Mercury mechanics please step in..

[Scott Amato]

I have a 1970 mercury 500, 50 horse. This thing is like it has been in a time capsule. I bought it from the original owner who had it stored in his basement since 1973. This thing is absolutely spotless, and I have done the normal maintenance on it getting ready, water pump, plugs, etc. Here is my issue... the motor runs great, but I'm getting some moisture on #3 plug. I have replaced all gaskets on the covers, and have done a compression, and leak down test on every cylinder. Compression gauge shows 145, and leak down is 2%. The only question is where is the moisture coming from in that cyl? Is it possible to have a crack in the cyl wall, or would the leak down show more leakage if that is true.
Thanks, Scott

[Guest]

Gater leak

The engine could have a leak in the exhaust baffle. This was common on some of the merc models. This was also an issue if the engine was over heated. If it is a salt water engine I would assume that is where it is coming from. With compression being good the cylinder head looks like it is fine but you might take the water jacket off the back and also inspect there.

Alan

[Mike45]

The engine could have a leak in the exhaust baffle. This was common on some of the merc models. This was also an issue if the engine was over heated. If it is a salt water engine I would assume that is where it is coming from. With compression being good the cylinder head looks like it is fine but you might take the water jacket off the back and also inspect there.

Alan

Totally agree and a gasket/seal here would be the first place I would look.

[Scott Amato]

I have replaced all of the gaskets already thinking the same thing. This has been a fresh water motor, with only about 20 hours total on it. This thing is spotless, and I went ahead and changed any gasket related to sealing off water just to be sure. The plug has a thin layer of milkshake oil on it after you run it for a few mins.

[Guest]

Water in 3

Does the engine have an aluminum exhaust baffle? If it has one I would clean it well and inspect for cracks. Most of the time the gaskets are fine. If you have the S/N of the engine I will look it up in my Mercury data and get you the part number.

Alan

[Scott Amato]

It does have an aluminum plate between the case cover, and the block. I did replace the gaskets on both sides of it, and cleaned it very well before I reinstalled it. It looked really good, but I'll say I didn't look at every square inch of it. I was thinking gasket more than cover.

[ricochet112]

Your motor has 2 outer gaskets on the exhaust. Bolted to the block surrounding the exhaust ports is a water cooled exhaust cover.

This is what needs to be replaced, and the only way to get to it is to remove the pwerhead. The water is most likely coming in cyl’s #3 & #4.

A blown powerhead base gasket can cause this also.

add- rebuild the carbs and fuel pump after you get the water issue under control

[trident]

Water Leak

Isn't that what he just said he did?

I have seen, ONCE, a Merc 500 of this approx vintage, that had a pinhole leak between the block casting and the sleeve itself, that would drip, drip, drip into the #4 cylinder from the port opening where its drilled though the aluminum casting and the sleeve. Factory defect. Motor ran for years until all the bearings rusted on #4.

Had me stumped. How do you prove where its coming from? Found it by sealing everything up and attaching a garden hose... sure enough, drip, drip drip into #4... could see the drops form just like that. I suppose it was fine while it ran, sorta. Damage was done when it stopped with moisture in there.

Junked it!

Jerry

[LarryR]

I, too, have seen this exactly ONCE.
It was an overheated merc600, 6-cylinder.
Not totally unrelated to your newer 4-cyl merc500.

No one has mentioned the lower crankshaft seal, which is a possible cause if water is on 3 and 4. If your sure that it is only on 3 and not 4, then it is not the seal.

[ricochet112]

Isn't that what he just said he did?


Jerry

The outer 2 gaskets yes, the inner one, doubtful.

gasket ref#15 and cover ref#16

http://www.boats.net/parts/search/Me...VER/parts.html

I would need the serial number to verify, but this old of a motor may not have the inner cover to cool the exhaust...

[trident]

Gaskets

Against my better judgement, I'll bite.

Say what?

Two gaskets plus another one where on the exhaust side?

There is one inner and one outer gasket, on either side of the exhaust deflector plate, under the water cooled cover.

I'm done. I'm not going to start a urinating contest.

[mercguy]

The outer 2 gaskets yes, the inner one, doubtful.

gasket ref#15 and cover ref#16

http://www.boats.net/parts/search/Me...VER/parts.html

I would need the serial number to verify, but this old of a motor may not have the inner cover to cool the exhaust...

Mike, you obviously have not had a 500 apart, as there is no separate inner baffle plate gasket on a 40/44...........only 2 gaskets....besides the pictures you show are from an 800/850, no where even close to a 500

the previous suggestion of lower crank seals is a good one though (if the other ideas did not solve the problem)....................seem that happen with a motor like Scott has, that had been sitting for a long time in a hot dry climate...

[ZUL8TR]

500 exhaust cover and gaskets

Here is an early 500 schematic with serial #'s that fall in the 70 to 73 year
serial numbers:

Merc 500 50
2736807 thru 3816307
1970-73

Obtained from:
http://store.oldmercs.com/category_s/827.htm

schematic below shows only 2 gaskets for the exhaust cover and baffle:

http://www.boats.net/parts/search/Me...BLY/parts.html

[ricochet112]

Mike, you obviously have not had a 500 apart, as there is no separate inner baffle plate gasket on a 40/44...........only 2 gaskets....besides the pictures you show are from an 800/850, no where even close to a 500

the previous suggestion of lower crank seals is a good one though (if the other ideas did not solve the problem)....................seem that happen with a motor like Scott has, that had been sitting for a long time in a hot dry climate...

I've had 2 motors in this year that had the inner baffle plate gasket go out and they are scavenging water into #3 & #4. It's been 10-12 years since I have rebuilt an antique like the 40/44ci 4 cyl. Merc.

With working 11-12 hour days I don't have the time to research the exact model without the correct serial numbers...so if the op had posted the serial number of his engine, I wouldn't even had replied. I would have been able to look up a breakdown of his engine.

Did you even read far enough to get to the last sentence of my last post?

[trident]

Exhaust Gaskets, 30, 40 and 44 Merc

OK, For the record, every small four Mercury ever built, KF9 up to the last 40 hp. 44, has an inner exhaust plate, with a water cooled outer cover with a gasket on each side of the inner plate. Early ones were a thin flat stainless plate, prone to warpage, later were aluminum with various 'splitters' cast in...

I don't mean to be rude, but if you don't know and haven't time to research it, what do you add to the discussion?

Sorry, I'll shut up now.

Are we all still friends?

We're all trying to help solve Scott's water leak problem. Don't want to lose sight of that...