Compression Test Results

[Micadogs]

Just performed a compression test on a mid 90's model yammie 70 HP 2 stroke. The results were Cyl 1. 120, Cyl 2.115 Cyl 3.108. Are these results ok?

[borrie01]

hi mate i would have to say shes very borderline fair amount of difference there on the last pot are you looking at buying the engine all comps should be eqaul or within 10%of each other di you try runnig it for a while then retesting it

[Micadogs]

Buying the boat from another location so had a boat mechanic perform the test. Didn't get a chance to run the motor myself. Not sure how the test was performed either. Will need to find out those details i.e. warm motor or cold.

[backlash08]

hard to tell really, depends on battery quality and other variables, static comp is ok indicator but not totally reliable, was 120 the reading the first and the 108 last?
either way, its not too bad, would prefer to see pressures closer but again, depends on the cranking etc when test performed

[borrie01]

ok mate would have to think if a mechanic tested it he would have done it correctly if it were me id be a little shy of it myself i would prefer to see them closer than that like 120 115 115 would be okay i know its not much but giving you a resonably good indication that there may not be that much life left in the old girl
thx

[FNQCairns]

Yea it's too borderline to consider, for sure it would run well ATM. I have the 90 yam same design of engine, the top cyl runs the hottest and the bottom the coolest, it looks carboned up to me, that engine is on the way to snagging/scoring the bore (if not already) a ring in the next 10s of hours or so.

Just for reference my engine is 130psi with no variation to be bothered relating.

cheers fnq

[Micadogs]

Clee - not sure the order the test results were performed. Does this make a difference? Are the decreasing results are a pattern of some sort. Is there a procedure that I should request the mechanic to perform to confirm results?

[Roughasguts]

I'm Kinda with FNQ I would like to see higher than 125 psi.
However as FNQ says shes probably carboned up, top cylinder the highest PSI cause the carbon gets burnt off better cause shes the hotest pot.
And the lowest coolest cylinder will have the most carbon build up there fore lowest comps. The carbon will be probably behind the rings causing the PSI to be down.

Anyway that motor should have the heads off by now to clean the cooling gallerys, they block up with salt.
But before that give her a good spray tthrough the spark plug holes with carby clean, turn the motor over a few times then spray more carby clean down her let soak for 5 mins then start her up to oil up the bores again.

Then give her another comp test and compare the figures if the same might be best to walk away or offer 1500 bucks less for repairs.

[borrie01]

Micadogs im also with fnqcairns mate if i tested that myself i would walk away with those results when you have variences like that it is definately telling you something isnt quite right and is very close to needing some major work the last thing you want is to outlay money then have to put your hand back in your pocket within a few trips what was the mechanics opinion on the test i guess the other thing is are you getting the boat at the right price is the some room to spend some cash repairing when needed and still walk away with a good boat at the right price
cheers

[Micadogs]

With the input above, I will talk to the seller. The seller is a good genuine guy, he has been nothing but helpful and willing to drive the boat around to mechanics, give the honest results etc. Didn't want to sell the boat without all the right checks being done etc. It's the unknown that we test for, and these results have raised some concerns. I will continue to talk with the owner and see what can be worked out.

Anyone else offer some more insights??

[Brumby]

With the input above, I will talk to the seller. The seller is a good genuine guy, he has been nothing but helpful and willing to drive the boat around to mechanics, give the honest results etc. Didn't want to sell the boat without all the right checks being done etc. It's the unknown that we test for, and these results have raised some concerns. I will continue to talk with the owner and see what can be worked out.

Sounds as though you have the makings of a good deal there - at least if you budget for some work right away you are far less likely to hit surprises further down the track.

Minor side step here - RAG, how about an intro to your avatar model???;D

Good luck with it,

Cheers

[Roughasguts]

RAG, how about an intro to your avatar model???;D

Yeah she's pretty rough ah.

Got a bloody big mouth slimy and wet, prickly to touch and taste horrible, definately not a keeper.

[BM]

Micadogs,

They are within 10% so I would not be overly concerned. Ideally a touch closer to one another is desirable but those numbers would not concern me.

Re, the lower comp numbers overall, this is quite desirable in terms of running on poor grade fuel. The higher the comp, the more susceptible to burning pistons.

Most V6 engines run about 100 psi comp.

I would buy that engine and I would also fit that to one of my customers boats and warranty it as well.... (Of course provided it ran well etc etc. I'm commenting only on the compression numbers)

Cheers

[Noelm]

agree with BM figures are just within spec, and may last for years, but may also crap itself first time ,that is what second hand is, the Motor is not new, and some TLC and maybe a decarb (even though I am not a big decarb fan) may work wonders.

