1978 Mercury 70hp Worth Messing With?

[Mc Tool]

Yeah, it just seems easier to temporarily move one wire than to move the coils.
I'd put things back of course.
But if when I do that, and #2 then has spark, I know it's not a bad coil.

yep . One of the things that makes a good mechanic ( or any repair man ) is the ability to diagnose the problem and one advantage a multicylinder motor has over a single banger is the immediate proximity and availability of substitute parts for diagnostic purposes ....by all means swap things over to see what it can tell you .

 

[Berdink]

yep . One of the things that makes a good mechanic ( or any repair man ) is the ability to diagnose the problem and one advantage a multicylinder motor has over a single banger is the immediate proximity and availability of substitute parts for diagnostic purposes ....by all means swap things over to see what it can tell you .

I'm learning.

 

[Mc Tool]

I'm learning.

I been at this lark 46 years and Im still learning

 

[Berdink]

Lower unit oil must have been typical gear oil and not marine gear oil, because about a half a teaspoon of clear water came out when I removed the plug.
Oil was black. The plug has a magnetic tip, but I didn't find any metal shavings on it or in the oil.
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I haven't started working on it yet. I just wanted to make sure the lower unit was good.
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I'll start a different thread if I run into anything I need help with.
But from all my searching; the compression being good, the lower unit gears likely good, I feel better about giving it a chance to live again.
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[Berdink]

Not a robust lower unit for a 70 HP.----...clip...

I'm reading the motor is actually only a little over 60hp at the prop.
But yeah, I just took it off.
It's a little light one.
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It's on a 1978 15 foot Britton BeachCraft Trihull.
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I think it being as light as it is - the whole motor being around 190, I think it'll be an advantage for me. Getting older.
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I hope to leak test it Friday.
It looks clean and free from saltwater use.
A couple bang-ups though.
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Oh, I should mention, the mounting nuts for the lower unit came off easy, and the lower unit dropped right off. The impeller is semi soft still.

 

[redneck joe]

looks fine to me.

i understand the older. decided to sell the 26 foot cabin cruiser on a trailer for an 18 foot aluminum with a 50 hp - about 1800ish lbs all in for that very reason and as fast as I'm going down hill it was the correct call.

 

[Chris1956]

Those are some pretty good bangs on the side of the gearcase. You might put a straight edge on 'em and see if there are dents in the casing.

 

[jimmbo]

It is apart, makes no sense to use the old Impeller, unless it is only a yr old, otherwise, replace it

 

[Crosbyman]

not to expensive books ... see the one for 1978 all models

Mercury Outboard Manuals and Models Directory

Directory of Mercury Outboard Boat Motor Models and Owners Workshop Service and Repair Manuals - for pdf Download

watercraftmanuals.com

 

[Berdink]

It is apart, makes no sense to use the old Impeller, unless it is only a yr old, otherwise, replace it

Yeah, I was only mentioning the impeller's softness for verification that someone had been in there not too long ago.

 

[Berdink]

not to expensive books ... see the one for 1978 all models

Mercury Outboard Manuals and Models Directory

Directory of Mercury Outboard Boat Motor Models and Owners Workshop Service and Repair Manuals - for pdf Download

watercraftmanuals.com

View attachment 419964

Thanks, I bought a physical service/shop manual.

 

[Mc Tool]

Thanks, I bought a physical service/shop manual.

yeah there is something about holding the book in your hand .... its the oily finger prints all over the pages of the section pertaining to the issues your having with your motor

 

[racerone]

Remove impeller from the cup.----Let sit for 24 hrs.----Post picture of it.

 

[Texasmark]

On the ignition problem, I'd loosen every connection from/to the packs and once slightly loose, work the wire back and forth which will shine up the contacting surfaces.......then re-secure the locking device....screw-nut. If the (ground) wires under the screw have corroded terminals....which isn't out of the question (especially since there is no corrosion shield like on the pack terminals), what I said will put you back in business, costing you nothing but a little time.

 

[Berdink]

On the ignition problem, I'd loosen every connection from/to the packs and once slightly loose, work the wire back and forth which will shine up the contacting surfaces.......then re-secure the locking device....screw-nut. If the (ground) wires under the screw have corroded terminals....which isn't out of the question (especially since there is no corrosion shield like on the pack terminals), what I said will put you back in business, costing you nothing but a little time.

The way the wires are crumbling as I remove them, including the wires coming from the stator, I plan on rebuilding the whole harness - all wiring - every wire and connection.

 

[Berdink]

Remove impeller from the cup.----Let sit for 24 hrs.----Post picture of it.

Done, I'll get back to you in 24 hours

 

[Berdink]

Those are some pretty good bangs on the side of the gearcase. You might put a straight edge on 'em and see if there are dents in the casing.

Yes, they are pretty scary looking.
However, they are just really significant scrapes. Scraping of the metal itself. Fortunately they haven't deformed the housing itself.
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I attempted a vacuum leak down test on the lower unit after work today. I couldn't get it to develop any vacuum resistance.
So I put my mouth on the hose and blew. Lots of air escaping around the main shaft.
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Google Ai keeps telling me because I get 130+/- compression, if the lower unit gears are good, it's worth moving forward with the fixes.
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Personally, I'm intrigued by getting it going. Will my time and money be worth it? No. But will I enjoy it? Yes. And as I have said before, the lady that gave it to me, along with the 1978 Britton BeachCraft TriHull it's on, said her husband loved this boat. I'm going to see this through with my sincere attempt.
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Google Ai says it's a really good chance this 1978 mercury was installed on the 1978 Britton BeachCraft when sold new. I see no evidence of any other motor on the boat it's on.
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Yes, it means nothing but a neato factor.
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I'm moving forward with, yes, this 1978 Mercury 70hp is Worth Messing With?

 

[Berdink]

Also, I watched a video where a guy said this really is just a 50hp with carburetion and timing advanced (RPM) to get 70hp.
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Interesting, Shop Manual shows the same Cubic Inch from 50hp to 70hp.

 

[jimmbo]

Just because the Displacement is the same, doesn't mean a change in Jets or Timing will turn a 50hp into a 70hp. Porting, Pistons, Exhaust Tuning, Carbs, Reeds, and other differences are things that are often different to get the differences in Hp. As for the WOT Rpm range being the same, that doesn't mean all 3 engines make their Peak Hp at the same Rpm, within that range.

 

[Mc Tool]

Just because the Displacement is the same, doesn't mean a change in Jets or Timing will turn a 50hp into a 70hp. Porting, Pistons, Exhaust Tuning, Carbs, Reeds, and other differences are things that are often different to get the differences in Hp. As for the WOT Rpm range being the same, that doesn't mean all 3 engines make their Peak Hp at the same Rpm, within that range.

Yeah , its not like they stretch a 50hp up to 70hp , they build a 70 hp and then detune it down to 50hp . Some things are hard to change ....like port timing or sizing , primary compression .