1985 90HP Merc Inline 6 Outboard - Opinions?

[bchaney]

Hey guys, I just checked out a boat with this motor that I'm considering purchasing so I'm hoping to get some more info on it to decide if I should proceed or stay away from it. I have read thru a lot of great info on this forum and elsewhere, and what I gather is that this is a good model year but the 90hp version doesn't rev as high due to airflow restrictions and was more of a workhorse for pontoon boats, as opposed to the more sought after 115. True?

The motor is on an 18' aluminum bowrider - will it be underpowered? I'm going to test drive it Friday so I'll know more then, but if its a terrible setup then I can cancel the test drive. Here's more info on the boat itself - https://forums.iboats.com/forum/gen...985-sylvan-sportster-w-merc-90-l6-ob-opinions

The middle carb was leaking fuel when it was running in the driveway - he tells me the merc dealer mechanics said it wasn't a problem but to me it sounds like it needs a carb rebuild and maybe a float adjustment or new float. What do you think? The owner says that the mechanics replaced the stator and 2 ignition switches. He may be using the wrong name for the parts replaced, I think of the ignition switch as the key receptacle and there's only 1.

Is there any way to check the oil in the lower unit on these motors without draining it?

Any opinions on this motor - is it OK or should I stay away? Is it reliable or am I going to have to tinker with it often? I'm pretty mechanically inclined so I plan to work on it myself when necessary but don't have a ton of time for it. I plan to do a compression test on it. Anything else I should check if I proceed with the test drive? Thanks!

 

[racerone]

They are a great motor when properly maintained.----If you buy it then install a new impeller before using it.

 

[jimmbo]

That cowl says 86, the 85s had ugly brown stripes


I had an 84 115, for 14 yrs. While it was far from trouble free(carb flooding, switchbox, trim pump motor, and bogging) it is still my favorite engine
The 90s were great engines for low end torque. and you are correct bout them running out of breath at speeds above 5000 rpm. The engine WOT range is 4500 - 5000, but it makes its best power around 4700 - 4800 rpm
On a light 18 ft boat, speeds between 34 - 39 mph would not unreasonable

 

[bchaney]

Thanks for the responses. Looks like you're correct it's an 86, I verified with the serial number. The seller sent me the paperwork on the recent engine work, they replaced the stator and 2 switch boxes as well as lube, which I assume is oil for the lower unit.

jimmbo what made it your favorite motor?

Are they pretty reliable? Parts available and inexpensive? Easy to work on?

Any thoughts on leaky carb?

 

[roscoe]

I've got a 90, and a 115.
They run well, look nice, parts available, nothing is cheap for boats though.

My 115 is a bit down on power because previous owner did some repairs, and put on manifold and carbs he had sitting in his shop. Never did determine what motor these carbs came from, but the guy was a wizard with motors and made it work. Bit still runs great.

Leaky carb is typical, especially considering vintage of motor, carb rebuild should fix it.
Possibly just needs a gasketl.

 

[jimmbo]

Thanks for the responses. Looks like you're correct it's an 86, I verified with the serial number. The seller sent me the paperwork on the recent engine work, they replaced the stator and 2 switch boxes as well as lube, which I assume is oil for the lower unit.

jimmbo what made it your favorite motor?

Are they pretty reliable? Parts available and inexpensive? Easy to work on?

Any thoughts on leaky carb?

It was quick on my 15ft crestliner topped out at 49mph. It was a prop rated 115. My crank rated 90 did 38 on the same boat. My crank rated evinrude 140, did 47-48 on a good day on that boat

Reliability... like anything mechanical/electrical, if neglected it will fail. Parts are still availabel, but Mercury is raising the prices of the internal engines into the Stratosphere. I think they are trying to discourage repairing/rebuilding of these 2 stroke and are trying to force people to buy a newer, cleaner 4 stroke.

Ease of working on... Well, that depends on your mechanical abilities, they are quite simple but sophisticated engines, The ADI ignition is quite easy to fix, once you figure out which part is fried, Trigger, stator, switchboxes, wires, plugs, wiring harness, ignition and safety switches are what make it it.

The carbs... the design was first used back in the 50s, maybe even the 40s, and again, simple but sophisticated. The floats on my engine had a tendency to get stuck in the fuel bowl and not rise to shut off the fuel going into the carbs thus flooding.
There is a feature on the carbs that should be disabled to help the engine survive todays garbage that is sold as gasoline. There is a short hose(less than 1/4" dia) on each carb, that goes from the top of the fuel bowl cover, over the top of the carb, to a fitting on the port side of the carb throat. Disconnect that hose from the fuel bowl, find a small ball bearing to plug the hose, small enough so it can be pushed into the hose, but big enough to plug it. With the hose plugged, reinstall it on the fuel bowl fitting.
This hose is part of auto lean circuit of the carb, designed in the mid 70s to lean the fuel-air ratio at mid rpm cruising speeds. It worked very well, but once the gas refiners starting adding oxygenators to their products, the resulting fuel mix is too lean and that can lead to various engine failures

 

[bchaney]

Well the guy sold it to someone else before the test drive. Very disappointing. Thanks for the good info though, maybe I'll find another one.