[rm]

Hello all.

I have a 98 Mercury 175 2-stroke with what looks to be a burnt piston (see pics). I was cursing yesterday and I felt the boat loose power. I pulled the throttle back and the motor shut off. Steam was coming out of the pee-hole so I think it got hot. When I got home and started troubleshooting I found out that the over temp horn was disconnected by the previous owner – not good.

Any idea if this piston damage is due to improper fuel delivery / mixture or excessive heat?

Also, is it possible to replace the damaged piston – hone the cylinder or do I need a complete rebuild? All other pistons look good.

Thanks,
Ron

[SoFloStone]

Some of the Pro's will step in, but it sure looks like you blew a head gasket to me. You should not have rust around the cylinder sleeves, which leads me to believe water intrusion.

did you take a compression check prior to removing the head?

[rm]

Thanks for the follow-up.

No. Piece of the piston broke off and prevented piston from going past TDC. I was able to turn the engine by hand (forwards/backwards) with the plugs removed. All cylinders did have compression except for the cylinder with the damaged piston.

I’m trying to narrow down possible causes before I make repairs. Do you think it’s safe to say the piston damage was due to a blown head gasket and not fuel issues?

Thanks,
Ron

[CharlieB]

Anytime you have to split the cases to deal with a bad cylinder there are a few things that SHOULD BE done to ensure reliability of the engine. Skipping some of these things to save a few bucks will often end up costing far more WHEN the engine fails again.

Check the block and heads for straightness, or take them to a competent machinist and have them checked

Carbs, complete teardown, detailed cleaning, adjustment, kits advisable but not always required.

Fuel pump rebuild, with kit

Fuel filter(s), replacement or installation if none

Fuel lines, fittings, primer bulb, and fuel tank pick-up tube, clean and inspect, replace any hose even possibly suspected of age or fuel related deterioration.

Water Pump, impeller required unless less than 6 months old, kit advised

Piston rings on all cylinders

Reeds, very close inspection to determine if any need replacement

3 sets of spark plugs, they will be dying during the oil-rich break-in.

Add oil to fuel tank to increase mixture to double oil for first 10 hours of operation, follow manufacturers break-in operation directions.

Also, I very highly recommend slightly retarding ignition timing 2 - 3 degrees froooom manufacturers maximum setting during the break-in period.

[CharlieB]

Close up pictures would be nice.

Look very carefully at the 3 o'clock position of the cylinder wall, I can't tell if that brightness is metal transfer or a 'notch' worn into the cylinder wall.

Also look carefully at the top and bottom cylinders, what is that brightness in the same position?

Significant metal transfer from piston to cylinder wall indicates an over temp in that cylinder, lean fuel, dirty carb or too much alcohol, a hair too much prop for the fuel and timing, or a combination of all the above.

[rm]

Hello -- Sorry it took so long to respond. I’ve been unsuccessfully trying to separate the crankcase block for the cover. Are there any tricks?

Here are close ups of the piston. The brightness on the wall is from the piston. All other cylinders and plugs look good.

Originally the motor was oil injected. The guy that I bought the boat from said the injection wasn’t working so he just mixed the oil with the gas. Do the carburetors need to be re-jetted if the fuel / oil is premixed?

Thanks,
Ron

[rm]

Hello again.

One of the reed valves for the damaged piston was broken-off. I can’t find missing piece. Could this have caused the piston damage?