner and put them on it with no change.It has to be an issue with the power head.The higher number says it should idle but the lower would explain why it doesn’t.I’m stumped.

Stanley, even with a compression of 70/70 psi, the motor should be able to start and idle decently without having to shoot fuel mix into the intake. Remember introduction of fuel mix into the crankcase takes sufficent amount of pressure differences with in the crankcase.

Things that can cause poor introduction of fuel into the crankcase.(excluding carburetor issues and compression)

-broken or fixed reed valves/leaflets
-obstructed or partial obstructed intake or exhaust ports.
-obstructed or partial obstructed exhaust housing or lower unit foot.
-worn or damaged piston skirting

These conditions can prevent proper air flow through the crankcase and thus prevent proper fuel/air delivery.

Easiest things to check and ruleout are lower unit foot and exhaust housing obstructions from trash, insect nest, build up of fuel/oil mix goo that clumped up over time obstructing air flow out.

May not be your issue, but if you have ruled out your carburetor, ignition, and you have decent compression, then these are possible issues.

Cheers,

Cajun

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