1977 Mercury 7.5HP impeller and spark plug wires

[ajcampbell31]

I know I need to replace the impeller on my motor, but I don't know which one, or if there are more than one. I've read in different places that this motor uses two different sizes based on inside dimensions. Is this the case, or does it just have one size?

Also, I can't seem to locate spark plug wires for this motor anywhere. I've found build your own kits, but nothing already made. Can someone provide a little insight to this?

 

[racerone]

The old impeller may have a number on it.----Ends with 80 or 81.-----That does not mean the last party installed the correct one.-------Measure the shaft with calipers and then decide which one you need.

 

[60sboater]

It should be Mercury 47-89981 or Sierra 18-3039 (0.456") . I don't know of a source for pre-made,custom-fit wires. The Mercury impeller fits better...Sierra has thicker vanes.

 

[racerone]

Different years lower units will fit.-------Best to measure the shaft to see which size you need !!

 

[ajcampbell31]

Here's a possibly stupid question - worth the risk.....since the pee tube is for water to go through if I blow the opposite direction of the line, should I see bubbles come out through the lower unit? I just tried this to see if there was any blockage and could blow through...but didn't see any bubbles at all. This leads me to think that:

a. This was a pointless test
b. This isn't a closed system like on a car

On the other hand, if I am supposed to see bubbles, what other issues may I have?

 

[merc850]

Wires are available here http://store.oldmercs.com/category_s/1996.htm the "pee tube" shows water flow thru the block and blowing air thru it might not enter the driveshaft housing, if there are any blades broken off the old impeller they could be blocking the watertube, so to test this better use a garden hose on the bottom of the tube and run water up the tube and see if it goes thru the block and out of the "pee hole". The cooling system is designed to drain completely when the motor is stopped.

 

[ajcampbell31]

I ran water into the screen at the bottom and no water came up through the pee hole. I reversed this and ran water into the pee hole tube and it came out through the rear of the prop.

 

[racerone]

Obviously this is NOT a closed system like a car.-------Water is pumped into the engine and goes out with the exhaust.-------Some of these motors have a plastic washer at the top of the water tube.--------On severe overheat this washer may melt and then block cooling water flow to the engine.

 

[ajcampbell31]

So if I am understanding this correctly, the pee hole isn't the water circulation, but a visual indicator to show that water is flowing?

 

[racerone]

Most of the cooling water goes out with the exhaust.-----Out the propeller.

 

[ajcampbell31]

Ok, well I ordered a micrometer off Amazon, so I just need to wait for that to get here so I can measure the shaft. Not that I don't believe what 60sboater said, and I'm sure they are right, but I don't really want to buy two impellers if I can help it. Carb seals are en route so I can rebuild it. With any luck in a couple of weeks I'll be out on the water again! Thanks for all the help, it's much appreciated. Fair winds and following seas, boys.

 

[ajcampbell31]

Another question. I got the old impeller out, which was destroyed, but now I am running into a small issue....the shaft measures out at .44 and the inside of the impeller measures out at .44...the choices of impellers are .438 and .456. .438 makes more sense to me as it will probably stretch the extra .002". What do you think?

 

[racerone]

The impeller will / should slide right onto the shaft.----How did you measure the shaft?----Pump housing installed or pulled out of the lower unit ?

 

[ajcampbell31]

I measured the shaft at the bottom right above the lower housing that holds the impeller and the inside of the impeller itself. Both using a micrometer.

 

[racerone]

Not sure how you would measure the inside of a hole with a micrometer.---But is there a number on the old impeller ?----How does the hub of the impeller fit on the shaft ?----Snug or very loose ?

 

[ajcampbell31]

I can see plainly 47-89981, and L 34, and what looks like it could be an 80 on it. That one is not so clear.

 

[racerone]

??-----So does it fit snug on the shaft or is it loose ?-----Last chance.

 

[ajcampbell31]

The new one hasn't been ordered yet. The old one wasn't real loose, but wasn't hard to get off of the shaft. Should it slide off easily?

 

[60sboater]

I just looked at a 47-89981 (0.456") impeller I have and the pn is printed into the rubber between two of the vanes. You might want to check the meas. again. If you're really seeing 0.44 then maybe you have the 0.438 shaft but Marine engine lists a '77 7.5hp using the 47-89981 or 18-3039 Sierra.

Oops,missed your post where you listed the impeller pn. There are vids on YT of imp replacement. I did my 1973 9.8hp about a year ago.

 

[ajcampbell31]

After doing a bunch of research, I ended up purchasing:

(1) 18-3039 Impeller - replaces O.E.# 47-89981
(1) 18-3139 Impeller Plate - replaces O.E.# 46895
(1) 18-7409 O-Ring

It arrives on Saturday, I'll update if it works.