How to do a compression test?

[keen_angler]

Hi All,

I found my fathers compression tester, but there are no instructions on how to use it. Can someone tell me what to do, I have a 2001 Yamaha 70hp 2 stroke.

Any info would be great.

Cheers

[FNQCairns]

Pretty simple with no need to overthink the procedure, pull a plug, ensure the kill switch is on, insert compression tester and turn the engine over until the tester numbers even out, dunno 2 seconds of cranking?

Replace plug then move on to the next plug.

Basic test no need for warm engine, throttle, yada yada.

If there is a problem noted further tests with changed procedures can help nut it out.

cheers fnq

[Noelm]

FNQ is pretty well spot on, you can test warm, cold however you like, the real readings you are looking for is large variations, or a very low reading, so how you do the test does not matter a whole heap. (sort of) as long as the test is performed on all cylinders in the same way.

[Roughasguts]

Just a guess but I think you should be looking at around 120-130 psi with that motor.

And if not! don't worry take the boat for a run, squirt some carby clean down each spark plug hole, take it for another run then check the comps again.

Most times you will get an increase in compression after the carby clean trick, that just cleans some carbon from the rings so they seal better on the bore.

[Spaniard_King]

RG,

cant see him gettin those figures..more like 110 if he's lucky

[Noelm]

also keep in mind the old saying "if it aint broke, don't fix it" if you are testing the Compression just because you have found a compression tester, do not get too fired up by the results and start doing some unecessary "surgery" the Motor is running fine, yes? then leave it alone (sort of)

[Hornet Rider]

RG,

cant see him gettin those figures..more like 110 if he's lucky

Hi, did a compression test on my 1998 Johno 70 hp 2 stroke, 370 hours. Results were, top 110 psi, centre 110 psi, bottom 108 psi. Can't find any OMC stats for this motor on the the web, but these figures appear reasonable. Any comments? By the way, motor runs very well although it appears to be a bit thirsty.

cheers, HR

[goldfish]

Pretty simple with no need to overthink the procedure, pull a plug, ensure the kill switch is on, insert compression tester and turn the engine over until the tester numbers even out, dunno 2 seconds of cranking?

Replace plug then move on to the next plug.

Basic test no need for warm engine, throttle, yada yada.

If there is a problem noted further tests with changed procedures can help nut it out.

cheers fnq

FNQ is spot on. but if I could add one more thing. on the tester there i relese valve when pushed the gauge returns to 0 push this valve before each test. I know its basic but i have seen a guy who didn't do this & thought he had even comps when 1 cly wouldn't blow out a match on a windy day.
cheers
rick.

[Hornet Rider]

Rick, thanks mate. I think I've got the how to down pat, but was just wondering with the figures I got on testing, if they are acceptable for that motor.

cheers, HR

[Spaniard_King]

Hi, did a compression test on my 1998 Johno 70 hp 2 stroke, 370 hours. Results were, top 110 psi, centre 110 psi, bottom 108 psi. Can't find any OMC stats for this motor on the the web, but these figures appear reasonable. Any comments? By the way, motor runs very well although it appears to be a bit thirsty.

cheers, HR

HR that 70 is in good health if the engine was starting to get tired you would be seeing pressures of 95 and below.

[Hornet Rider]

HR that 70 is in good health if the engine was starting to get tired you would be seeing pressures of 95 and below.

Thanks Garry. One expert told me I should be getting 120psi but the googling on the 70 Johno didn't support that advice, & you've confirmed it. Much appreciated.

cheers, HR

[goldfish]

HR that 70 is in good health if the engine was starting to get tired you would be seeing pressures of 95 and below.

Hornet. as above. saver me typing. lol

[cormorant]

For a more accurate reading




pull all plugs out
attach and reset compression tester - most errors are whith it not sealing or seated properly
Open throttle butterflys fully and choke as well if not already open
turn over on starter 5-7 revs
A good reading is where the compressions are even. Do it at every service and record it and note any big changes as it is a warning signal all is not well.




A couple of warnings

Some electronic ignitions / computers don't take kindly to no plugs earthed

Readings between guages are often huge (up to 10 to 15psi), always test with yours and borrow someones to check it is close.

Some unscupulious people have 2 guages - one for selling motors and one for buying motors - I'll let you guess which guage reads high and low!!

[Hornet Rider]

For a more accurate reading




pull all plugs out
attach and reset compression tester - most errors are whith it not sealing or seated properly
Open throttle butterflys fully and choke as well if not already open
turn over on starter 5-7 revs
A good reading is where the compressions are even. Do it at every service and record it and note any big changes as it is a warning signal all is not well.




A couple of warnings

Some electronic ignitions / computers don't take kindly to no plugs earthed

Readings between guages are often huge (up to 10 to 15psi), always test with yours and borrow someones to check it is close.

Some unscupulious people have 2 guages - one for selling motors and one for buying motors - I'll let you guess which guage reads high and low!!

Thanks cormorant, did the testing with all plugs removed after I gave it a 5 min warm up, throttle in WOT but didn't do the open choke. Will try that bit next time to see if the reading vary. I guess it pays to have a quality guage & take it with you if you're going to look at a boat for sale.

cheers, HR

[Spaniard_King]

Thanks cormorant, did the testing with all plugs removed after I gave it a 5 min warm up, throttle in WOT but didn't do the open choke. Will try that bit next time to see if the reading vary. I guess it pays to have a quality guage & take it with you if you're going to look at a boat for sale.

cheers, HR

HR your engine has an electronic pump type choke it will have no effect on the compression test. Cormorant is refering to choke systems that use butterflies in the throat of the carby (this type of system has 2 butterflies.. 1 for choke and one for throttle)