Re: mercury 1976 1175 v6 2 stroke outboard melted piston
A "dead hole" will definitely make it run poorly in the water!!
Most likely #5 had been running lean for quite a while and probably burnt up from detonation.
What I'd recommend is servicing ALL the carbs while the motor is down for repairs. Then you know this isn't going to happen again. Also check all the fuel lines for rot/restrictions. Rebuild the fuel pump(s), you can get a diaphragm/gasket kit cheap enough.
There are a few problem areas which would be good to address while it's apart; if you're replacing rings in all cylinders, you'll be replacing both head gaskets. That's a Good Thing as it's not uncommon to see head gasket problems crop up on older motors. Even if you don't open the motor up that far, be sure to do both heads.
Take the heads to an automotive machine shop and have them surfaced/trued up. You can even do this yourself by using sandpaper on a metal surfacing plate or piece of glass.
Another problem area lies with the exhaust manifold. The gaskets dry up and then they leak water into the engine. Not good for running either! New gaskets will ensure you don't have any problems with the motor down the road. Inspect the inner exhaust baffle for cracks, perforations, and warpage. This sealing area is (besides the heads) just about the most critical.
Be sure you replace the wristpin and conn rod big-end roller bearings when you renew that piston. Note you may be able to find a very servicable piston/rod assy on eBay, it's worth a look anyway.
When renewing the dead cylinder, if the conn rod has body-fit bolts with nuts, you can re-use the rod bolts. Degrease and put a drop of Red Loctite on the nutted area of the threads.
If the conn rod bolts thread directly into the rod, without nuts, you MUST renew the bolts as they are "stretch" or Torque-to-Yield bolts and cannot be reused. If you do they are virtually guaranteed to break.
Be sure to torque the conn rod bolts, head bolts, and exhaust manifold bolts to specs. You'll need to pick up a manual to help with this. These torques are extremely critical and can cause you to lose an engine if not done correctly.
When you're in there, check the upper and lower crank bearings for roughness, inspect the crankshaft and rods for pitting/wear/etc. Renew the upper and lower crank seals.
You'll also want to time/synchronize the motor to specs, this'll ensure you don't destroy it from detonation caused by too much spark advance.
One last thought, I can't recall if the 175 had chrome bores, but if it does, don't use any kind of hone or 'glaze-buster' on it.
If they're just steel liners, it's OK to use one of the "dingleball" type of deglazers to 'rough up' the bores so they'll seat-in new piston rings. Only run the glaze-buster enough to get a good cross-hatch, and no more.
The V6 actually isn't that bad of a motor to work on, it's just a bit on the heavy side. Don't try lifting it off the motor by yourself, get a strong friend, an engine hoist, or eat a bunch of Wheaties before you start. I did it in my younger days but no more, that's fer sur!
Oh, and this site is a great place to look up parts/diagrams for your motor:
http://www.mercruiserparts.com/SelectModelType3.asp?class_id=2
HTH & Happy Wrenching...........ed
