1992 Mercury 90HP - Seeking Opinions

[Mscanon99]

Hi All,

I have owned a few boats over the past 10 years, this site is a valued community to me.

I am looking at a boat on Sunday with a 1992 Mercury 90hp motor. I had a 1999 Mercury 115, which was a 4 cylinder 2+2 set up. It was not a bad motor, the 2+2 was a bit funky trying to throttle it at certain speeds, however I have no complaints.

I am seeking some feedback on the 1992 models. The cowl is different, however I am not sure if it is the same setup. I belive the 90 is a 3 cylinder, however not sure.

The PO had the boat 6 years, had regular maintenance and the impeller replaced, so I have that much history at least. I want to do a compression check, so I need to know what it should come back at.

Any other suggestions/opinions of this motor would be very welcomed. I have learned that all motors need to be maintained, however some are better than others. I am hoping some folks with expierence on this motor can share their thoughts on it.

Thanks to all,

MS

 

[Silvertip]

Re: 1992 Mercury 90HP - Seeking Opinions

75 and 90 triples are essentially identical motors from an appearance and maintenance standpoint. They do not have the same funky throttle setup as the 115. They are very sound and reliable motors. Compression numbers are rarely published but anything over 100 pounds and all cylinders within 10% of each other will be fine. They have a 2.34:1 gear ratio so they can twist a prop with a fair amount of pitch. Wide open throttle with an average load will cost you about 8.5 - 9 gallons/hour. 2/3 throttle (4000 rpm or so) should net 6.5 gph or so depending on the boat and load. I have a 75 with a fuel flow monitor.

 

[achris]

Re: 1992 Mercury 90HP - Seeking Opinions

All of what Silvertip said +... They are the worst idlers on the market. At idle they sound dreadful, but are solid engines.... Should give you many years of good service. About the only nasty item is the switch in the oil tank. The magnet falls off the float and the alarm stays on. The only way to fix it is to replace the tank, complete!

Cheers,

Chris...

 

[Texasmark]

Re: 1992 Mercury 90HP - Seeking Opinions

All of what Silvertip said +... They are the worst idlers on the market. At idle they sound dreadful, but are solid engines.... Should give you many years of good service. About the only nasty item is the switch in the oil tank. The magnet falls off the float and the alarm stays on. The only way to fix it is to replace the tank, complete!

Cheers,

Chris...

Exactly why I didn't buy one and bought a triple instead. I mean, why buy a 4 cyl engine only to have it run on 2 when the smoothness of the 4 is most important. I know why they did it but I personally think their design mentality absolutely STINKS.

My Merc manual for later model engines, says anything below 120 and start expecting an overhaul. Also, as weird as it sounds, they say within 15% (not psig, and not 10) on readings. Compression tests are flukes in a sense as you can get a bad reading and it may have nothing to do with a worn out engine; uncalibrated meter, defective measurement seals, mismeasurement techniques, carboned up cylinders-rings.

Mark

 

[achris]

Re: 1992 Mercury 90HP - Seeking Opinions

Exactly why I didn't buy one and bought a triple instead. I mean, why buy a 4 cyl engine only to have it run on 2 when the smoothness of the 4 is most important. I know why they did it but I personally think their design mentality absolutely STINKS.

Mark

Mark,

The 4 cylinder engine, running on 2 at low speeds, is one of the best idlers around. At low speed they are very, very smooth. When I mentioned the rough idle I was referring to the 3 cylinder 90s, which are the exact opposite to the 4s.


Cheers,

Chris.......

 

[Texasmark]

Re: 1992 Mercury 90HP - Seeking Opinions

Mark,

The 4 cylinder engine, running on 2 at low speeds, is one of the best idlers around. At low speed they are very, very smooth. When I mentioned the rough idle I was referring to the 3 cylinder 90s, which are the exact opposite to the 4s.


Cheers,

Chris.......

Well I've been wrong before. What makes them smoother than a 3? My 3 is somewhat smooth, nothing like the tower I once had, but I have 3 big jugs here and when one goes bang something happens. Since the 4 cyls are just the 3 with another cylinder, just don't see how 2 cyl's are smooth.....unless the unused cylinders act like a large flywheel and dampen out the ignition impacts.....which is possible.

I grew up in the days of the OMC alternate firing twins but back then, before the days of high hp engines with jack plates, hot dog props, TT and all the goodies of today forced OEM's to make the rubber engine mounts more rigid a twin could sit back there and do it's thing and you hardly knew it.

Mark

 

[MattinTally]

Re: 1992 Mercury 90HP - Seeking Opinions

I have a 1989 90hp..... blew the foot..... checked compression and after some seafoam, got it up to about 105 each cylinder. This seems very low me. Is it worth spending any more cash putting a new lower unit on it? I was expecting the comrpession to be around120. It started at 90 - 92, but after one seafoam treatment jumped up to 105ish. As the earlier post said, it sounds rough at idle, but revved up sounds great. It smoked like crazy after the seafoam and sounded even better....

 

[Texasmark]

Re: 1992 Mercury 90HP - Seeking Opinions

I have a 1989 90hp..... blew the foot..... checked compression and after some seafoam, got it up to about 105 each cylinder. This seems very low me. Is it worth spending any more cash putting a new lower unit on it? I was expecting the comrpession to be around120. It started at 90 - 92, but after one seafoam treatment jumped up to 105ish. As the earlier post said, it sounds rough at idle, but revved up sounds great. It smoked like crazy after the seafoam and sounded even better....

Your 120 expectation is right at the lower limit Merc says (in the service manual for that engine) of the pass/fail divide.

Since you did the Sea Foam thing and got positive results, it is highly possible that your compression problem could merely be stuck rings. That engine, like the original OMC loopers uses combustion pressure to force the ring against the cylinder walls. If the rings are caked up and stuck, they can't move out and seal the chamber and obviously compression will suffer.

Do some more treatments and give them time to work....like several tank fulls of gas, then worry.

Mark