[SunnyCoastMark]

Hey Micadog,
Often a slight variation in compression can be attributed to sticky rings - esp in a motor that has sat for a while. Usually a spray in through the spark plug ports with CRC or WD40 does the trick.

Even if you have to remove the head and de carbonise or deglaze the bores and replace the head gasket - it's a pretty easy job and possibly one you should think about doing (or having done) in any case. It's pretty easy to clean out the cooling galleries/channels and replace the thermostat at the same time.

That way you know all is OK with the top end of the motor. - You can't put a price on peace of mind.

Regards,
Mark

[backlash08]

Micadog, order of cylinder test will make a difference if the starter motor or the battery are a little weak, as you crank the engine the revs decrease as you move from cylinder to cylinder - it can affect the result but as I said, the spead is not too bad
cheers
Craig

[pursuit001]

hi thats quiet a difference there myself working around alot of outboard engines.id have to say shes on her way out normally 5psi difference in a outboard engine is ok. but when you start getting those results.your best of walking away from it as in the long it will loose power run like rubbish and then go bang.
cheers shane.

[polky]

Hi fellas, this compression test was done on my motor. A bit of history on motor bought in Brissy 4 yrs ago c/t 127,126,126, 250hrs on motor. I then put 50 odd hrs pushing vc 4.7. Restoration done on boat motor sat in shed for 9mths, put back on boat and have done 10hrs. Mechs think rings gummed up advice was to fill 80ltr tank with preium fuel spray carby cleaner in pots,give motor a good run on 5000 for a few hrs, do c/test if not good lift head clean or replace rings. Polky. Apprently 310 hrs is stuff all on motor, and doubtfull rings worn that quick.

[FNQCairns]

Polky what engine is it to assume if overall low?? Looks pretty good to me on the surface.

cheers fnq

[Noelm]

polky, does your motor acheive full max RPM with a normal load?? if not, it could very well be carboned up and need a decarb, and then check RPMs and swap prop to get full WOT.

[dnej]

From the internet
David
Compression is the heart of an outboard. If one or more cylinders are damaged due to improper timing settings, lean fuel condition, overheating, or stuck rings from carbon buildup, your outboard is about to become history.

Does a compression test alone always determine that my outboard is in good shape? Suprisingly the answer is "no".

A good compression reading does not completely determine internal conditions. To correctly diagnose condition of pistons, rings, and cylinders, a leakdown test should also be performed.

Using a quality leakdown tester, each cylinder should not show more than 10% loss. If any do, there is certain to be an internal problem in the making. For example, a compression reading of 120 lbs would be viewed upon as a good cylinder by the average person.

This may or may not be the case. If a leakdown test of the same cylinder indicates say...20% to 25% or more leakage, it would be a good bet that excessive wear, scored cylinder, the cylinder scratched, and/or the rings stuck or damaged in a way that does not show up with a simple compression check.

In any case, wear or damage is present and advisedly requires attention before further damage and/or exceptionally poor performance and efficiency results.

Most common effects from diagnosis of excessive leakdown would be in the idle range of the motor. If all other motor functions check out attempting to diagnose an idle problem, then its a good chance that blowby is the culprit. It is advised that leakdown tests should be performed at the beginning of the compression stroke (rings just above exhaust port opening, which is the critical point), and NOT at TDC. Make sure you have flywheel locked down securely when doing a leakdown test. If you try to hold the motor from turning with a socket and breaker bar, you are putting yourself in a dangerous position should the breaker bar slip from your hand. Purchase a compression gauge and leakdown tester to go in your boat's toolbox and check cylinders a couple of times a year to ensure there is nothing unusual going on internally. Using proper grade of gasoline and outboard oil is essential to preventing internal problems. Proper winterization procedures will also do wonders to prolong the life of the internals of your outboard. If compression check shows more than 15 lbs difference on any given cylinder from highest reading of other cylinder/s, or if leakdown test shows excessive loss, you have a problem in the making! DO NOT continue running the motor until problem is diagnosed and repaired! It could result in causing much more internal damage than you already have.

[polky]

Thanks fellas, FNQ 1994 yama 70hp 2st, Nolem will check full revs on weekend, plan to fill with preium fuel and take her on a good long run. Nolem when you say decarbing, is this somthing I can do or should the Marine tech do this. Since the c/test I have had 2 buyers keen to buy boat,both say c/test is ok. But if it not to hard of a task I would like to decarb motor for my own learnings and peace of mind. Again fellas thanks for your imput, Micadogs thanks bud. Polky.

