4th Cylinder - No Compression. Pulling the head questions.

[deener]

Hi all,

I have been lurking in this forum for a while now and have come across quite a few very informative posts. Unfortunately, my first post is a fairly large problem - perhaps someone can help?
This is my first boat and I am new to outboards. I am a 'hobby' auto mechanic, with decent tools.

I think I have run the gamit on troubleshooting some significant power loss (low RPM at WOT) to my 1969 Mercury 500 Thunderbolt 50 HP - serial 2360051, I only wish I had checked compression first! The compression on cylinder 4 is about 2-5lbs!! The rest are 119, 120, 120. I found this out AFTER checking carb linkage, rebuilding the fuel pump, checking fuel pressure, checking for an airbound fuel system, replacing spark plugs, checking spark gap (shoots a blue arc arcoss 7/16 no problem), fuel tank(s) cleaning and checking for restrictions, throttle cable adjustments, replaced an ugly ground wire, and reassembled the mercontrol. Whew.

My next step is going to involve pulling the power head. Before I just run out and buy a new engine, I want to see exactly what the problem is. I confirmed that the piston has travel in the cylinder - it travels as far as the others so I my lucky guess is a ring problem...(?). I tried deep creep into the carbs while running and even sitting in the cylinder overnight to unstick a possibly stuck ring. Yeah I know...if is was just stuck i'd have more than 2-5lbs compression, but the optimist in me had to try.

Regarding pulling the powerhead, I have some questions:

1. Do I need to drop the lower unit? Doesn't look like it.
2. Do I have to pull the flywheel?. If I do, does it only go back on one way (perhaps with a notch of some sort)?
3. Would removing the plate on the port side allow me to possibly see the cause of the issue or does it sound like a full pull is in order?
4. I've heard to watch for the water tube and shifter pieces. Any other tips or suggestions that may save me from breaking anything else while yarding the powerehad out?
5. If I have to replace the powerhead, is there an interchange rule I can follow for this year/model?

Many thanks in advance.

 

[Iceman66]

Re: 4th Cylinder - No Compression. Pulling the head questions.

Get your self a repair manual(seloc or Clymer)

1. No,but driveshaft can be really good seized/corroded in the crankshaft.
2.Yes, and can only fit one way(a key on crankshaft)
3.Try this first and observe,if you can get the Exhaus coverplate bolts out(might break)
4.Correct observed.

5.Check the "new" powerhead properly.

 

[deener]

Re: 4th Cylinder - No Compression. Pulling the head questions.

Thanks IceMan. It turns out that I have a Seloc. It came in handy to get the head off. Lower gasket and exhaust gasket are in great shape so I suspect some work was done to it fairly recently - I've only had it for a year.

I discovered that the piston in that cylinder has a broken skirt! What would cause such a thing? I have never overheated it and it pees really well, instantly and strongly from startup. I didn't notice any water on the plugs either. I guess I will know more about the cause when split it open, but I am just cuirious about what condition to look for that would cause such destruction? If I end up putting this thing back together with a new kit, I hope to eliminate any possible underlying cause.

 

[canoemang]

Re: 4th Cylinder - No Compression. Pulling the head questions.

Could be a ring snapped and got hung up in a port and skirt came down on it n got chewed up..

 

[Faztbullet]

Re: 4th Cylinder - No Compression. Pulling the head questions.

Excessive cylinder clearance will cause this along with excessive rpm. When you pull it apart have all the cylinders miked for wear around the ports.