Re-powering a 1978 21' Starcraft Mariner - Engine Help!

renoboater

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Hi - I recently purchased a fully restored 1978 21' Starcraft Mariner. The existing engine on it is a 1979 Johnson 140hp. I want to re-power to a 2 stroke direct fuel/oil injection or a 4 stroke so I will be able to take the boat in the lakes in the tahoe region. The boat is rated at a 150hp and has a dry weight of only 900lbs. The current Johnson weighs in at about 310lbs. I found a few newer 150's but all are about 460lbs - quite an increase. I'm concerned on the additional weight. Any suggestions? For comparison - a 70hp 4 stroke is about 310lbs - half the horsepower! Thanks! Chris
 

renoboater

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What's wrong with the existing motor? With maintenance it will out last all the new motors
They do not allow 2 strokes that don't meet 2006 emission guidelines on the lakes in the Tahoe region - so it severely limits where I can take it. I had a 1984 Evinrude 150hp on my 22' center console that I sold to my brother when I moved - that engine ran beautiful - just winterized it and tuned it up each season. Unfortunately if I stick to the 140hp I am just a little more limited..
 

Chris1956

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The only thing I can think of is to get a newer 4 cycle/DI motor with near the same HP, but fewer cylinders. Your current motor is a 4cyl, with about the max HP that could be had. It will be hard to match that power/weight ratio.

The brand new motors are all 350#+ for much less HP than you have. Maybe you can find an older one that still meets the emission standards (mid 2000's Optimax in 115HP or so), and is lighter? It is a guess....
 

renoboater

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The only thing I can think of is to get a newer 4 cycle/DI motor with near the same HP, but fewer cylinders. Your current motor is a 4cyl, with about the max HP that could be had. It will be hard to match that power/weight ratio.

The brand new motors are all 350#+ for much less HP than you have. Maybe you can find an older one that still meets the emission standards (mid 2000's Optimax in 115HP or so), and is lighter? It is a guess....
Thank you - I’ll narrow my search there like you said. The current engine is a short shaft as the transom well is cut low. I’m assuming a heavier engine on a jack plate would be worse as it would create a larger moment due to the height. Appreciate the help.
 

Scott Danforth

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and the jack plate pushes the motor aft about 4-5" so that really squats the boat.

I would stick with a short shaft on the boat.
 

Chris1956

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A 140HP motor with a short shaft would be unusual. You might measure from antiventilation plate to transom clamp. A SS motor is about 15". A long shaft motor would be 20", which is much more standard for that year and HP.
 

airshot

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A 140HP motor with a short shaft would be unusual. You might measure from antiventilation plate to transom clamp. A SS motor is about 15". A long shaft motor would be 20", which is much more standard for that year and HP.
Can't say I remember ever seeing an over 100 hp motor with a short shaft !! Measure carefully as a large motor can make a shaft look shorter than it is..
 

Moserkr

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Hey Reno,

Im not far from where you're at, fairly close to tahoe. They are definitely strict with boats on the lake, especially when you have to pay them to inspect it!

I have a 16' starcraft that I restored. The new motor is a 75hp mercury and it weighs 357lbs. The original motor was a 75hp 2-stroke johnson weighing 260lbs. Point Im making is that your 21' mariner should handle 460lbs easily on the transom if it holds 300+ already. I moved my gas tank forward too and the balance on the boat actually is better now - no more porpoising as I cruise.

Just my .02. Maybe Ill see you out there one day when its calm enough for my little boat haha.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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Can't say I remember ever seeing an over 100 hp motor with a short shaft !! Measure carefully as a large motor can make a shaft look shorter than it is..
plenty out there in the 70s and 80s
 

renoboater

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Apr 7, 2024
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Hey Reno,

Im not far from where you're at, fairly close to tahoe. They are definitely strict with boats on the lake, especially when you have to pay them to inspect it!

I have a 16' starcraft that I restored. The new motor is a 75hp mercury and it weighs 357lbs. The original motor was a 75hp 2-stroke johnson weighing 260lbs. Point Im making is that your 21' mariner should handle 460lbs easily on the transom if it holds 300+ already. I moved my gas tank forward too and the balance on the boat actually is better now - no more porpoising as I cruise.

Just my .02. Maybe Ill see you out there one day when its calm enough for my little boat haha.
Thank you! Makes me feel a lot better that you’ve had success doing something similar! Just look for an orange center console with a yellow interior!
 
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