3 cyl Mercury 70 to 90hp conversion?

[an86carrera]

What parts do I need to do a 3 cyl Mercury 70 to 90hp conversion?

Thanks,
Len

[Mark75H]

I don't know which 70 you are talking about, but I think all the 3 cyl 70's are actually different motors than the 3 cyl 90's

[an86carrera]

Oops, it is the 1987 model which as I understand it is the same block from 70 to 90

[Mark75H]

Yep, those 2 are the same

[sosmerc]

The only difference is in the carbs...and possibly the max spark advance. It may be as simple as running a larger main jet and running a few degrees more spark advance at WOT.
The 70 is a weak 90....the 90 is a strong 70 :)
I like these motors, but they are not powerhouses. When Merc went to the one piece top cowl, they bumped the cubic inches up some and they got a little stronger...but then, they later ones didn't idle as smoothly!!

[The Big Al]

I have personal experience with both engines in question.

Carbs and tuner were different. Thats all.

Boysen did make reed valve for this engine. And helps.

Bumping timing does help.

If you are putting engine on a very light boat, find a lower unit with better gear set. that engine has a above 2.? gear set.
A lower unit of a 115 will work and have better gears set.

[The Big Al]

Changing the jets will not help. The carb Ventura on the 70 are smaller big time smaller.


Had engine on a 14ft Panther stick steering boat what a blast:eek:

http://forums.screamandfly.com/forums/attachment.php?attachmentid=113659&d=1156566243

[sosmerc]

Yes, the 70/80/90 all had 2:3 to one gears. The 100/115/125 have 2:1. Otherwise, the gearcases are the same.
I have used the Boyesen reeds for the L3 and L4...I didn't see much top end improvement, but I thought the idle quality was a little better. One thing I did once to an L4 that made a BIG difference on the top end...I simply had my machinist carefully match the exhaust opening of the cylinder sleeve to the opening in the block. The factory porting is pretty crude...maybe it doesn't make that much difference in a fishing motor....but the 115 I built gained alot at the top end...we had a hard time finding a high pitch prop that would keep the rpm's down on the 15 Tahiti it was on....yet still have some decent bottom end for skiing. (I think the port work we did actually HURT the bottom end...though it still idled ok.) I wonder what this would do to an L3?

[The Big Al]

.
I have used the Boyesen reeds for the L3 and L4...I didn't see much top end improvement, but I thought the idle quality was a little better. ?


Same with me, but I did see better torque. Idle was a lot better!

I also had the bigger 90 later what a piece of crap. But it would run on the big end.

Al

[an86carrera]

Thanks to all I just bought a set of ninety carbs. Will be here this week.
Also I am getting a mid section because the steering and tilt is very sloppy so I will be pulling the powerhead and I will check the exhaust porting.

I am running a 12.75 x 22" and can not get it above 5000 but I have alot of room to go on the jackplate.

Thanks again
Len

[Markus]

I have a friend with a 90-hp Merc. Recognizing that the monocast block is not the best performance platform, are there any hop-ups?