1978 Mercury 800 Water Ports

[1970Thunderbird]

Hello again,

I've removed my lower unit, as water was not exiting the engine when I had it attached to a hose with earmuffs. My water pump was shot, as I expected. While the lower unit was off, I decided to attach a hose to the water tube leading up to the power unit, just to check that water flowed unrestricted through the block. While running, I took some photos.

I've never seen a Mercury run, so I'm not exactly sure where the water exits the motor, once it has run through the block. I saw a Johnson about the same age as my motor run, and when it did, water exited from a tiny port up top, as well as two rectangular ports near the top of the lower leg, like the ones I've shown here

Mine, however, does not have water exiting here. It has a small port at the top where water exits, but water also pours out the bottom of the lower leg. It appears that the water would exit through a series of holes, similar to the water intake in the lower unit. What I think are the exit ports are just above the anti-cavitation plate, before the lower unit attaches to the lower leg.

Here's a photo with the water running

Are those rectangular ports water exit ports? I had the motor running for a very short amount of time, less than 5 seconds, and when I put my hand over the ports, I felt pulses of air, similar to exhaust, coming out. I know the exhaust comes out through the prop and the water acts as a muffler. What are these rectangular ports for? Also, what is the little hole just below the 80HP sticker in the first photo? Is that for water, as well?

I want to make sure my motor is cooling properly and that the water is flowing freely and exiting where it is supposed to.

Thanks a ton.

 

[Chris1956]

Re: 1978 Mercury 800 Water Ports

When the water leaves the powerhead it is divided in two. One side mixes with exhaust and flows thru the prop. This same air/water mix can exit the two square ports high on the midsection, as exhaust relief when the motor is in neutral in the water. The rest of the water exits the gratings above the AV plate, as you have guessed.

 

[1970Thunderbird]

Re: 1978 Mercury 800 Water Ports

So, does it sound as though the cooling system is working effectively? I'm concerned I may have a clog (engine sat for a couple of years), since no water is flowing through the two rectangular ports. When I saw that Johnson run, it had water pouring out of those, even with the engine off.

I guess my concern is that I go through the lower unit, get it all squared away, put it back, and I still have a water issue in the power head.

Where should water be exiting from if the engine isn't running, but has water flowing into it? Where should it exit if the engine is running? Do you think I can test the motor as I have it in the pictures, on a stand with a hose hooked to the water pipe? I just don't want to fry the thing.

Thanks again

 

[Moody Blue]

Re: 1978 Mercury 800 Water Ports

From your photo, I'd say you have nothing to worry about.

Chris gave you a great explanation. The telltale output is plumbed at the "end" of the cooling system and since you have a good stream of water, there is nothing to be concerned about.

Only time you will see water exiting those two square ports is if the exhaust has encountered back pressure, ie when the LU is immersed in water, and then it will only be a spray.

 

[1970Thunderbird]

Re: 1978 Mercury 800 Water Ports

Thanks for the advice, guys. I really appreciate it. Now to go into the lower unit!