1970 Mercruiser 120

I picked up a pretty much turn key boat with a Merc 120, I have two issues with it. It runs smooth and has 140 psi compression on all 4 and excellent oil pressure but lacks power in my opinion. It may well be all that it was every capable of too. It will cruise at 30-35mph, at wide open trottle, it don't have a tach or speedo, so the rpm is a guess right now. I get 33-34mph on my GPS. I like the boat and it's in really nice shape with no rot anywhere. Is there a motor swap option with that drive? Did merc ever use the ford 2.3 OHC? I'd prefer a more modern motor if possible. This is a sea water cooled motor and while it looks fine now, I am sure that over it's 38 years it probably has some internal corrosion. (I had an earlier Merc 110 that had the cylinder head rust away just behind the thermostat housing or water neck, and that was a pretty well maintained motor). I got this boat super cheap, at $500 for a running boat and trailer with a full tank of fuel that I was able to drive home to the local ramp here to load there has to be at least that in parts. It came with 4 or 5 new spare props, a new bimini top, partial enclosure and all new seats. Everything works, with the exception of the tilt up function.

The other problem is that the power tilt won't always go all the way up. It's not hydraulic, since it when it works, it works fine, but when it won't, it won't raise at all. The pump and cylinders both look new. Is there a safetly switch somewhere or am I just missing something here? It worked fine when I first brought it home, and it works fine on the trailer, but it wouldn't work the other day on the water. It's not running at all in the up or trailer positions. It has three buttons for the controls, none are marked.

I see a limit switch outside on the unit, but I don't see that shutting the whole 'up' circuit down. I tried it in neutral and forward too with no change. Is there another switch or could the main controls switches be getting intermittent? Is there a simpler set up or something more modern to control the tilt with other than the three button set up?

Re: 1970 Mercruiser 120

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Re: 1970 Mercruiser 120

Re: 1970 Mercruiser 120

The year is a 1970, the motor and drive all all original but the boat looks pretty much like new. It runs fine as it is, but lacks any sort of acceleration. It's pretty sluggish getting going with just me and one passenger. Compression and oil pressure wise the motor is healthy, but I am a bit leery about it being a 38 year old sea water cooled motor. It didn't come from saltwater, but I don't doubt it's most likely seen some in its day.
I had two an older Merc 110's that rotted out the head right behind the top water neck, both were fresh and brackish water only boats.
The former owner seemed pretty diligent with maintenance, but how much corrosion is just unavoidable over time.
The 120 motor looks to me like the old Chevy Iron duke from a Chevy II or similar. Mine isn't the newer style, more modern motor.
I like the fuel economy of this thing and it does cruise fine, but it would never pull a skier and has trouble in a strong headwind. I have several different props that came with it, the best all around seems to be a 15 3/4" with an 18 pitch, both power and fuel wise. I have a 17 pitch but it revs too high with that one. I did used a hand held tach to check RPMs but I don't care to run this wide open simply since it's an old motor for which I don't have any history on.
I am afraid to be out on the water with it and have a major block failure or something, or worse a problem with the drive or gimble bearing. All boots were new two years ago, I did that myself, the shaft, u joint and gimble bearing are also new. A buddy of mine has a very similar boat which has never left the lake he lives on on PA since the day it was new back in 1971, he's had major bock corrosion problems just last year, his also rusted out the head and flooded the cylinders. The head on both of my 110's and his 120 all rusted through just behind the water neck into the intake or exaust port of the motor.

I tried pushing the two buttons at once, but nothing, when it does this the only way to get the drive up is to go back to the solenoid on the pump and jumper wire the motor up. There is a limit switch of some kind on the outdrive near the one pivot pin, and there's sensor or switch in the tank itself. I can see where the one in the tank is wired in, but don't see where or how the outside switch is wired. Those wires head forward towards the dash. The whole rear wiring harness looks fine. I can't see much at all of the forward harness, even the dash wiring is pretty well hidden, it would take removing the steering completely to access that area as is it's a closed bottom dash pod. There's only a small hole where the wired enter through the gunwale side and the panel bolts up under the chrome bezel beneath the steering wheel, so I can't even remove the switches to check them easily.