[Noelm]

a decarb is done with a "miracle" can of various chemicals, most say that use the one made by the Engine manufacturer, may cost $5 more, but at least it is the "proper" one, as a general guide, an engine run with the correct prop (that can acheive full RPM) has used quality oil and good fuel, will not need a decarb, but I guess it is one of those "it can't hurt" things so why not? I have seen some absolute miracles from old engines that have used cheap oil, dodgey fuel and bad maintenance, and after a decarb, (and smoking out the entire neigbourhood) the motor went like a dream, but as a general rule I am not a big decarb fan, I guess because I always use synthetic oil, premium fuel and so on, it never seems to make any difference to my Motors, My son in law did my Subaru yesterday, he thought there would be smoke and the car would go like a dream (it did before) but there was no smoke, no mess or any difference!

[Roughasguts]

Don't know about miracle chemicals but my old man use to De-carb side valve motors and un stick the valves by pouring Brake fluid down the carb.

You can get some impressive smoke doing that I can tell you, seems to work un sticking valves too.

[Noelm]

there is lots of those 'do it yourself" things around, one guy reckons that if you get the Motor very hot, then introduce plain old water while it is running (via the manifold) it will do the same thing, who knows huh! not too sure I would be trying anything but manufacturers recomended though, unless the Motor was stuffed anyway! for the sake of $30 why take the risk, kind of like saving $2 a litre on oil by going to a Supermarket or something (don't want to start an Oil war here!!!!) better off to use the proper stuff from new and be done with it.

[Roughasguts]

Yep I'm guilty of the above, well I was playing with water injection on a new Sigma (country boy with nothing better to do) Anyway the car did have more squirt and used around a litre in 2 miles cause I had it hooked straight up to a spare vaccum line in the manifold.

Looked like a steam engine flying down the road but when I looked behind the car the boot was covered in soot.

So yep It works Oh yeah the old Boeing 707 use to be water injected on take off 10% more power and all the smoke you see was due to the water De- carbing.

But they stopped carrying water for that cause it needed to carry more fuel to lift the water, Still scratching me head over that one how come some one took so long to work out it was false economy.

[Noelm]

hehe, must have been a sight, a steam Sigma! maybe Steam power may make a comeback, we could just stoke up the fire half hour or so before we need to go fishing, carry a Coal/wood pile for fuel, plenty of water around when Boating, might be a tad smokey and hot, and polute the air, but at least it will be cheap!, the wood pile could be just a bit inconvenient, as well as the extra Spiders and Cockroaches and stuff in the Wood, but you never know!

[FNQCairns]

Yeah water works wonders and has for near a century, a de-carbon is the go (if there is anything wrong with it) if only for the experience of doing it.

If this is for sale it's good to go! If you have the report and pass it on to the new owner you are totally in the clear, for all we know the mechanics guage could be out, the mechanic down the road might get the same spread around 129psi with his.

Just for interest, it has been known for dealers etc to own more than one guage, the known low reading one and a high reading one can you guess when either may be chosen for use!!!!

Cheers fnq

[Noelm]

that is very true FNQ i was once at a Dealer to check out a Motor for a friend of mine (a V6) and the shonky Dealer ( I am not inferring that all dealers do this by the way) had two gauges to do the test while I watched, there was two guys, one did one bank, while the other did the remaining bank at the same time, not too sure why I "twigged" that something might be a tad dodgey, I asked if I could just test the second bank again, and the guy reached for the gauge, but I asked if we could use the same one on both sides, the "mechanic" went all sort of flustered and said he would prefer to use his own method, but I sort of insisted, and guess what, one bank was way below the other, and I mean really way below spec, just left with the usual "I'll let you know" and never have been back there ever since.

[FNQCairns]

That's a telling story! the whole area come under the weights and measures regulations I would suspect, the only guage a person can trust is their own or a known re-calibrated one.

For $30 Polky can do his own test, takes less than 15 min and on a 3 cyl is about as hard as making 3 cups of coffee.

cheers fnq

[BM]

Yep, a cheap gauge is fine. I have 4 cheapies and one more expensive one. Only bought the expensive one when I was caught out without one and needed it in a hurry.

I'm surprised we are still discussing his engine - its fine!! :) :)

Cheers

[polky]

Good onya BM I agree the engine is fine and I will be readvertising the boat and stating the engine is fine. Polky.