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-79 Triple 70hp lost thrust revs fine

johanwahlberg

johanwahlberg

Member
I’m back with new Merc triple trouble. (Mariner 1979 3 cyl, 70hp)
Last evening, after putting the boat in water after winter, we went for a test run and suddenly, at 25 knots, the boat slowed and the engine ran away in ”neutral”. We lost thrust completely, no gear forward or backward. Wires still work, engine revs fine, but no gear whatsoever. Got towed home by friendly neighbours.

By the time we got home it was dark so I didn’t get the chance to do any examinations.

Do you guys have any ideas on what to start looking for?

We first thought it was the shear pin, but then we remembered that there is no shear pin on this model, only a friction clutch in the propeller, which should at least leave a hint of thrust at the propeller at low rpm, right?
 
My first thought is spun hub.What type of hub do you have? Solid rubber or the Flo Torq type?The flo torq type are less likely to give any propulsion than the solid rubber.
If not spun hub,then mechanical,that's a lower unit strip down.So hopefully spun hub.
 
haventaclue said:
My first thought is spun hub.What type of hub do you have? Solid rubber or the Flo Torq type?The flo torq type are less likely to give any propulsion than the solid rubber.
If not spun hub,then mechanical,that's a lower unit strip down.So hopefully spun hub.
Click to expand...

I'm not sure but I would guess solid rubber. It sounds older and i don't recall seeing the Flo Torq name in the service manual. How can I identify this and how can I test to see if the hub is spun?
 
Thank you for your suggestions. I have a short time to spend this weekend so I'll need to prepare for the next steps as well.
I'll start with checking if the hub is spinning. I'm assuming the hub isn't spun since I get no propeller rotation at all. So what would the next step be - put it on the trailer and checking the gear oil for metal particles?
What will turning the prop manually with/out forward/backward gear tell me?
 
Stripped teeth off the gears.----Broken driveshaft.-----Stripped splines in the crankshaft.-----Step one is drain the oil.----See if there is oil or water in there.
 
Hi again,
so the spun hub test passed, and so did the transmission oil check - no debris or water. Took the lower unit off and the gears seem to engage, both forward and backward behaves as expected at a first glance (turning the prop and the driveshaft). What doesn't is the flywheel. With the lower unit reinstalled, turning the flywheel clockwise in forward gear doesn't turn the propeller. The top of the driveshaft was dirty but seemed intact. Lots of mud around the exhaust and water pipes but nothing that looks wrong.
Any ideas?

Edit: Here's a photo: https://ibb.co/eXdTbT
Isn't it strange that there are dirt packed into the splines? (And worse on the backside not visible in the photo) Wouldn't the splines in the crankshaft clear the mud from the splines in the driveshaft when inserted correctly?
 
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Found a diagnose: The splines in the bottom of the crankshaft are completely rusted away. The seal is long gone and all that's left is a smooth mess of dry lube and dissolved iron. I guess this is the end.
 
Plenty crankshafts about,lots of outboard breakers.All you need is the part number and I'm sure you'll find one. If you can do it your self,not too expensive,otherwise,deep pockets will help:(:)
 
One warning: Merc used two different firing orders for these motors (1-2-3 and 1-3-2). That one on eBay is a 1-2-3. They both fit, but the wiring has to be changed to make it run.

I have a good 1-3-2 crank if you need it. Similar price.

Jeff

PS: You can determine which one you have by rotating the flywheel and putting a Phillips screw driver in the plug opens to feel the pistons coming to TDC.
 
How difficult is this operation? How long time would it take?
Im not a mechanic and I don’t have access to a proper workshop, only my back yard.
I’ve changed impeller and fuel pump diaphragm, lv electronics overhaul and some basic service, but I’m better with electronics than mechanics. Would I manage?
 
You mean finding out the firing order? Two minutes, tops, with simple tools.

If you mean changing out the crankshaft, it's a major job that requires special tools and experience.

Jeff
 
fastjeff said:
You mean finding out the firing order? Two minutes, tops, with simple tools.

If you mean changing out the crankshaft, it's a major job that requires special tools and experience.

Jeff
Click to expand...

Firing order is printed on the block. 1-2-3. I meant changing the crankshaft. Sounds like deep water for me...
 
fastjeff said:
It IS a big job.

Where do you live?

Jeff
Click to expand...

In Sweden, unfortunately. I really appreciate your support, and I can probably get my hands on a replacement crankshaft off the bay, but even with a good service manual I'm afraid I'd run into so many obstacles on the way (due to inexperience) that I'd soon give up...
 
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