What can you see ?

Uxboat

Uxboat

Petty Officer 3rd Class
Joined
Nov 14, 2006
Messages
75
Hi,
I have a 1977 Mercury 500 50 hp outboard,with no compression on the 3 rd cylinder down.Debating what to do with this animal as I worked the problem backwards trying to get it to rev up under load.First I changed the impeller,gear oil then the plugs and finally rebuilt both carbs and fuel pumps.Also one coil before I did the all important compression test.
So if I remove the side cover can you I see the side of the pistons ? I borrowed a Rigid SEE SNAKE but it was too large to enter thru the spark plug hole.From the manual I have looks like the cylinder head and block is one,is that correct ?
Thanks Mike
 
Uxboat

Uxboat

Petty Officer 3rd Class
Joined
Nov 14, 2006
Messages
75
Re: What can you see ?

Forgot to mention the lower carb I rebuilt feeding the 3rd cylinder was missing the little black plastic venturi.So I replaced it, was wondering if it caused a too lean condition and fried the rings or even put a hole in the piston.If you remove the exhaust cover on a mercury 4 cylinder can you see a portion of the rings ?
Thanks Mike
 
C

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,158
Re: What can you see ?

Pull the lower transfer cover on the starboard side of the block You will be able to see the piston and most of the cylinder. Look for broken or missing rings and scratches on the cylinder wall.
 
Uxboat

Uxboat

Petty Officer 3rd Class
Joined
Nov 14, 2006
Messages
75
Re: What can you see ?

Thanks a million,that,s the answer I was hoping for.A guy told me on mercury 4 cylinders if the top cylinder went bad that was overheating,if the bottom cylinder was bad it was likely the lower roller bearing that had broke up. Did you ever do one of these motors ? Block and head I think from the manual pictures are one piece.
Thanks Mike
 
C

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,158
Re: What can you see ?

Block and head are one piece. Top cylinder scratching can be caused by overheat, too much timing advance or lean fuel-air mixture. Bottom cylinder issues are usually bad crank seals. The lower bearing gets plenty of oil, so it is usually pretty robust.
 
Uxboat

Uxboat

Petty Officer 3rd Class
Joined
Nov 14, 2006
Messages
75
Re: What can you see ?

Thanks guys, think this will be a winter project.Just can,t tell people enough ever decision to buy or job to repair should start with a compression reading.
 
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