Logo
FULL SITE MENU

1977 Merc 500 - Need Help

travis0332

travis0332

Member
Hello, I have a 77 Merc 500 that I got runing by replacing some of the wiring, but it runs like crap. The engine had good compression, 112 on all 4 and has spark. After letting it idle (very roughly) I decided to pull plugs and take a look. Plug 1 had a little wet oil on plug. Plug 2 started smoking (white smoke). Plugs 3 and 4 looked steamed clean. It looked like water dribbled out of plugs 3 and 4 as well.

I'm pretry certain I have a major water leak. Is this a major overhaul not worth fixing in an engine this old?? Or could a DIY semi mechanically person have an attempt to repair this kind of issue. I have only done basic maintenance before - (carbs, water pump, stator, rewiring).

Thank you for you assistance!
 
faztbullet said:
Could be lower crank seal if 3/4 have water...
Click to expand...

3 and 4 definitely looked like water came out. 2 was smoking when plug removed (that can't be good lol). Im looking at the parts diagram and reading forums regarding water leaks- I was hoping this could be a bad exhaust gasket instead of lower crank seal....but I have no idea on what to check from here
 
If you're brave (and lucky) pull the exhaust plates off and replace the gaskets. That might do it. I doubt if it's a cracked block since two cylinders are getting water in.

Good luck.

Jeff

PS: If you want to do this, get back to use. We'll show you how to remove all those bolts without breaking them off in the block.
 
Thank you Jeff. Going to attempt to remove this plate and take a look this weekend. I have the manual and parts diagram ready. I'm reading that removing these bolts can be interesting, they break easily?

Do you think I need to remove the back plate covering the cylinders too, I'm still learning but I guess it's not a true head gasket, manual calls it cylinder block cover & gasket.
 
..."Do you think I need to remove the back plate covering the cylinders too,"

No! That's just water jacket.

Many methods exist for getting the bolts out without snapping them off. My favorite is to use an impact wrench on its lowest setting and "rattle" the bolts loose. They'll usual unscrew, but sometimes it takes several minutes of rattling away before they start to move.

If they won't move, do NOT slap a socket wrench on them an grunt away--you'll break them for sure. Instead, center punch the bolt head and drill it off with a drill the size of the bolt (usually 5/16 ". It helps to drill a small hole first--about 1/8 ".) You can 'steer' the drill when you start drilling if it's off center: just angle the drill about 45 degrees to 'move' the hole sideways, then drill vertically again. (Easier to do than explain.)

Good luck! Remember: PATIENCE is the watch word here.

Jeff
 
Update, went at it lightly with WD40 and a socket wrench, very lightly turning and spraying, turning and spraying, etc. Surprisingly have top 7 bolts out, not too bad. Going to need to a quite narrow socket to get to ones near bottom of cowl, and by looks of it, will need to remove shift linkage as well. Nonetheless, good start.
 
Good. Remember: PATIENCE!

Jeff

PS: When removing the throttle assembly (necessary) do your self a HUGE favor and have a 3/8 coarse nut ready. Squeeze the throttle assembly parts together as you remove it and immediately bolt the assembly together (with the nut) so it doesn't fly apart on you. It's a major pain to get back together. Ditto on reassembly.
 
1.png2.png3.png4.png5.png
 
Last edited:
I had 112-115 compression on all 4, only reason I took a chance on it. At this point, would you advise I give up on it given the look of it? Not too sure what I'm looking at - I am assuming lines in piston are score marks?

I'm thinking I'll clean these plates really well, replace gaskets, and then see if I can get a little run time on this beast. Hopefully this at least solves the water leak issue...
 
Last edited:
Yes that piston is scored..running it in that condition will only increase its chance of destructing itself. Doubt you will ever get it to idle or run without dying when shifting
 
See all that clean aluminum? That's water blasted metal from a leak somewhere (probably in the cover assembly.

My olde eyes can't see that "bad piston". Could you add a close up of it (in several locations in the bore)?

Jeff
 
Thank you guys for all your help! After skimming the manual, I could take the motor apart, doesn't look too bad, but there is 0% chance I'd be able to properly "rebuild" internal components. Additionally, I'm reading a mechanic would charge $1500+ for this type of repair.

Going to patch it up, check compression again, and run it until it dies (if it runs).
 
It's hard to tell in the pics, but the passages are completely clogged with debris/sand/gunk/deposits...so that likely explains the failure...

Question though on reassembly after cleaning this out. The manual doesn't have any details but do you add a gasket sealant to these two large gaskets, and anything added to the bolts? The manual just says torque to 200 in lbs.
 
1: Look around for a "parts motor" that is OK 2: Guys are fixing those motors all the time (even older ones), pistons, gaskets, seals and bearings are available; you can use a sealer on the gaskets [don't glue it together with silicone] - something like Permatex or Perfect Seal for fasteners. If you have a Mercury dealer close you can get what you need from him.
You will have to take the flywheel off there's a puller for that or if you take the flywheel apart {not recommended} mark the flexplate to the hub.
 
Last edited:
Be damn careful who you select to do the boring. Since this is a closed end block, it's much harder to do properly.

Jeff
 
Decided to replace water jacket and see if it would run. It actually idled on the muffs to my surprise. All plugs looked the same. Problem is I decided to check warm compression and read 121/121/121/104 from top to bottom. I am guessing it will need an overhaul.
 
Some motors are shy about peeing in public in a driveway.------Did you install a new impeller ?-----Is the exhaust cover staying cool ?------Then test it after you launch the boat.
 
racerone said:
Some motors are shy about peeing in public in a driveway.------Did you install a new impeller ?-----Is the exhaust cover staying cool ?------Then test it after you launch the boat.
Click to expand...

Replaced complete water pump kit a couple months ago. Exhaust plate felt nice and cool but cylinder cover felt fairly hot to me as well as the plugs themselves. I noticed water leaking a little around outside of plugs. Can that cylinder cover plate and gasket affect the telltale?
 
I couldn't get engine to stay running in a tank so I did spark check and saw spark on all 4. I pulled carbs to take a look and decided to taken a quick look at intake. Pretty strong puffs of air escape intake as I rotate flywheel. One gasket flew off. Is this normal or could be weak reeds? It would also explain this whiny fluttering noise I hear when it's running.
 
Back
Top