1986 Mercury 75hp Mod.

[N1265]

Hi Guys :

I just bought another project witch has a 1986 Mercury 75Hp , 4 Cyl. that has no spark. After reading several posts it seem like the first thing to check is the wiring condition at the controls and the tilt switch and I will be doing that first thing in the spring.

Being that this is my first Mercury OB, I am wondering if it would be easy to up the 75HP to maybe a 85HP by simply changing out the jets in the carb, or any other quick and easy Modification ? I would also like to know what the "Achilles heel" of these motors are and if any repair parts ( especially electrical ) are hard to find or grossly over priced . ( that way I can keep an eye out for them and pick them up before I need them )

Many Thanks,

 

[Chris1956]

The Achilles heel is the cooling system and the carb condition. Both cause overheat and cylinder damage.

Do change the waterpump impeller, wearplate and two gaskets every 3 years. if you suck up sand, change it right away.

Clean the carbs and rebuild with new gaskets, inlet needle and seat and new floats.

The ignition system is pretty standard and very reliable. Check engine wiring harness for missing insulation. If she has good compression, replace the engine wiring harness.

I do not know of any way to up the HP. I think the 85HP motor is 4 cylinder, 66 cu in.

 

[Faztbullet]

You have the 67ci (1098cc) Inline 4. Sorry but no upgrades as this motor was originally a 85HP.. then when BIA propshafted rated HP it became a 75. Dang good motor as long as as kept cooled and not dogged on RPM. Prop it to the high end 5800RPM for best performance

 

[racerone]

That used to be an 85 HP then changed to propshaft rating at 75 HP.

 

[Scott Danforth]

I am wondering if it would be easy to up the 75HP to maybe a 85HP

as others stated, if you want to change it to a 85hp, change the rating system back from prop rating to flywheel rating.

other than that, you either need more cubes or you need to look at building a custom engine with custom trumpet exhausts like the racing 1250BP's of the late 60's.

granted you would only have 2 trumpets for your 4-banger (could not find pics of the 850XS's with the trumpets)

you also need lots of engine work.

best to simply use the motor as intended.

 

[racerone]

I had an old Evinrude Big Twin with home made " trumpet exhaust " ------One day I let a friend drive the boat.-------I could not believe the amount of noise coming out of that engine.----Was not that bad when you were in the boat.-----I took that set-up off the next day.-----Still have it somewhere.

 

[N1265]

What year did the start rating OB motors at the prop ?

 

[Faztbullet]

Around the early 80's

 

[roscoe]

each manufacturer switched ratings on their own schedule

Merc for example rated their motor at 50hp in 1985, then renamed it as the "Classic 50" rated at 45 hp in 1986. Some hp motors may have been switched in different years.

Pretty sure all makes, models, hp, were switched by 1990., the last being the Merc/FORCE motors.

 

[N1265]

That's good info to know about the "classic 50" , thanks for posting.