How does the rear limit switch work? What circuit does it interrupt? I assume is keeps the drive from going too high while in gear? can it be malfunctioning and preventing all upward movement? If so, can I jumper the switch for test purposes? If it's a bad limit switch, thats an easy fix, if its a bad dash switch , thats going to take some doing both to find a new switch and to get at the wiring and switch.

Re: 1970 Mercruiser 120

The 120 motor looks to me like the old Chevy Iron duke from a Chevy II or similar. Mine isn't the newer style, more modern motor.

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Ayuh,....

That's the Same basic motor that Merc,+ everybody else is Still selling as the 3.0l Today.......

Re: 1970 Mercruiser 120

Just to clarify - the 120, or 2.5 liter; is not the same as the 140, or 3 liter. They are interchangeable, but the engines are different. As I understand it, the 2.5 is the old "Iron Duke", while the 3 liter was not used in a car. (I know there are some exceptions, I believe with old mail Jeeps, but by and large the 3 liter was not used in cars).

I know Bondo knows this, but I think (without meaning to put words in his mouth ) that he means they are essentially and practically the same since they are interchangeable. Right??? Or am I completely wrong in my understanding?

Re: 1970 Mercruiser 120

Right??? Or am I completely wrong in my understanding?

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Ayuh,..... That's pretty darn Close......

As I understand it, the 2.5 is the old "Iron Duke",

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That part can be Debated,......
Problem is,.. It depends on Where you're doing the Debating,...

The Hot Rod crowd tends to call the Pontiac version of the 2.5l as the "Iron Duke".....
That's the motor in the GM Cars,.... 151cid, rather than the 153cid of the Boat motors....
Totally Different motors btw....

Re: 1970 Mercruiser 120

Yeah, I remember reading up on the "Iron Duke" a few years ago and discovering that there is a different definition depending upon who you talk to or read. That's the problem with nick names, they tend to mean whatever the one doing the talking wants it to mean.

Bottom line for reelfishin is to know that there is a difference between the two engines. The 3 liter/140 will give you a noticeable increase in power than the 2.5/120.

Re: 1970 Mercruiser 120

Correct me if I am wrong, but..............

That old 2.5/120 was rated under the old 'crankshaft' rated system.

The newer (current production) 3.0/140 is 'prop shaft' rated.

BIG DIFFERENCE

Re: 1970 Mercruiser 120

Ayuh,.....

The 3.0l came to life in the begining as the 140hp......
Then is was called the 3.0l.....
Which has been "Rated" at anywhere from 130hp to 150hp to God knows What....

Basically, IMHOpinion,....
The HP figures touted by the manufacturers are just something to fight about at the Dock,.....
Or in a Forum.....

So,........ No Big Difference,...... At All......

Re: 1970 Mercruiser 120

I'm not so much interested in the "ratings" either. I just know that my Dad's old boat, a 1984 18' Sportcraft, originally came with a 3 liter engine. It had reasonably good acceleration, etc. He had someone replace the engine years later and they put in the 120 /2.5 liter. That thing was a dog compared to the 3 liter! You could tell a definite difference in performance in that boat.

Re: 1970 Mercruiser 120

reelfishin,

What size, type is your boat? I have a 73 SeaSwirl (old heavy 16.5' tri-hull, 3200# including trailer) with a 140HP Mercruiser and the only time it felt underpowered is if the boat was heavily loaded (1000#) or way up in elevation, 9000'. It cruises 30+mph at 1/2 throttle, 3300rpm, with a 15 x17 prop, had a buddy skiing, fighting heavy seas, no problem.

I blew the engine recently and during tear down found a few of the cam lobes worn and a lot of cylinder, so I imagine my ol' worn out 140HP probably had as much power as your 120HP,,, if the 120HP is in sound mechanical condition.

When I got my boat I had to go through all of the systems to make sure everything was in good condition, 30 years of wears and tear... I bought it from a buddy who wasn't too mechanically inclined, so things NEEDED to be fixed. Have you checked out the ignition, springs and weights in the distributor might be rusty and not advancing, plugs, wires, points, condenser, cap, rotor, coil, dwell, timing? How is the carb, fuel filter, fuel pump, hoses and lines? It might need a good tune-up.

