low compression 90hp 1989 2 stroke motor

[eisenralph]

purchased 2 years ago/boat has been sitting for years/motor very clean. Put in water twice last summer/not much time for boating yet/first run-found plugged discharge/took home and cleared/second time-took for a good run/motor idles poorly/sluggish at low rpms/once up and planing ran smooth/45mph and 450rpm and up/ back at the dock/stalled out. Starts and runs but need to keep the revs up. Checked compression/only at 85lb.wet-checked with throttle open of course. Tired rings??.Checked cyl.walls and they are smooth and clean. All fuel/vac lines are good but can blow and suck from line to block. Is the check valve causing a problem?? Clymers manual somewhat vague. Could a faulty reid valve cause the problem. All cylinders are low. Would like to know any other possibilities before I pull the motor off and apart to do the rings should it be something else? Thanks for any help you can give me. Motor in question is clean and almost looks new/3 cyl/3 carbs/90hp

[200090tlry]

seems like low comp reading

Try a full decarb, run sea foam in tank, mercury quick cleen works pretty good. Then once your done spray some deep creep in cyl's try and loosen a stuck ring. Next I would look at head gasket. But first do cleaning then try comp readings again, dont think you need to open throttle on 2stroke get readings. Also make sure motor is warmed up before checking. Something you should know never use automotive spray's like carb cleaner on outboard it washes away oil in cyl, use marine products and sea foam is good.

[robert graham]

After sitting for so long, my best guess is that the carbs just need to be disassembled,cleaned and set, probably varnish and debris in the jets, etc. You could buy a manual and the carb rebuild kits and try to do it yourself or have it done by a Yamaha technician. Motor should run wide open throttle in the water with a normal load at about 5500RPM's, if it's set up right with the correct prop. My motor is a 1999 Yamaha 90HP, and if you can get it clean and set up right it's a darn good motor with almost no problems. Install a water/fuel separator type filter to avoid any future fuel/water/ethanol/debris problems. Good Luck!

[johnstanleyho]

Compression gauges can vary as much as 20% I have two and they are my "high" and "low" gauges! Most importantly is the even readings. 85 across the board could mean 95 with another gauge which is not so bad for a 1985 in my book.
The carbs have an idle circuit which is usually the first thing to malfunction. It's covered by a little rubber cap inside the carb. Unscrew this jet and clean it until the little holes permit a decent air stream to pass. Since your motor runs well at high speed, the most likely culprit is a slightly clogged set of idle jets. All it takes is one of the three to cause a little stumble. It's easy to do once you know what to look for.
Good luck

[eisenralph]

Thank you for all the input. I appreciate the effort taken. Always nice to have a good running motor when you are out in the middle of the lake.

[robert graham]

After you get your carbs cleaned and set and the motor is running fine, start adding the yamaha Ringfree in the gas in the "deposit removal" dosage for a couple of tankfulls to remove most carbon, then drop it to the "deposit preventive" dosage from then on, to keep carbon off rings, ring grooves, pistons, ports, etc. Also learn how to drain your carbs once in a while to remove any water or debris that may accumulate in those float chambers. It's a very easy 5 minute job on the 90HP and great preventive maintenance. Good Luck!

[rodbolt17]

wet test on a two stroke is a teat on a boar hog.
what are the actual numbers?
I actually went through this monday on a 2003 115.
customers gauge showed 105 on all 4, mine varied between 135 and 140 on all 4.
my leak down test showed less than 10% on all 4.
was a difference in guages but the leak down wont lie.
cleaned the carbs and all was happy happy.
we did the tests within 20 mins and I watched him with his.
what your looking for is the variation with the same tester.

[200090tlry]

My 96 force only had 105 105 107

Maybe my gauge, dang were did I hide that sucker lol I cant find it. I will do another post on where to mount water fuel separator on my crestliner and should I go 10micron or cheaper unit.

[robert graham]

Forget the compression for now, get carbs cleaned and set, walmart sells a pretty good water/fuel separator for $27. Good Luck!