Merc compitibitly lower units? Please Help!!

[wakeski]

Hello all I have 1979 merc 90hp with a bad lower unit.(grinds and skips in forward with more than 1/2 throttle) I found one off of a 85hp(not sure of the year). I have both serial numbers is there a way to make sure they fit before I buy it and have him ship to me?
1979 90hr Ser # is 5220684
? 85hp Ser # is 2352299

Thank you very much!

 

[wakeski]

Re: Merc compitibitly lower units? Please Help!!

The Ser# for the 85 is off the lower unit. thanks again for any assistance

 

[Anchor Marine Repair LLC]

Re: Merc compitibitly lower units? Please Help!!

Hi

The 85 was a 4 cylinder in that serial number range. Your 90 is a Inline 6 if it's a '79 (no serial # so it's speculation). They won't bolt up.

The good news is that Inline 6 cylinders always fail on the powerheads, so there's plenty of good used lowers out there.

 

[emckelvy]

Re: Merc compitibitly lower units? Please Help!!

If the 85hp was indeed a 4-cylinder motor, then the lower unit Most Certainly will bolt up to a '79 90hp Inline Six!

The 85hp lower unit has a different gear ratio (2.3:1, think "underdrive"), so if you were to use the 850 lower unit with the prop that's now on the 90hp Six, it would behave as if you had a lower-pitched prop. Which may not be a bad thing, typically most motors are over-propped anyway.

One peculiar thing about the '79 90hp, it has very small ports and although it makes lots of low-end torque, it tends to fall flat on its face after reaching approx 5000 rpm. Just doesn't like to rev like the 1350, 1400, and 1500 motors, for example. But uses a lot less fuel, too!

The point of all this is, if you're propped perfectly for your 90hp now, the 85hp gear ratio will definitely affect your propping. If you have a tach you'll be able to better tell what's going on.

HTH & G'luck with the L/U hunt...........ed

 

[wakeski]

Re: Merc compitibitly lower units? Please Help!!

Ok now I am a little confused? I appreciate your help. If 85 lower unit will fit my 79 90hp inline 6. I am new to props. I understand gearing from motorcycles and bicycles. Are you saying that the 85 lower unit will have more top end gearing but give up a little torque at lower RPM? We are using this boat for wake board/skiing so the 90 was more than fast enough. But We could always use a little more "power out of the hole to pull the skier up to plane"
I need to get a new prop so if I use the 85 lower unit on my 90.

What prop would give us a little more torque willing to lose a little on top end?
Again thank you very much for helping this rookie

 

[Chris1956]

Re: Merc compitibitly lower units? Please Help!!

Wake, The 850 gearcases had a 2.3::1 gear ratio. The 90HP gearcases have a 2::1 gear ratio. If you have the right prop now, and were to install that 850 gearcase on your motor, your top speed will decrease, but the time to plane will decrease.

 

[wakeski]

Re: Merc compitibitly lower units? Please Help!!

Ok thank you very much!!

 

[Anchor Marine Repair LLC]

Re: Merc compitibitly lower units? Please Help!!

OK, well sorry if I was wrong. I just looked up several parts to incude the driveshaft and gear set. Those 2 are different for the mentioned models. I hope it works out for you.

 

[wakeski]

Re: Merc compitibitly lower units? Please Help!!

I found a local 1969 1000 merc motor with a good lower unit. I have a 1979 90hp in need of a lower unit.
Im pretty sure my is a long shaft he is not sure about his. His is off the motor. Does anybody know the shaft lenght of a long shaft?

Will they bolt right up and if so will I need a different prop?

My1979 90 Hp Serial number is 5220684
His 1969 1000 Serial number is 2573572

 

[wakeski]

Re: Merc compitibitly lower units? Please Help!!

Does anyone know the real sizes of short, Long extra long shafts for Merc outboards?
Each person I speak with gives me different sizes. Is there a web site that will tell me this.

Again thank you very much for helping this newby