OMC/Johnson 120 outboard RPM / Mounting

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PSiedTSi

Seaman Apprentice
Joined
Jun 12, 2007
Messages
36
I have an 87/88 Four Winns 160 freedom with the 120hp OB motor. I supposed I bought it, oh 7-8 years ago. It's been a great boat but I recently got curious about the RPM it was supposed to be running. My tach goes to 6k and some people have been suggesting it should run almost that, if not more. Is that correct? I only run about 4700-5k(always ran this way since I bought it). I generally get the last 300 rpms with trim. The boat rips out of the hole, it's almost hard to trim it far enough to get it to launch without picking up the front end. It's not really a dog by any means as I just walk away from my buddies 200X Maxum 1800 I/O.

Anyway, I have noticed some stuff about raising and lowering the motor on the transom to get it in the right position. How is this done? I've seen people talking about it, but couldn't find great information on it. Someone driving it broke a prop a few years back so we replaced it, and I want to say it was either a 19p or a 21p that replaced it. I know that is a concern and I will check on it this weekend when I hit the lake(how can I tell? I'm assuming it says something on it?). It also has cavitation when turning decently hard. I have a large hydrofoil as well...and I have tried with that both installed and not installed. My concern is both of the issues(RPM and cavitation) could be cause by poor mounting, no?

Any information will be greatly appreciated!
 
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PSiedTSi

Seaman Apprentice
Joined
Jun 12, 2007
Messages
36
Re: OMC/Johnson 120 outboard RPM / Mounting

There isn't a way to do this without an $800 part? lol. I see that you joined and this is your first post...advertisement?
 
Bob_VT

Bob_VT

Moderator & Unofficial iBoats Historian
Staff member
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May 19, 2001
Messages
26,065
Re: OMC/Johnson 120 outboard RPM / Mounting

That outboard is equipped with various mounting positions. Take a look...... you can adjust it by moving up or down a hole. You are correct. You do NOT need a big buck part or a jackplate. Jackplates have their place and it is not on your boat.

If you are cavitiaing on turns then you should actually lower the motor one hole which will decrease speed and rpms...... I suspect the hydrofoil is the culprit based on the hull design.
 
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steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: OMC/Johnson 120 outboard RPM / Mounting

Cavitating in a turn is one of normal reactions that you can avoid by trimming down for the turn
and possibly reducing throttle slightly.Certain props will resist venting as well.
Raising or lowering the motor is a little tricky but doesn't require any special; tools.
I've never done this but feel confident I could.
With the trailer blocked in place: Using the tongue jack you set the motor skeg on a block of wood so that as best can tell motor is supported on the block.Then carefully loosen the mounting bolts leaving the adjustment bolts in place.Remove the mount bolts and slowly loosen the adjustment bolts just enough (don't remove them)Once loosened very carefully raise or lower the tongue jack to slide the motor up or down on the transom.
Hopefully someone has done this and can fill in some actual details.
 
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cdn-pk

Petty Officer 2nd Class
Joined
May 20, 2009
Messages
162
Re: OMC/Johnson 120 outboard RPM / Mounting

Hold on hold on your jumping ahead here.

first determine where your cavitation plate is now:

1) out on level surface motor down
2) long straight edge under boat on keel (6' at least)
3) move straight edge to side of lower unit and determine if you cavitation plate is above or below keel
4) take tape measure and measure distance from cavitation plate to keel (top of straight edge) +/- 1" is the zone we want to be in
if your already above and from what you tell us i would not raise any higher, cavitation will get worse and you risk sucking ait into the water pump at WOT.

if you below and have holes left on your mounting bracket to adjust to then heres a good procedure

1. Park on a level, paved surface
2. Block car wheels so it can't move
3. Motor vertical
4. Place wood blocks under skeg until only small space
5. Remove upper engine to transom bolts
6. Raise tongue jack on trailer until slight load on skeg blocking
7. Loosen, but DO NOT REMOVE lower engine mounting bolts in slotted part of motor bracket
8. Continue extending tongue jack until hole you want lines up with bolt hole
9. Replace upper bolts
10. Tighten all bolts
11. Retract tongue jack
DONE

Credit to wilde1j for this


also don't forget to seal ALL 4 bolts with adhesive sealer 3m 5200 (i think.... can't remember)

cheers
 
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Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: OMC/Johnson 120 outboard RPM / Mounting

Only use 5200 if you don't plan on ever taking the bolts out or moving the motor again.
 
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cdn-pk

Petty Officer 2nd Class
Joined
May 20, 2009
Messages
162
Re: OMC/Johnson 120 outboard RPM / Mounting

thanks dhadley... what other sealant then? 4000 or 4200?
 
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PSiedTSi

Seaman Apprentice
Joined
Jun 12, 2007
Messages
36
Re: OMC/Johnson 120 outboard RPM / Mounting

Awesome, I will have to measure that stuff this weekend and check it out. I think I remember at the mounting adjustment area and it looked like there was one hole left from the top and a bunch towards the bottom.

I also noticed I had a 19p prop when glancing at it today. Should I adjust that at all?
 
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