Yamaha 55hp lower on 70hp

[maartenk79]

Hello,

I've just put a 55hp Yamaha /Mariner shortshaft lower on my 70 to build a ces style motor. It works but it doesn't pump water.
Should I have used the 70 waterpump in stead of the 55? Or is there anothe issue i'm missing?

Also the exhoust doesnt line up so it uses the ehaust dump above water a lot. Is this a problem?
I'd like to run the smaller lower unit to get away from the 2.33 gears and the drag the big foot causes.

Thanks in advance

[maartenk79]

The lower unit is on and works. But damn, it's loud! I'm in a populated area so I have to work on the exhaust to line up.
Tried a chopper2 18" and got 50+ mph at 6500, not bad.

[Hounddog]

You likely are not running the short shaft muffler. That is the reason for the noise. The muffler is still available. The 70CES model had no muffler. The 50G and 70B short shafts had the muffler. You will have to get the part from Japan, mega zip part sells for approximately $145.00 US. 6H3-14711-01-CA

[hydrospeed77]

I run a 55/60 2cyl lower on a 70hp 3cyl. It is also loud. I am pretty sure it is that the gearcase casing has a wall/baffle in a different location from the stock 70hp lower, does not line up with the 70hp mid baffles. This allows some of the exhaust to flow to the back of the top of the gearcase and exit the vent holes above the cav plate (you can see in the above pic).

Or, if you are running a surfacing prop at surface piercing height, the exhaust won't be fully muffled by the water.


Here is a pic of a strange 2cyl lower that has both baffles/walls. 2Cyl location is in between the bolt holes, the 70hp 3cyl location is back by around an inch:

[maartenk79]

Here is a pic of a strange 2cyl lower that has both baffles/walls. 2Cyl location is in between the bolt holes, the 70hp 3cyl location is back by around an inch:

Mine only has one baffle. I'm a bit puzzled by the one with two baffles as the exhaust gasses have nowhere to go.
I was thinking about putting in the extra baffle at an angle and remove part of the front one to direct the gasses to the exhaust.

This is the 70hp 15" big lower


And my 55hp 15" small lower

[hydrospeed77]

So based on your two photos above, I have the exact same configurations..
You can see that some of the exhaust from the 70 3cyl mid will flow rearward of the baffle/wall between the two bolt holes, and not down to the prop shaft area. Whereas with the "big" 70hp lower, all exhaust will flow down to the prop shaft..
I'd be curious to see what a real 6J2 (factory 70 CES) gearcase casting looks like with regards to the above..

I think your idea of cut/add will probably work. My only concern would be does the smaller lower have enough exhaust flow down to the prop shaft for a 70hp 3cyl.. I mean I hope it isn't a restriction. Because the big case does have a larger passage all the way down to the prop shaft.. So for the small lower, maybe some exhaust is meant to be relieved out above the cav plate.. Not 100% sure on that.

Not sure why the strange aftermarket one I have has both baffles/walls, but maybe depending on what motor it will be used on, you are meant to grind/cut out the one between the bolt holes.. That's the only reasoning I can think of.