[Crankstomp]

Motor is a 1977 175 Evinrude model#175749S. I purchased it a few months ago with very little knowledge of it's history. Last weekend I was running across the lake at full throttle, engine slowed down, I backed off the throttle and it started knocking/rattling so I shut it off and cruised in on the trolling motor. I pulled the plugs and the #6 plug had aluminum on it. I suspected something bad, so I did a compression check(previously 115-125 on all cylinders) all checked out good except #6 cylinder. It was only about 20 psi. Pulled the head and the top of the piston was melted off and deposited on the intake side of the piston. There is some scoring, I'm guessing a few thousanths, to the clyinder walls. All the other cylinders look fine(well sort of, more on that later). I have two questions. First, what are the most likely causes of the melted piston? Is it because of running too lean? Also the engine was previously rebuilt with oversized pistons. My service manual says the piston to cylinder clearance should be .0055 to .0075 but mine is huge(just eyeballing I'd say .015 to .02), but the cylinders look good. You can still see the crosshatch from honing.. Can the pistons be this worn out with very little wear on the cylinder? Also, can the pistons be this worn out and still have 125psi compression? Sorry about writing an entire encyclopedia. Any advise is appreciated.

[Dhadley]

Lets look at some things. What spark plugs were in it whaen it burned the piston? What fuel?Had the motor been sitting? The piston to wall clearance can only be determined by removing the piston and measuring the cylinder and skirt. Dont worry too much about how much the piston moves right now.

[Crankstomp]

The boat had been sitting for about a year when I bought it. I removed the fuel tank/line, cleaned out tank, replaced line, cleaned carbs thoroughly. Gas is fresh 87 octane mix 50:1 with quicksilver TC-3. Plugs are UL-77V. I had previously checked both pick-up and full advance timing.

[Dhadley]

Describe how you set / checked full advance timing.

[Crankstomp]

I used the method prescribed by Joe Reeves where you advance the timer base to it's stop with the engine not running. You then crank it over and set the timing a 4 degrees less than spec. I just checked mine and it checked out OK

[Dhadley]

What did you set it at?

[Crankstomp]

I set it at 24 degrees with the engine cranking, 4 less than the book spec of 28 degrees.

[Dhadley]

To get to 24, did you have to change it from what it was?

[Crankstomp]

No

[Dhadley]

Did you verify it under a load afterwards?If you put the head back on and got a new spark plug, will the motor still run?

[Crankstomp]

I would assume it would, it was running after I melted the piston, but only on five cylinders.

[Dhadley]

If it still runs it might be a good idea to try and isolate the cause of the burnt piston now rather than after you have a rebuilt powerhead.