Re: 1970 Mercruiser 120

fishrdan said:

reelfishin,

What size, type is your boat? I have a 73 SeaSwirl (old heavy 16.5' tri-hull, 3200# including trailer) with a 140HP Mercruiser and the only time it felt underpowered is if the boat was heavily loaded (1000#) or way up in elevation, 9000'. It cruises 30+mph at 1/2 throttle, 3300rpm, with a 15 x17 prop, had a buddy skiing, fighting heavy seas, no problem.

I blew the engine recently and during tear down found a few of the cam lobes worn and a lot of cylinder, so I imagine my ol' worn out 140HP probably had as much power as your 120HP,,, if the 120HP is in sound mechanical condition.

When I got my boat I had to go through all of the systems to make sure everything was in good condition, 30 years of wears and tear... I bought it from a buddy who wasn't too mechanically inclined, so things NEEDED to be fixed. Have you checked out the ignition, springs and weights in the distributor might be rusty and not advancing, plugs, wires, points, condenser, cap, rotor, coil, dwell, timing? How is the carb, fuel filter, fuel pump, hoses and lines? It might need a good tune-up.

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The tune up is fresh, the distributor is working and the compression is even. It 'runs' fine with no real signs of any missing or rough running, but it feels 'heavier' than it is power wise. I am mostly comparing this to my 90hp Starcraft with an outboard that jumps up and goes like a rocket. The I/O boat is about 800 lbs heavier, but it's also supposed to make more power. I don't think that the motor is lacking much from when it was new since it does still cruise at a decent speed. It just feels like it needs another 50 hp or so to run like I'd want it too. I don't think that I have any serious problems, but would like to upgrade the motor if I can. The brochure lists both a Mercruiser 120 and 140 for that year, so I take it either was available then?

That leaves a simpler question, will the older drive handle a larger motor? 140hp (3.0L)?

What is the 170hp/3.7L 4 cyl.? That looks like a totally different motor again? A buddy has one in a 1985 19' bowrider, his boat runs away from mine, I can't even begin to keep up with that boat.

My boat weighs nearly exactly what the 1970 brochure says it does, so I can rule out wet foam or actual excess weight some place too.

I guess what I am really after is that if this does end up with a corrosion problem, can I use a passenger car head? Or is the marine version different in some way?
I just don't have much faith in these motors after having so much trouble in the past. When I had my other boat and saw how the head had rotted out, there just isn't much metal there to afford any rust at all. The 'wall' of cast iron that separated the water jacket and the cylinder head port was only about 1/8" thick at best, so even with proper flushing, I would guess that a 30 year old sea water cooled motor is bound to have some corrosion there or elsewhere where it can't be seen.

Has anyone else had problems with seawater cooled 120-140hp Merc. motors?

Re: 1970 Mercruiser 120

That leaves a simpler question, will the older drive handle a larger motor? 140hp (3.0L)?

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Ayuh,..... Same Drive.....

What is the 170hp/3.7L 4 cyl.?

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Ayuh,... They're Junk,+ Won't Fit,..... Don't even Think about it....

can I use a passenger car head? Or is the marine version different in some way?

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There is NO automotive head that will Fit,....
These are Marine/ Industrial motors....

Re: 1970 Mercruiser 120

Bond-o said:

.......There is NO automotive head that will Fit,....
These are Marine/ Industrial motors....

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Several after market parts books all list it this way:
* 1962-1969 Chevy II
* 1967-1971 Postal Jeep
* 1968-1974 Marine

All of the internals are the same as the 62-70 passenger car engine. The headgasket and exaust gaskets are different in the marine gasket set and the water neck is a different part number for Mercruiser applications. The after market lists some head gaskets the same, I suppose there are some that won't hold up to saltwater, so those aren't listed for the marine motor. The crank is the same for the Chevy II, GMC and Chevy Vans, and Jeep motors, as is the harmonic balancer. Mercruiser lists only one drive adapter for all, fron the 153 up to the V8 small blocks.
The only thing I don't see is whether or not the actual head casting is the same. If I did decide to redo or replace the motor, I would build it myself, I just was hoping that someone here may have had one apart and knew for a fact whether they they were beefed up in anyway for marine use. I did find some major differences in a few older Chevy marine small blocks, but only in the 400ci motors. Every other small block V8 I've had has used standard passenger car castings and internals.
Even the cam shaft in the 153 Chevy II and the marine motor are listed as the same, as are the after market cams.

