Motor Change on my 1973 Mariner 18'

[MonkeyBird747]

Upgrading from my 1985 50hp merc. I've read the boat is rated up to 125. I found a 2001 Merc 2-stroke 125hp in good shape for $5k. It's a little more than I want to spend, but it will also plug and play with my existing controls. I know 4-strokes are heavy, but have 2-stroke engine weights changed that much in 50 years that I'd worry about going up to the maximum hp rating? Would I be better off with a 90 or 115? Also a factor is only 12 gallons gas on board.

How should I inspect the transom to make sure it is in sound condition before upping the weight to 350lbs. I have no way of knowing when it was replaced last.

Cheers!

MB

 

[SHSU]

I think its going to be hard to figure out what the weight of a 50 year old 125 HP motor would be. Maybe somebody has a chart to give you an idea, but think it would be a guessing game...

As for the transom, the normal checks of the knee brace to make sure you have no cracks and inspecting all the through holes for any signs of damage/rot would be a start.

Other option is to rust replace the transom, then you know exactly how well it has been put together.

Take it for what its worth, but that is my 2 cents....

SHSU

 

[MonkeyBird747]

Thanks SHSU, that's food for thought indeed.

 

[BWR1953]

You can find the motor weights here: https://www.nadaguides.com/Boats/Outboard-Motors

I did a quick look and it seems to me that you'll be fine with the 2001 Merc 2-stroke 125hp.

 

[MonkeyBird747]

Thanks for the link!

 

[DLNorth]

I suspect if you look you'll find they got heavier, 1st with the change to loop charge and then to injection.
The old OMC V4's and the Merc I6 towers were both in the 300 +/- range, when OMC went to the V4 loopers they got heavier, as did the Merc when the V6's came out.
After these generations I don't know specifics but I think they are heavier yet.


"but have 2-stroke engine weights changed that much in 50 years"

 

[roscoe]

2002 125 , 350#
1990 115, 326#
1986 115 inline 6, 315#
1977 115, 300#
1974 115, 270#

A little heavier, but not that much.
I'm running a 1986 115hp and 315# on my Mariner without issue.

You won't see much difference in performance ( if any ) between a 115 and a 125.
And really not that much difference if you would drop down to a 90 hp, as the hull is not fast and there are limits to how fast it will go.

So if you can find a better priced deal on a 90 or 115, go for it.

 

[MonkeyBird747]

Decided to hold off on the 125 and see if I can get something cheaper. The price tag was hard to swallow. I don’t mind an older motor, so I’ll poke around and see if something else comes along. For now I put on a new SS Mach 3 turning point prop. The PO had a 15 pitch aluminum on there, which seemed a little much for this combo. Went with a SS 12 pitch after conversing with TP on the phone. We will see how it does this weekend.

 

[Watermann]

Yeah, that's too much I have a low hour 1995 Merc 115 that is the same motor as that 125. I put it on my SS 18'r but I bought it on a rotten glass boat that had a heavy duty galvanized trailer under it, all for 1500 bucks. So look for boats that have the motor you want on it that people are trying to unload on the cheap. It seems motors that are off the boats are way more money than the rotten package deal.

 

[MonkeyBird747]

Yeah, good idea. I just missed such a deal that had an older ‘82 115 Merc. I’ll keep an eye out.

 

[Watermann]

Yeah, good idea. I just missed such a deal that had an older ‘82 115 Merc. I’ll keep an eye out.

I'd stay with later model motors than the 80's at least 90's for Mercs that is.

 

[MonkeyBird747]

Happy to report the new SS Turning Point prop worked out great. On plane much faster with the 12 pitch, and actually gained 2mph with an extra person and 6 more gallons of gas In the boat than the last outing. Went through about 8 gallons of gas in an hour hour day of fishing, with a fair amount of cruising between fishing holes. Still running basically WOT, as backing off drops off plane. The idle mixture adjustments have it really purring though.

This motor will do until I stumble across a nice used 90hp.

 

[SHSU]

Good to hear!!!! Any pics of the fish or just fishermen tales???? lol

SHSU

 

[MonkeyBird747]

Lol, no pics this time. Just a few perch with eyes bigger than their stomachs.

 

[MonkeyBird747]

Well, I took a chance on a 2003 75hp Merc 2-stroke. One popped up down the road from me for a good price and came with controls. I will more than likely be taking it to the local boat mech for the engine swap, rigging, and electrical. At 300lbs I’m not set up for a diy swap. I’ll need the wiring harness as the PO apparently reused on their new motor? Also need the trim switch installed, and since I have no gauges whatsoever I’d like to at least have a tach gauge installed.

Hopefully the steering controls are compatible. At least they look to be so.

 

[Watermann]

Re-used the wiring harness but gave you the controls and cables, that's really odd?

The steering if remote should just be the tilt tube and steering rod attached to the motor is standard to attach to the end of your existing cable.

It's easy rigging the motor using a lifting eye and come along or even renting a cherry picker if you don't have a structure to use. You know what boat shops charge an hour, you'd be in DIY less than half an hour of their time. I can see them charging 4 or 5 hours and 200% mark up on any parts they use.

It should be a good motor if you keep the boat light, otherwise it's going to start struggling under a heavy load on an 18'r.

 

[MonkeyBird747]

Yeah, I thought it was weird too about the wiring harness. He bought a new 4-stroke Merc, and I doubt the harness was the same. But he had the dealer do the swap, So who knows what they did with the original harness.

The current motor is clamp on, and the new is bolt on. Holes will have to be drilled for mounting. I’ve only ever dealt with clamp on and the though of screwing up the mounting process is a bit intimidating. There looks like two different sets of mounting holes on the motor bracket: upper and lower.

I would want to build a test stand first to make sure everything is good with the motor and go ahead and wire it up to check all the connections. So as not to drill unnecessary holes.

Really it’s just the mounting part that worries me. A little F.U.D creeping in I suppose. I would like to do it myself, so I’ll give it a little more thought.

 

[Watermann]

I had the same issue on my SS, it had non BIA standard outboard mounting holes drilled through so I had to cover them with a plate and drill standard holes. Here's the template.

 

[SHSU]

Sounds like the dealer just cut the harness and trashed it....

Making a little stand isn't that hard, plenty of designs floating around on the forum.

https://forums.iboats.com/forum/eng...327-building-an-o-b-motor-stand-instructional

As for an engine lift, you know a friendly mechanic? Engine hoist work well otherwise you can use a sturdy tree/branch. The thing to think about either way is it goes up easy, but comes down fast. So plan the process before you do it and account for how you are going to control lowering it into place.

In the end, doing the work yourself isn't that hard, plus you know how it was done and where everything went. Makes it easier when you are doing future enhancements/repairs.

SHSU

 

[MonkeyBird747]

Ok, I’ve been inspired. I’ll give the swap a shot on my own. Sounds fun!

i wonder if the wire harness on my ‘85 will be compatible?

Im getting together a shopping list.

• Water pump kit since I’m in the maintenance mode
• marine grease
• lower unit oil
• Carb kits - same as above reason
• 4200 and 5200 adhesives
• epoxy for bolt holes
• Drill template
• Drill jig?
• grade 8 mounting bolts
• some stainless hardware for hole patching (another thread)
• ring puller or similar for hoisting
• wire harness tbd
• gauges. I have no gauges at the moment. I’d like at least an rpm gauge. I don’t know what all else is available for these motors. Temp? Trim gauge? spedo?
• Maybe a MicroJacker

What else would you add/remove from the list? I’ll start digging in to repower threads.


I have a chain hoist already.