57 Johnson 7.5hp impellor replacement

[PatSea]

I bought a '57 Johnson 7.5 hp for use on my inflatable. It runs great (most of the time), and is a real conversation starter as my wife and I cruise around the marinas. Twice this fall it overhated as I was idling. I noticed it immediately and shut her down. After cooling it started and ran OK. I haven't done any damage to it. It never overheated when running above idle.
I decided to replace the impellor and have it torn down at the moment. The impeller was intact, no missing pieces, but the only thing I noticed was it was quite rigid and hard, not very flexible. It may be quite old, I just don't know.
The teardown was easy, thanks to some of the postings I found on this forum.
I have a new powerhead gasket, but I think I will go back to the dealer and get a new o-ring and cork gasket for the seal at the top of the drive shaft.
I have several questions:
1. How do I hold the powerhead gasket and cork gasket/brass washer in place as I lower the powerhead onto the intermerdiate housing? I can't see in there to make sure all the parts are properly aligned as I lower the powerhead into position.
2. Which direction do the openings face on the water inlet plate? It seems they should face forward and act like a scoop, but somewhere I thought I read that they face toward the rear.

 

[raczekp1]

Re: 57 Johnson 7.5hp impellor replacement

hi.
1. i put some grease on cork seal and metal washer. grease acts as glue and just glue this parts to powerhead.
2. here is a wiev of water intake:

its good to read this articles :http://www.sschapterpsa.com/ramblings/OMC_6hp.htm
its about 6hp model, but its almost the same ase 7.5hp, some parts are different but some are likely

 

[rolmops]

Re: 57 Johnson 7.5hp impellor replacement

The picture you are showing us is not the water intake.The water intake is the little extension that is behind the propeller.it sits right before the exhaust which is also part of that extension.The propeller forces water towards the water intake when you go forward and the impeller pumps it farther up and into the engine cooling system.When you idle,your propeller does not force water into the cooling intake and in your case the impeller clearly cannot do it by itself.The impeller should be a fairly soft rubber.
Me thinks that you should check all the water passages and remove any debris.Often the intake gets all clogged up.Then replace the impeller and while you have it open,take a garden hose and connect it to the copper pipe that comes down from the block and push some water through the system to make sure that all the internal passages are clear.
While reinstalling the the water pump and propeller make sure everything is done according to the manual or you might end up having to do it again.

 

[raczekp1]

Re: 57 Johnson 7.5hp impellor replacement

well, i just lookde at photo of 57' joh 7.5hp,and midsection and Lower Unit is the same as mine 6hp, and for 100percent i can tel thet piece of LO just above propeler is exhoust outlet for burned gas.
water intake is smal plate just above cavitation plate,
am I wrong?

 

[F_R]

Re: 57 Johnson 7.5hp impellor replacement

Yep, you are wrong. Look at the third picture you posted above. It clearly shows the water intake. It is a screen on the leading edge of that exhaust outlet that you are talking about. It is located so that prop thrust forces water into it. However, when running in reverse gear, the prop sucks water away from the intake screen. That is what the rectangular plate above the cavitation plate is for---it lets water in while running in reverse. Water actually flows out of those holes in that rectangular plate while running at speed in forward gear and water is being forced into the main intake screen by the prop.

 

[raczekp1]

Re: 57 Johnson 7.5hp impellor replacement

so, i lower my head down.
but what about 6hp 72'.
i did rebuilt of water pump and i could swear that water chanel leading to water pump just goes from water screen placed above cavitation plate.
in this case am i also wrong?

 

[kfa4303]

Re: 57 Johnson 7.5hp impellor replacement

I think you're both right. the water is forced into the screened, leading edge by the prop, as F R said (everything F R says is right ) Once it reaches the water pump it is sent up the water tube to the motor. To quote an Evinrude service manual: "The wash from the propeller blades and the forward motion of the lower unit through the water assist in directing water to the pump. After water reaches the pump, it is forced through the cooling system under positive pressure." However, the side plate ("bypass cover") you refer to also leads to the water pump. It can and "should" be removed when testing your motor in a tank along with removing the the normal prop and replacing it temporarily with a special test tank prop, but I've never done either of those things and my motors all pump plenty of water in a test tank :/ As for the the direction of the slits, I would listen to F R on that one. Some models seem to just have holes without any particular orientation, go figure. good luck with the motor.

 

[PatSea]

Re: 57 Johnson 7.5hp impellor replacement

I'm still trying to understand the water intake in both forward and reverse. I just looked at the exhaust extension on my unit and I now see the small screen on the forward edge of the exhaust extension. I understand that is the primary water inlet going in forward gear, correct? The small plate above the cavitation plate is the water inlet going in reverse. It would seem that water would also be pumped in through the small screen in reverse since reverse speed should not be very fast.
If the water inlet in reverse is the small plate then it would seem the scoops around these two holes should point to the rear. Hope this makes sense!

 

[F_R]

Re: 57 Johnson 7.5hp impellor replacement

You are trying to make it overly complicated. The screen on the exhaust outlet is the main water intake. From there, it goes behind the plate on the side, and on to the water pump. The holes in the plate are just an auxillary inlet, for in case the main screen is starving for water when running in reverse. That really is an issue only when running in reverse at above idle speed, such as when "backing down" (brakes) or when manouvering a heavy boat.

"Most" motors don't even have scoops on the plate. I don't consider them to be a big deal.

 

[rolmops]

Re: 57 Johnson 7.5hp impellor replacement

I would go even beyond what FR says.The little plate on the side of the lower unit is not really an intake.It is really an overflow for when too much water is pushed into the water intake by the propeller.
Normally when the outboard sits on the transom of your boat,the lower unit is submerged to the point where the water pump is submerged as well and the impeller will be under water,so it has no problem pumping water into the cooling system,and once the flow has started it does not need any help to continue