1975 650 Mercury - Zero Compression on top cylinder

[JunoNormandy]

Firstly, please excuse my naivety with this subject as I am new to boating and outboards.

To cut a long story short and get straight to the point, I own a 1975 650 Mercury ?Thunderbolt Ignition? that no longer runs attached to a 16ft runabout.

Compression Test Results: Two cylinders both have 120psi, 3rd cylinder has 0 psi.

I suspect a broken rod as this was owned by a young lad before me (over revving perhaps) never assume!

Ignition Test: No spark from plugs.

Fuel Test: There is plenty of fuel getting to the carburetors.

It?s pretty clear this whole motor needs a good overhaul however, my main issue here is the compression and whether I am about to embark on a wasted journey to try and fix whatever the problem maybe. (read too many negative stories)

I would be much appreciated If anyone can pass on advice or experience they have had with compression issues and if they were successful and what my next steps should realistically be regarding the compression issue.

I appreciate your time in answering me and hope one day I will also be able to help others.
Cheers!

 

[emckelvy]

Re: 1975 650 Mercury - Zero Compression on top cylinder

Not very likely to be a broken rod if the crankshaft turns smoothly (and I assume it is, since you got compression on 2 cyl's).

Most probable is that the motor overheated and fried the piston/rings. Likely the cylinder will be scored but you won't know that for sure until it's torn down.

If this is a fresh-water motor, you may have success pulling the exhaust manifold cover for inspection. Use caution if any of the cover bolts appear stuck. You'll need to apply heat via propane or mapp gas torch.

On the other side of the motor are the intake port covers, if you remove these you'll be able to peer into the ports and see the exhaust side of the cylinder.

One caveat on these 3-cyl's, they are very highly-stressed and when they overheat the block can crack. Typically on the dome of the combustion chamber of #1 cylinder. You can pull the cover over the back of the block (the one with holes in it for the spark plugs). This cover is not a cylinder head, it just covers the area where water circulates around the cylinders, which are integrally cast into the block.

With the cyl block water passage cover removed you can inspect #1 cyl for signs of cracks. Unfortunately this kind of damage isn't repairable. I've seen attempts to weld up cracks and the motor might run for a while, but will eventually crack again. Of course this time, the water pump's in great condition, and it'll force water right into the cylinder.

If you find there are no cracks in the block, you may be able to fix the bad cylinder. Just depends on how much you want to spend to fix and what you find when you tear it down.

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

 

[JunoNormandy]

Re: 1975 650 Mercury - Zero Compression on top cylinder

Thanks, I'm going to take your advice and strip it down to try to determine the condition of the bore and I will hopefully learn something in the process. It is very likely this is a fresh water motor as it shows little or no oxidation.

If the bore is cracked or scored, will this be clearly obvious and defined as a crack or a score? Is there a potential for hair line cracks that are not visible to the eye?

I'm unable to buy another motor and am hoping (if I can do almost all the work myself) to get this motor running for under or near a $1000. If taking into consideration the worst case scenario, I suspect I may be hoping for a miracle, who knows?

Once I get to see inside the cylinder (in about a weeks time) I'll get back on here with the outcome.

Thanks again

 

[rodbolt]

Re: 1975 650 Mercury - Zero Compression on top cylinder

typically your going to find a scuffed piston and a scored cyl liner.
the piston can be replaced and the liner can be bored or resleeved as nessasary.
cooling system failure is most likly what took your top cyl out, that coupled with a small design flaw.
look at the very top aft end of the block, if your tell tale outlet fitting isnt there then when you rebuild it relocate the tell tale to the top of the block.
that engine had a tendancy, when the impeller was failing due to old age and getting weak, it would develop a steam pocket at the top of the block leading to #1 cyl failure with no other symptoms.
later 650 motors had this done at the factory.

 

[emckelvy]

Re: 1975 650 Mercury - Zero Compression on top cylinder

If the bore is cracked or scored, will this be clearly obvious and defined as a crack or a score? Is there a potential for hair line cracks that are not visible to the eye?

The crack, if there is one, will be in the very top of the combustion chamber and visible in the casting from the outside. But you would have to pull the back cover (over the spark plugs) to see this.

The ones I've seen crack had chunks busted out so I expect it'll be obvious. Especially if it's the cause of your zero compression. I suppose you might see it from the inside of the cyl, too.

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

 

[jumfleet]

Re: 1975 650 Mercury - Zero Compression on top cylinder

i have low compression on number 1 cyl. it runs but it goes up then down up then down. like boggs out and goes good. back and forth until it dies, then does not wanna start for a while. what should i do.