1975 Mercury 500 Fuel mixture issue?

[t-swans6]

I have a 1975 Mercury 50 hp serial number 4286394. Now that it is spring I got my boat out for the first time this year and everything went great. The motor ran really well and only smoked a little bit, but I was on a no wake lake so I couldn't open it up at all to possibly clear it out. I noticed when I got back that there was a milk shake colored fluid coming in between the prop and lower unit. I am mechanic and know that, that usually isn't good, but I am new to this whole boating and outboard motor thing. I removed the lower unit thinking that it was probably a seal in the lower unit seeping gear oil out. What I found was that the exhaust section from the power head to the lower unit was caked with this very thick milk shake type fluid. My first thought was that the fuel mixture had to much oil in it and it just built up in there. I have read a lot of forum posts and you guys sound like you really know what your talking about. Just wondered if anyone has seen this or knows whats going on and could point me in the right direction. Thanks for your time.

 

[emckelvy]

Re: 1975 Mercury 500 Fuel mixture issue?

Most likely the exhaust manifold gaskets have dried up and are spraying water into the exhaust (thus the milkshake), and/or the aluminum exhaust baffle has perforated and is spraying.

With that much water you should be seeing droplets on your spark plugs also.

There's an off chance that a bad powerhead gasket could do the same thing but more likely to be coming from the exhaust side.

You'll have a very difficult time getting to all of the bolts with the powerhead in place; best to pull the entire powerhead if you need to get into it. Also a good time to check/renew the crankshaft lower bearing seals (2 ea) and bearing cap O-ring. You can pull the bearing cap with a harmonic balancer puller or other suitable puller, without removing the block front cover.

Take a look at the plugs, if they appear Super Clean it could be from the water injestion, it'll steam-clean all the carbon off the plugs.

If this motor is a Salty Dog it's all the more likely to have perforated the exhaust baffle plate. And the bolts will be frozen in the block; use of heat and penetrant, and in extreme cases drilling the bolt heads out, is about the only good way to get the covers removed then you Vise Grip the bare studs (or weld a nut to them) and apply more heat/penetrant directly to loosen them up.

Worst case, busted bolts require drilling out and fitting with Heli-Coils. You can't tap stainless out of an alum block as all it does it gall; you have to completely drill them out and the Heli-Coil makes a nice repair.

HTH & let us know what you find..........ed

 

[t-swans6]

Re: 1975 Mercury 500 Fuel mixture issue?

I got all the exhaust cover bolts out and found the gasket very hard and brittle, and also found the baffle plate in between the outer cover and power head very corroded. But did not find any obvious sign where water was getting through. I am pretty confident that is the issue though. I would obviously replace the gaskets but would also like to replace the baffle plate as well. I found some kits that have the gaskets but am having a hard time finding anyone that has a new baffle plate. Are there any good web sites you would recommend checking out?

 

[emckelvy]

Re: 1975 Mercury 500 Fuel mixture issue?

I looked up the part at www.boats.net and it's listed as an active part, not cheap though at $69.07. But considering retail is over $85, could be worse!

http://www.boats.net/parts/search/M... CAPS, MANIFOLD AND EXHAUST COVERS/parts.html

You might check at www.crowleymarine.com as they have good parts diagrams also.

Torque on the 5/16-NC stainless manifold cover bolts is 200 in-lb. If you spread a thin coating of Permatex #3 Aviation-Type gasket dressing on the bolt threads, they'll never stick in the block. Plus it seals them real good against salt intrusion or other corrosive materials.

HTH...........ed