1988 Yamaha 90 HP Compression Numbers

jodee95

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Jul 28, 2010
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Hello,

Can I please get some help identifying compression number on my 88 Yamaha 90 hp outboard. I just did a compression test with the motor warm and all plugs out. I got between 80 and 82 on all 3 cylinder. I understand that I am looking for all cylinders to be in the same range.My question is I have see used Yamaha 90 hp motors for sale that had number over 100? Could this be an issue? Not a ring but crank seal etc? The motor has been running very well and appears to have plenty of power mounted on a 17 ft Boston Whaler and moves it well. I have had to Boat for a few years and I do know the cylinder head was replaced but other than that I have kn knowledge of a rebuild or any other work. I just want to make sure if I have an issue lurking I can get a grab on it in the off season.Thank you for any help you can provide.
 

JoLin

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Re: 1988 Yamaha 90 HP Compression Numbers

The good news is that your compression numbers are even across the board, and that's important. If it doesn't act or sound strange, that tells me it's had reasonable care and is simply wearing out. Worn piston bores and piston rings are responsible for the low compression readings (that means reduced power and increased fuel consumption), but the unseen stuff (like bearings) are also reaching the end of their useful life. Your engine could run acceptably for several seasons yet, but eventually you'll want or need to rebuild or replace it. I'd start svings my pennies.

My .02
 

Capt Ken

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Re: 1988 Yamaha 90 HP Compression Numbers

Try using a different compression gauge. No two will read the same. That is the reason they give no psi numbers to shoot for, only a percentage of difference between the highest and lowest cylinders.
 

JB

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Re: 1988 Yamaha 90 HP Compression Numbers

Three cylinders do not go bad in a 2 stroke by the same amount at the same time. It is much more likely that the seemingly low readings are caused by the gauge, technique, battery or conditions. Perhaps a decarb would bring them up, but it is more likely that a different gauge would read higher psi.

In 60 years of messing with outboards I never, never saw one "wear out". I messed with a lot of 20 to 50 year old outboards.
 

robert graham

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Re: 1988 Yamaha 90 HP Compression Numbers

With the readings nearly the same and it's "running very well", then just use it and enjoy it. Might run a "carbon-removal" dosage of Yamaha Ringfree through a couple of tanks of gas just to maybe remove some carbon from piston rings and grooves. Sounds like you have a healthy motor. Good Luck!:)
 
Joined
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Re: 1988 Yamaha 90 HP Compression Numbers

I know on my gauge The o-ring that seals off threads it had a crack in it causeing lower readings ( leaking compression by) . and just make sure that it is tight enough ( hand tight) . Just my info . and from my past experince.
 

jodee95

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Re: 1988 Yamaha 90 HP Compression Numbers

Thanks for everyones info I did notice this year I could not get the tach over 4300 rpms WOT but like I had said the motors really runs well. I beleive it should top out over 5000. This has got me scraching my head I got another new compression test with the same results.
Maybe it is slowly wearing out as stated earlier. Thanks again to ALL
 

robert graham

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Re: 1988 Yamaha 90 HP Compression Numbers

Thanks for everyones info I did notice this year I could not get the tach over 4300 rpms WOT but like I had said the motors really runs well. I beleive it should top out over 5000. This has got me scraching my head I got another new compression test with the same results.
Maybe it is slowly wearing out as stated earlier. Thanks again to ALL
Have you ever had the carbs cleaned?...4300RPM's WOT is not so good!....Did it used to run 5000+RPM's?, like it's supposed to?
 

jodee95

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Re: 1988 Yamaha 90 HP Compression Numbers

I have never cleaned the carbs. It idles and powers up as it should. It also strange it never stumbles and has never not started.I guess that would be a good idea to do. I am just puzzled by an earlier post "I never, never saw one wear out" I know I am the novice here My question is it possible due to the age of the motor that it is slowly coming to the end of its life cycle? Showing the even lower compression numbers .I did buy another NEW gauge with similar results. If so I would not hesitate having it rebuilt. I can not tell you how happy I have been with the combination 90 hp Yamaha on a17 ft Whaler I love the power to weight ratio and has always been good on fuel. I wish yamaha still made the 2 stroke 90 Hp
 

99yam40

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Re: 1988 Yamaha 90 HP Compression Numbers

like the others have already said, decarb the motor to see if that will bring compression up. maybe just carboned up rings

Or maybe motor is not turning over fast enough or you did not let it turn over long enough to build up all the compression on the gauge.

Replacing the head may have something to do with compression #s, depends on the size of combustion chamber and thickness of head gasket. Just a thought

You did not answer the question on did this motor ever run 5500 RPM at WOT like it should, if not maybe it is over propped.
If it did before but will not now then you have a motor problem.

Over heating the motor and/or running lean due to plugging jets/passages are what usually causes motors to fail.
has this motor ever seen any of this?
Leaking fuel pump dumping fuel into crankcase washing oil off of moving parts is something else that will take out the motor.
Always good to keep up the preventative maintenance to keep motor running properly
 

robert graham

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Re: 1988 Yamaha 90 HP Compression Numbers

Yep, I love my 90C Yamaha, simple, dependable, great power to weight ratio. I agree with JB that these motors don't "wear out" as such, but usually fall victim to some mechanical failure like burned pistons caused by lean fuel mixture caused by plugged carbs, or some other major issue. If your carbs are clean and set up correctly and your prop is correct pitch and diameter for your boat/rig, then you should see a WOT of 4500 to 5500RPM's. A Yamaha Dealer can clean and synchronize your carbs for maybe $200,and check your electrical timing,etc.....could make a big difference in how it runs! Like I say, mine is a 1999 model and it runs strong just like new...can show me 5500RPM's easily, although I rarely run it that hard...drinks too much gas!;)
 
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