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120HP Force questions (prop, tilt and trim, numerous questions)

Rjcamel2355

Contributing Member
Hey guys,
I've got a 120hp force, not exactly sure on the year I am guessing between 89-95, but again that is just a guess. It seems to be more mercury than anything (thunderbolt ignition, lower unit, the usual force carbs though).

Currently, I am turning a 13 3/4" x22P four blade prop. Pushes the boat around 48-52MPH (I don't have a tach installed at this time, but judging by the sound it isn't really revving that high I'd guess around 4000 @WOT)

Can anyone tell me what Diameter/spline count/ pitch prop I can turn? (I'm used to early model johnsons, this is my first non OMC engine)

The tilt and trim works, but I believe it is bypassing somewhere (Running WOT, I can trim the motor up, but it stops at a certain point and won't trim any farther.) I've been told it SHOULD trim completely out of the water (if I were to hold the button long enough)

Running a compression on the engine (cold) #1 - 128lbs, #2 126lbs, #3 130lbs, and #4 130lbs) is this good compression? I wasn't sure what they came from the factory with.

Also, the engine was ran on a pontoon previously, and it has a "spacer" on the lower unit to extend the length (I assume?) can that be removed? and would it be worth the hassle of doing so?

I'm running this engine on an 84 tidecraft spitfire V15 (it's 14.75 I believe, around 800-1000lbs) It has a 6" jackplate also, it is jacked nearly all the way up due to the extension on the lower unit.


Thank you.
 
There is a foundry stamp on the block under the pack and coils.
The center of the stamp is the year.
Really need to figure out the model and year??

The 22p means nothing.
You need to figure out the rpm's at wide open.
The size of the boat and the load affect it too.
I never met anyone who can figure out the rpms by ear??? :)

The trim/tilt isn't designed to come out of the water under load.
It will get to a certain point and stop. Normal.

The normal comp is 145-150# But the thing your looking for is that all are within 5# of each other.
You gauge might be reading low.
Your #s are real good.

I didn't know they put spacers on any Forces?? Not that they didn't I just never saw one???
Post a few pics of the motor and lower unit with the spacer??

Depending on the year?? The drive shaft was one piece. The early 90's they put in a 2 piece shaft but it was only for a short time??
The were made with 2 sizes a 20" and a 25"
 
Just a complete guess on the rpms. Just doesn't sound nearly as wrapped up as the omc I had before.

That's good to know on the tilt and trim thank you for setting me straight on tthat

Installing a tach today, and will be able to know the exact rpms this weekend.

Not sure if the picture is going to upload, but if not I'll post a link .
 
That is an extension.
That's how mercury made the 20" into a 25"
But the drive shaft is a 1 piece 25"
If you can find a lower unit with a 20" shaft you can probably shorten it that way.

Finding a used 20" shaft is unlikely and nobody wants to work on them anyway.


A new cost $500+
 
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Would it really make any difference? In the tilt and trim or anything? I don't mind it per say, just have to be more careful in shallow water.

Do you know the diameter prop I can run? Largest and smallest?

Thank you by the way .
 
Same system on a shorter one.
It should be able to tilt out at slow speeds.
The 13 3/4 is prob the max diameter. An alum prop might be a bit bigger?
But the 4 blade is for power and probably not used on a pontoon.
Hook up the tach and post the results.

Try Michigan propeller they would know what you need.
Make sure you tell the rpm's to them.
 
It does tilt out at lower speeds, just wanted to tilt it a little more for higher wot speeds. And to throw a rooster tail.

I was just looking at going to a 3 blade higher pitch for some more top end.
I'll definitely get the tach installed, and see what I'm getting rpm wise prior to moving forward with it.

With the 4 blade it gets on a plane fast and easy, seems to have tons of bow lift, but the boat is so light I don't need it, it forces the nose down after I'm on plane and if I don't trim up it almost feels like its pushing the nose down into the water.
 
Was looking forward to getting a tach reading this weekend. However, after hooking the tach up, it reads erratically (on any of the 1-6 settings on the back of the tach) at idle it shows no reading what so ever, if you barely touch the throttle it jumps to 3-4K (on a hose in neutral). I'm assuming it has to do with the regulator/rectifier? according to my fish finder voltage even at WOT it's only carrying 12.3-12.4 volts.
 
What tach?
What did you hook it to?

12.3 to 12.4 is normal battery voltage.
Get a meter and measure voltage while motors off(make sure it's fully charged) then measure when it's running.
I think the motor needs to be revved up to activate the charging system.

Might have the wrong tach? How many poles? could be a 12 or 20 pole depending on the ignition??

Pics of the ignition system??

Under the ignition system on the block should be a foundry stamp with the year in the center of the stamp??
 
I'll pull the ignition system off today and look for the stamp (weather permitting)

It's a faria tachometer: Hooked up in this manner "Tach wire from the harness to the SIG terminal, ACC wire from the harness hooked to the IGN terminal, and the ground is hooked directly to the battey"
Tach has settings for
4cyl, 6cyl, 8cyl, and 12pole OB (That's all I can think of now, I will look when I get home to verify)

Motor off voltage reads 12.4 volts, motor running WOT throttle, idle, half throttle, anything it reads 12.3-12.4 (Running it all day, it has never acted weak starting, and I have yet to be stranded.) But I run pretty decent sized batteries.

I will also get the requested pictures too.
 
The only stamp I could find under the ignition system .It was in two places reads 8196(or5)94. So is this an 95-96 motor depending on that second digit?
 
Yessir, It was all bolted onto one plate (took 3 nuts off) and the plate came loose. Those are the only numbers I could find anywhere (but I'm not sure if that was the correct stamp)

Aside from the stamp on the side (not under said plate) that says "made in USA"

There is a "serial tag" on the clamp that goes to the transom, but it's wore off and I cannot read a single thing on it. (If that helps at all)

The tach issue I discovered out of my own ignorance... it says in big black letters on the back "4 Cycle use only) so I'm betting that's why it's not reading correctly.
 
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