1978 Mercury 70hp Worth Messing With?

[racerone]

Has to do with factory / production / economy.----And prop rated verses powerhead rated power.

 

[jimmbo]

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 .

Actually the opposite was the case for decades, more hp was squeezed out of a given displacement, just about every model yr. Some examples of many were; OMC's 70.7 inch block was originally, a 50hp, later becoming a 60, and finally a 65. Same went for their 2bbl 89.5 inch block, 75hp, then 80, finally 85. A 4bbl version of the block, 90hp, then 100, and as a limited edition 115. The 49.7 inch 3 cylinder Looper started as a 55, then 60, 65, 70, and 75.
Mercury also did the same. the 99.9 inch inline 6 started out as 125hp, later 135, 140, 150, and 115. The 4 cylinder 66.6 inch started as an 80hp, later becoming an 85. Prop Rating dropped it to 75hp. Merc's 3 cylinder 49.6 inch block started as a 65hp, became a 70, and a highly modified version was rated at 90hp

 

[Chris1956]

The gearcase should be pressurized to 10 PSI or so. Another case of AI being wrong.
The driveshaft seal in the waterpump base often fails. It is easiest just to buy a new waterpump base, with the seal already installed. Clean the driveshaft with fine sandpaper and lube it with oil before installing the new pump base. Make sure to use the correct oring and pump base gasket.

Driveshafts of that era often have tapered bearings. You might push down on the driveshaft to load it and see if it has any play. Does the end of the driveshaft have a spring-loaded pin?

 

[BWR1953]

I had a pair of '82/82 Merc 70hp engines, one had been running when I got it years ago, the other wasn't. I did the carb rebuild on the running engine a couple years ago and found out that the fuel bowl float was no longer available, even in the aftermarket. I bought a couple of them using the correct part numbers, but they were completely different from the ones used on my Mercs. There were no correct parts available anywhere. I put the old floats back into the carbs on the previously running engine and got it running again. Then sold it. The 2nd engine I gave to my friend to use for scrap metal. And I highly recommend switching to ChatGPT. The Gemini AI that Google uses is an idiot and will lie right to you. I dumped it a couple months ago. Best of luck to you.

 

[Berdink]

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

I will be putting in a new impeller.
I was only mentioning it's softness as possible evidence someone had been in there in the recent past.
But yes, it needs replacing.

 

[Berdink]

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.

Sorry, yes, I should have known that. It's similar to my Johnson 90 having the same ci as the 115.

 

[Chris1956]

Yes, cracks in the rubber give it away.

 

[Mc Tool]

I think somebody here would get another 12 years out of that ....just keep an eye on your water pressure gauge

 

[Berdink]

I've been digging into this and cleaning things up. As mentioned earlier, it has good compression - 130+/- at each cylinder. Mechanically it's looking good, and cosmetically it is nice also.
Sometimes you have to determine at what point is the teeter point between it's too old to be worth it and it's nice enough to keep it going.
I'm not sure if I mentioned it in this thread or another, but this 1978 Mercury is on a 1978 Britton BeachCraft. I pulled the the motor off the boat today, and as best I can tell from the bolting and wear patterns, this boat came with this motor.
.
People dump money into classic cars and never recover their time and expenses - I know for my 65 Chevy Truck I never did, but it was still fun.
.
I washed up the boat some, and cleaned up the motor some.
I think it's going to be a nice looking survivor when I'm done.
.

 

[Texasmark]

I've been digging into this and cleaning things up. As mentioned earlier, it has good compression - 130+/- at each cylinder. Mechanically it's looking good, and cosmetically it is nice also.
Sometimes you have to determine at what point is the teeter point between it's too old to be worth it and it's nice enough to keep it going.
I'm not sure if I mentioned it in this thread or another, but this 1978 Mercury is on a 1978 Britton BeachCraft. I pulled the the motor off the boat today, and as best I can tell from the bolting and wear patterns, this boat came with this motor.
.
People dump money into classic cars and never recover their time and expenses - I know for my 65 Chevy Truck I never did, but it was still fun.
.
I washed up the boat some, and cleaned up the motor some.
I think it's going to be a nice looking survivor when I'm done.
.
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If that's a 15' Tri-hull that's a good match. Back in the early '70's when they were popular, lots of folks had that length with a 60 HP. Could pull a skier with a couple of folks in the boat at a good enough speed for everybody to have a good time.

 

[Mc Tool]

I've been digging into this and cleaning things up. As mentioned earlier, it has good compression - 130+/- at each cylinder. Mechanically it's looking good, and cosmetically it is nice also.
Sometimes you have to determine at what point is the teeter point between it's too old to be worth it and it's nice enough to keep it going.
I'm not sure if I mentioned it in this thread or another, but this 1978 Mercury is on a 1978 Britton BeachCraft. I pulled the the motor off the boat today, and as best I can tell from the bolting and wear patterns, this boat came with this motor.
.
People dump money into classic cars and never recover their time and expenses - I know for my 65 Chevy Truck I never did, but it was still fun.
.
I washed up the boat some, and cleaned up the motor some.
I think it's going to be a nice looking survivor when I'm done.
.
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Cool wheels on that ute

 

[nola mike]

I bought this:
.

Amazon.com: Electronic Specialties - Leads (640), Factory : Industrial & Scientific

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.
Will that do the same thing?

I've got the same one but was a bit cheaper when I bought it. Works fine

 

[Berdink]

I'm thinking the Stator and Trigger were original. The wires just crumbled away as I removed them.
I didn't want to risk it. I bought a new Sierra Stator and Trigger. They should be here Saturday.
I'm building a new Wiring Harness.
I stripped the motor down then degreased cleaned the motor.
I'm cleaning things up as I put things back on.

 

[Mc Tool]

If its the same old rubber insulation we used to use it here decades ago to wire up houses , called TRS ( tough rubber sheild ) and it used to give me the 5hits , specially on oven installs usually 2 phase with 400vac across them and the stuff would just crumble as soon as you moved anything connected to it . Wasnt unusual to see 3 bare wires ( that used to be a 2 core and earth cable ) in a wall or ceiling cavity with a wee pile of rubber underneath them

 

[Chris1956]

Check your spark plug wires. At the age of your motor, the insulation can break down. The conductor is stranded stainless steel. The boots and springs can be replaced for cheap. If the springs are rusty, it makes sense to replace them.

Are the coil ends push in? If so, you could make up a set from bulk Merc-style wire pretty cheaply. Likely Sierra or CDI make it.

Other than that the motor looks real good, tilt tube not withstanding.