There is also some similarities with the Merc 140 or 181 CI motor, but I can see that the crank and rods are different, as are the pistons and rod bearings.

I took a better look at my buddies boat, the Merc 170 is an aluminum block, and the cylinder head is a Ford 460 head as are most of the bolt ons. It don't look like too bad of a set up and it's served him pretty well for all these years with I am sure very little maintenance on his part. It sure makes a lot more power than mine does and is in a much heavier boat. He runs circles around me. I can see where a head change would be in order to save some weight though, they used an alloy block and iron head, I would have thought that an alloy head would save loads of weight and gain some performance as well.

His drive unit doesn't look much different than mine does? In fact, other than a few minor details, they look the same. His is an Alpha drive, mine is earlier. His is minus the small eye hole or loop atop the upper drive, and the tilt and trim pump has a different solenoid set up and uses less wires. The exhaust boot to the drive inside is different, but if I was to make a change, wouldn't that just have to match the motor?

From what I can see the 170 also mounts about the same. I am a big fan of the Ford 460, and from what I can see the 170 through 225 all use 460 internals other than it's own crank and block. What was the problem with those?

It also looks like a V8 swap would be fairly easy with the 153, leaving me to only need to relocate the mounts, but I don't want to add that much weight or ruin my fuel economy that bad.

It looks to me that the only thing I'd have to deal with to install the a Ford 2.3L in marine trim would be to find or fabricate an engine adapter for the drive and connect the exhaust boot to the drive?

One reason I asked about the 2.3L Ford is that I have an abundance of those here and would love to find a use for them. It's a motor with a good power to weight ratio and good torque and plenty of after market options for it.

It's starting to sound like the whole boat is more trouble than it's worth if the motor was to fail. I've got just under $600 in this thing right now, but if it was to lose a motor, I think I'd have to consider it as $600 or more worth of parts. It has all new seats, new cables and a good trailer, so I did fine either way. I'd just swap the interior into another boat and part what was left out. The lower unit and other misc parts has got to be worth far more than I paid. What would hurt is that its such a clean old boat and its really good on gas. Even my bass boat with a 50hp outboard uses far more fuel to run the same stretch of river and back. This thing runs all weekend, on well under 20 gallons of fuel. That's its best feature with the way fuel is lately, the last time I filled up at a marina last fall gas was well over $4 per gallon, I've been told a few are gouging as high as $6 now on the water. I bought a rolling fuel pig that can haul and pump up to 25 gallons, so I can fill the tank at the dock if needed. I really got it for my premix outboard boats, it saves guessing on how much is in the tank, I just mix in 25 gallon quantities.
It's getting far to expensive to run a bigger motor these days, even at pump prices of regular gas I can easily spend $75 or more in a day out fishing. With the I/O boat I can cut that to about $20 or so and not have to buy 2 stroke oil.

Re: 1970 Mercruiser 120

From what I can see the 170 also mounts about the same. I am a big fan of the Ford 460, and from what I can see the 170 through 225 all use 460 internals other than it's own crank and block. What was the problem with those?

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Do a search on this forum for 470.
That is the style of the 3.7. It's NOT a good choice, you will also be looking for an exhaust tube, different gear ratio for the outdrive (no, a prop changed won't do it).

Re: 1970 Mercruiser 120

Bond-o said:

Ayuh,..... Same Drive.....


Ayuh,... They're Junk,+ Won't Fit,..... Don't even Think about it....

There is NO automotive head that will Fit,....
These are Marine/ Industrial motors....

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About junk i dont know but i know they fit. I have a 120 1983 and i use a outdrive 3.7 ( dont know year ) and work just fine

Re: 1970 Mercruiser 120

eddiekeller said:

About junk i dont know but i know they fit. I have a 120 1983 and i use a outdrive 3.7 ( dont know year ) and work just fine

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Ayuh,... Welcome Aboard,....


You're confusing the issues,... This discussion is about the Motors, 'n transom assemblies...
This discussion also is darn near 4 YEARS OLD,...
Don't be postin' in the archives, they're for Learnin', 'n Not posting too...

Also, yer runnin' the Wrong Gear Ratio...