'75 Mercury 7.5 hp water pump questions

[Buzzwindrip]

I pulled the LU on my bargain outboard after noticing that my indicator output was more steam than stream. I guess I was a little shocked to see how much rust there was on the drive shaft where it enters the pump housing. I was scraping off the remnants of something metallic; was that the centrifugal slinger?
Edited to add: The shaft is pretty corroded above the pump housing, and inside the pump top cover, the shaft is pitted. I wonder if the seal that sits on top of the pump cover will even work with all the missing metal. I'm starting to think this $25 motor is just a hole to throw money into.

 

[emckelvy]

Re: '75 Mercury 7.5 hp water pump questions

The seal on the top of the pump doesn't really matter. It'll pump without the seal. There's a rubber 'slinger ring' which slides down the shaft and rests atop the upper pump housing. All this does is keep silt/debris off the top of the pump, hence the 'slinger' name.

The real sealing is done between the round ridges molded into the rubber impeller and the stainless plate at top/stainless 'cup' insert at bottom.

Check the center of the plate and also the cup, if there are large grooves worn into them, or obvious wear anywhere else, replace.

Clean up the rusty driveshaft so that the new impeller slides smoothly all the way down. You don't want it to bind on the shaft or you'll never get it properly in place.

Pull the gearcase oil drain plug and check the gear oil for water instrusion. If the oil is full of water, the driveshaft seal is probably shot. The lower pump body and copper water intake tube just pull out. The pump is sealed into the gearcase with an O-ring, and the oil seal is pressed into the pump.

Drive out the old seal and drive a new one in. A socket sized to just barely fit the diameter of the pump will easily drive the oil seal into place. Be sure to note which way the lips of the seal face and install the new seal the same way (should be with lips of the seal facing up).

This is just a basic overview, if you do a search you'll find more comprehensive directions on the job.

And as far as the motor goes, you can't complain about a $25 motor that runs! Even if the lower unit is marginal, you could test run the motor to see if the powerhead runs good, if so you could always upgrade to a newer lower unit with stainless shafts. Plenty of them on eBay or check your local www.craigslist.org for used bargains.

HTH & G'luck with the repairs..........ed

 

[Buzzwindrip]

Re: '75 Mercury 7.5 hp water pump questions

Thanks for the reply Ed. Actually, the top plate looked great, and so did the cup, till I nicked that trying to chisel off the frozen on impeller. That is the part they leave out of the manuals I have seen!
I ended up pulling out the lower pump base, and see where the oil seal contacts the shaft as it is still in decent shape, if not slightly seal cut. I think I will order a new cup and top plate, impeller, and water seal grommet, as it is so stuck to the water tube that I will have to cut it off.
Another question: The lower pump housing appears to be OEM, as it has the Mercury logo on it. Are all replacement impellers, cups, and top plates compatible with the OEM pump casing?

I had actually changed the LU oil before I ever fired up the motor, and it looked good, no milky appearance, no metallics.

The motor does seem to be a good runner. I ran it in a barrel after I first picked it up. Looked like it had a good indicator stream. Then I mounted it to a 12' Mirrocraft Resorter that I just picked up, launched it, and shut it down shortly after, when I saw the intermittent stream/steam thing. I never even left the launch.

 

[emckelvy]

Re: '75 Mercury 7.5 hp water pump questions

Are all replacement impellers, cups, and top plates compatible with the OEM pump casing?

Yup, long as the parts you ordered were spec'd for your motor's serial #, you can mix-n-match as you please with aftermarket/OEM parts.

BTW the best way to get a stuck-in-place impeller out of there is to split the bronze center with a sharpened screwdriver or equally-thin chisel. The thin blade gets right past the rubber and easily splits the center.

Cheers.........ed