Mercury Mark 25

This is my first post so forgive me if I'm asking questions that have been asked many times over.

So I have already learned from this site (and the hard way) to change the impeller every 2 years to be on the safe side. I didn't do this on Wednesday

I have a Mark 25, SN: 868788, that hasn't run in about 2 years. I got it started after 4 pulls, saw water coming out of the coolant holes and thought, "I'm good to go!"

I went up the river at about half throttle and after about 1/4 mile the engine began decreasing RPMs and shut itself off. I looked at the motor and could see I could cook an egg on the side of the powerhead

I make sure the flywheel still turned, which it did and let the motor cool while I drifted back downstream. As I neared the dock, I started the motor and IDLED over to the dock. No water was coming out of the coolant holes this time.

So, here I am on your forum, humbly asking for help. I'm in the process of removing the lower unit to change the impeller but I'm wondering if there is anything else I should do while I'm working on it?

Does anyone know what the compression should be on this motor? I hope I didn't ruin the rings....

Thanks for reading my ramblings. I feel like an idiot

Re: Mercury Mark 25

it happens to the best of us. i would also change my lower end fluid. good luck

Re: Mercury Mark 25

Keikauffer said:

Does anyone know what the compression should be on this motor? I hope I didn't ruin the rings....

Click to expand...

125 lbs if memory serves me well. Your probably ok, they were pretty tough engines. If they did scuff it will be on the exhaust side of the piston.

Re: Mercury Mark 25

Thanks for the help guys! I got really nervous when I saw that gasket between the powerhead and the lower unit bubbling from the heat
I wonder how I will know if that gasket is toast? Something tells me I will know when it's bad...

As far as the impeller change goes, I got the lower unit off and I see a round housing/cover that the vertical driveshaft goes through, but I'm not sure how to spin that housing/cover off. It has smaller holes going around the circumference of the cover. Is the cover threaded on? Should I use a screwdriver to turn the cover counter-clockwise?

I greatly appreciate all your help!

Re: Mercury Mark 25

there should be screws holding that cover down. there are a couple videos on youtube showing how to do it. http://www.youtube.com/watch?v=Fj1c4oYpHAw&feature=related

Re: Mercury Mark 25

Hi,

The cover is threaded into the housing. The impeller is under it. Well under a stainless plate that is in there too. You can buy an adjustable spanner wrench to spin it out with or pound it around with a punch and hammer.
The spanner wrench is best. you can crack the cover using the punch around method unless it has been off lately and was greased when it was put back in. You could try putting a punch or drill bit in the hole. Then take a flat bladed screw driver, put it between the punch and driveshaft and twist it off.
They are usually in to tight for this way to work, but you could get lucky.
The impeller will come out with a little pry up. Put some grease on the shaft to hold the pin it to make it easier to get the new impeller installed. I usually turn the shaft and impeller as I push it in. The blades fold back and fit in easier this way. A light coat of vaseline or grease on the blades helps a lot too. Once it is in trun the drive shaft to make sure the pin is locked into the impeller. Then put the SS plate and Al cover back in. Grease the driveshat spline, shift shaft spline and the water pump copper tube so it does not cut the seal when you put it back on. You should be good to go then.

Re: Mercury Mark 25

corm and jriegle04-

Thanks for your help, I greatly appreciate it! I'll have to let you know how it goes.

Once I get the impeller job done, I'll move on to replacing the bearing/seal that is on the propeller shaft, because I noticed it's leaking gear lube. Any thoughts on that job?

Most appreciative in Illinois,
Keikauffer

Re: Mercury Mark 25

Hi,

Bearing, does the propshaft move side to side or up n down? If so then I guess you had better take the threaded cove off and replace it. The cover comes off the same way as the pump cover.

Re: Mercury Mark 25

no slop in the prop shaft, just leaks gear lube. Thanks again for the help, corm!

So I got the impeller cover off using a punch and a hammer and it worked fairly well!

