90's force 70hp questions

[rk970]

I have a 1969 Merc 800 that even though runs really good (compression is 150 +-2lbs) I only have a few years left in the power head until it succumbs to corrosion.
One of my questions is on power ratings, I know the merc800 is 80hp rated a the crank and the force is at the prop. Will I loose top end/acceleration going to the force 70hp.
What year did mercury use the igniton and lower unit on the force.
What is the difference between the 70hp and 90hp.. I can an early 90's complete 70hp and a broken down 1994/5 90hp (in a box)
This is on my 1965 16' pacific mariner that tops out at 38 with a 17P prop.(5500rpm) with the old merc800.
Thank you for any info.
Robert..

 

[SkiDad]

well i would say the acceleration would be similar but the top end will drop a few mph. I think the props are a smaller diameter.

 

[roscoe]

All the 1990's Force 70 hp used the Merc lower unit.

Several ignitions were used in 1990-1992, the last were a merc system.

1993 and later used the Merc ignition with a switchbox with bullet connectors on the wires.

The 70 is a single carb engine.
Very simple setup, easy to tune.

Props are 10.5" diameter. Same as the ones used on Merc 40-50-60 hp motors for decades.

I ran mine for 9 or 10 years, and only had to change water pump impeller and gearcase lube.

 

[jerryjerry05]

My "opinion" the 70 Force was poorly designed.
​The one carb to feed 3 cylinders wasn't their best effort.
It seems like the top cyl. starved for fuel/oil.

I'm sure someone who likes it will disagree with me.

The 90, IF?? fixable would be the better motor.
If the cyl. are in good shape then use it.

Again my opinion.

 

[rk970]

Thank you for all the help.
I found a 91 90hp for $200 with low compression in one cylinder, It has a new starter and the lower unit had all the seals replaced. And another complete 1990 for $40 minus the trim pump...
So since I like a challenge I am gearing up for a winter project.....Both have the duel exhaust lower units. I don't mind tearing one down for over sized pistons.
All brands of outboards have weak areas.. Do these have have any known failure points that I should take extra care with..
Robert...

 

[jerryjerry05]

The only real weak spot is the fuel recirc system.
​The system has a reed and a screen to catch debris.
The screen can clog up and cause problems and eventually cause a piston to go bad.

The next would be the air/fuel mix at idle.
If not set right and checked it can burn up a piston.

When rebuilding: don't replace just the one piston.
Keep the good pistons and change the rings.
Wiseco sells the best setup. But you need their rings and wristpin setup.
Their rings won't fit any other pistons.