One cylinder has low reading...

ocuyler · #1 02-27-2005, 08:23 PM

I know this is basic stuff, so please forgive my ignorance on outboard motors.

#1 cylinder of 4 cylinders on my 1967 Mercury 500 50 HP short shaft reads 25 PSI, where the other three are 125 PSI each.

Before dropping it off for service, I ran this motor through it's paces while still attached to it's host, a 1963 Boston Whaler 13 Sport. With fresh fuel, she started right up, shifted smoothly in both directions and seemed to have reasonable, but not over whelming power. It sounded fine at all speeds, including WOT. I was actually impressed with the lack of 2 stroke smoke.

What is the significance of this cylinder reading? Will it run reliably like this for awhile? Can anything be done short of a teardown to improve the compresion in #1 cylinder?

As much as I'd love to hang a 2005 e-tec on there at $4,000, that's NOT going to happen to this boat. Your comments will be appreciated.

Bigshrimpin · #2 02-28-2005, 12:13 AM

Otto - Sorry to hear about your bad luck. I'm certainly no expert . . . but you should not run the engine in it's current state. If you do continue to run the engine, you'll destroy the block

If you take the head off . . . you'll be able to inspect the gasket and the piston for damage.

1. head gasket blown
2. cylinder walls scored or worn
3. piston rings are worn or broken
4. piston is blown

with the exception of a blown head gasket, you'll need to tear the engine down.

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Bigshrimpin · #3 02-28-2005, 01:08 AM

you might be able to find another powerhead

Here's one for sale $195

http://www.craigslist.org/eby/for/60224499.html

FELLOW-SHIP · #4 02-28-2005, 04:18 PM

Ha Otto
I would have it professionally checked and not run her until you do. If it’s just the gasket great, but usually it’s the rings that have broken due to cokeing on the cylinder walls.
FellowShip
[img]/images/graemlins/confused.gif[/img]

Scott · #5 02-28-2005, 07:42 PM

Quote:

What is the significance of this cylinder reading? Will it run reliably like this for awhile?

The readings should have been within a few psi of one another. Like Bigshrimpin and Fellowship mention ...get it to a mechanic before anymore play time or you cause anymore damage. I'm surprised you didn't notice it when you ran it unless something went south when you were running it.
Did you have compression checked before or after you ran it ??

Finster · #6 03-01-2005, 07:51 AM

Aahh, just run it! Lean out the fuel and you'll be fine. If you run it long and hard enough you may be able to bring that one bad cylinder back to life.

John R · #7 03-01-2005, 09:35 AM

Quote:

Aahh, just run it! Lean out the fuel and you'll be fine. If you run it long and hard enough you may be able to bring that one bad cylinder back to life.

Or at least get the compression more equal across the board. Will turn over real fast then too [img]/images/graemlins/tongue.gif[/img]

Fr. Frank · #8 03-01-2005, 11:51 PM

That engine has a blind bore, so no headgaskets. Sorry for the bad news. The Merc 500 50 hp was flywheel rated, and actually produced about 35-37 hp at the prop, which is how all ne ob's are rated. Rebuilding the engine is easy IF the cylinder only needs finish honing. Most machine shops are not willing or equipped to bore the pre '88 inline 4 and inline 6 Mercs.
On the other hand, There's a guy I know in Fairplay SC who will rebuild your powerhead for under $1G if the block is still good. All he does is inline Mercury OB's
Email me if you want his contact info.
My email is [email protected]

lonewolf7396 · #9 03-03-2005, 06:34 PM

Four_Boat_Theory
I would try using decarb,you may get lucky and have sticky rings on no# 1 cylinder,hay you have nothing to loose except $5.00 buck for a can of this stuff,also let it sit overnight and then recheck the compression on the cylinder

ocuyler · #10 03-03-2005, 08:10 PM

Gentlemen,

Thank you for you kind and prompt replies. I have terrific news! I spoke with the Merc mechanic (instead of the service manager)to follow-up on the approaches you-all have mentioned. I got a different story!

Cylinders 2,3 & 4 were 125 PSI, #1 was not 25, but 25 LESS THAN THE OTHERS. That's way different. So, we're back on track and every other thing checks out. Lower unit checks out, New plugs, impeller and good to go.

The only bummer is that they could not get the prop off. They quoted me 1 to 2 hours labor to "hack" it off. I think a file job will have to do. Hilton Head, here we come.

Thanks for your support.