I used the punch and hammer on the cover surrounding the prop shaft too but now I'm not sure how to get the bearing off and replaced. There is no slop in the shaft at all but I thought I might as well replace it while I'm at it, what do you guys think?

Here's a picture of the prop shaft and bearing...


And I was wondering what the single hole is toward the front of the lower unit for?

Re: Mercury Mark 25

Hi,
I love it when people put pictures up of the project. Cool!

Put the prop nut back on and gently tap on it (not with steel)
to get the propshaft to come out. I would not pull it all the way out. Just far enough to get the bearing carrier out. That is the part with the seal in it. I would not replace the bearing unless it has a lot of space between the needle bearings. It can be pushed out with a 3/8 socket extension and a socket that fits in the hole. It need to be a fairly tight fit so you push as close to the out side of the race as possible. The seals can be punched out with a regular bladed screwdriver. Put the seals and bearing back in with a socket. Not the old chewed ou ones you let the kids use either. Use the good set. You need complete contact when you push in the bearing in. Make sure you measure the distance that it is set in so you are running on the same surface. Otherwise you will not get full contact of the needles on the shaft. There is always some wear to deal with. Once you have the bearing in the right spot push the seals in. If you have a rust ring on the surface that the seals were running on try to reposition them a tiny bit so they are riding on a smooth surface. put it back together, grease it up and go boating.

Re: Mercury Mark 25

Well, I got the impeller reinstalled and while I was at it I replaced the seal in the center of the impeller cover. I also changed the prop shaft seal and thanks to corm that went smoothly. I noticed that the motor doesn't want to go into reverse though. If I turn the flywheel on the top of the motor, the propshaft rotates clockwise, and when I do the same thing with the motor in neutral, it rotates counter-clockwise. No clicking noises when I do this, either.

I'm wondering if I tightened something down too much when I put the propshaft bearing and seal back in???? Anyone run into this issue?

I'm gonna attach some pictures of the motor, since corm said he likes pictures!!

Below is the lead seal, still intact after about 42 years!!!


I don't know if you can read this plate but it says it's made in Cedarburg, Wisconsin.


Here's a sticker on the powerhead saying it was inspected "OK."


Here are the two spanner wrenches I had made to make reinstallation of the impeller cover and propshaft bearing cover a bit easier.


And here's a picture of my nekkid Mercury Mark 25!!

Re: Mercury Mark 25

thats cool but i bet its nothing like my merc500 lol wish i had a camera id be uploading so many pictures

Re: Mercury Mark 25

Hi,

I noticed by you last picture that the paint on the engine block had not turned color for over heating so you may have got lucky with the pistons. They are probably not hurt enough to make any real difference. When that paint turns brown you will have gotten the engine hot enough to stick the rings for sure.

Nice job on the tools!!

As for the reverse problem.
The shaft turning ccounter in neutral is caused by bearing drag which is normal.
If the LU bearing carrier is in the same position as it was in your 1st pic then I would look at the shift shaft. I can not remember if that engine had a flex shift shaft. (like a flex drive for a ratchet) If it does you may need to lube the reverse lock or see if it is sprung. Going in to reverse add pressure to the system so it may not let it turn all the way. This is the piece that is the LU shift shaft slips into. You could always drop the unit again and try shifting the LU by hand to see if it will go into reverse being careful not to mark up the splines. I use an end nippers to grab and twist the splines. I get a good grip and it does not chew any metal up.

One other thing the o-ring that goes between the bearing carrier and the reverse gear bearing race holder did not fall out of place when you put it back together? Also did you get the cover tight enough to push the reverse gear in place.

Re: Mercury Mark 25

that is some oldschool **** right there, cool that your restoring it but it looks weak lol. show a video of how it pushes the boat its going on. my old johnson 25hp was the toughest motor ive seen but it was 20 years newer (1982) I sold it though since i figured another year or two pushing a boat rated for an 80hop would cause some damage. yet it was on there for 5 years or so