Piston erosion

I have a 1981 mercury 70hp outboard. It was rebuilt and started eating the #3 piston in less than 15mins on break-in.( Aluminum debris fouling spark plug). Engine was torn down, again, and piston & rings were replaced (bore was not marked). Generous oiling and doubling quick silver oil in tank and in less then 10 minutes on controlled break-in, could see piston deposits on end of spark plug on #3cyl and the top cylinder looked very dry, again.
Has anyone had an issue with the #3 cyl. eating pistons ??
PS. Both carbs were rebuilt.

Re: Piston erosion

help!!

Re: Piston erosion

Is it the pistion on one end or the other? If so, I would be looking at the crank seal at that end leaking air and causing a lean condition which is worse at the closest piston.

Re: Piston erosion

It could be timing too far advanced or it could be lean mixture on the carb servicing that cylinder. Timing too far advanced would have the same effect on all cylinders while a lean carb will usually only affect one cylinder.

Re: Piston erosion

I would be looking at the crank seal at that end leaking air and causing a lean condition which is worse at the closest piston.

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Good idea and seals need to be replaced during a rebuild....

could be lean mixture on the carb servicing that cylinder

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# 2 would be lean also as each carb feeds 1-1/2 cylinders so doubt it s carb problem

You need to index the flywheel and check timing on #3 as it sounds like a bad trigger or switchbox. The bia's circuit maybe bad in box advancing timing on that cylinder..

Re: Piston erosion

Typically a lean condition would exhibit runability issues from normal you could detect, if it was a new problem to the engine and you. So my quess based on your description of both the piston and plug is you have detonation damage going on. You mentioned a new rebuild. Detonation can be brought on by many things including a lean condition, spark timing issue, excessive heat, high compression, improper or bad fuel for engine combination etc.

If you are using a good plug reader and seeing piston material detonation will usually show as "specs of pepper as well".

By any chance was that particular piston changed or over bored and changed etc. ? I have seen this.

Remember...doubling the oil in a given amount of fuel is leaning out your jetting. Fuel helps cool the piston crown. No need to go beyond 32:1-50:1.

8hygro

Re: Piston erosion

8hygro said:

Typically a lean condition would exhibit runability issues from normal you could detect...

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I disagree. If it was very lean then yes. However, you can be lean enough to do the damage described and still not be able to detect it. I ride snowmobiles and a bad crank seal will cause the damage described and the engine runs great until the damage gets worse. I would pressure test the engine before I ran it again. I pressure test my 2-stroke snowmobile and boat engine at the begining of each season. Here is a link on how to pressure test.

http://www.duncanracing.com/TechCenter/2strk_presstest.pdf

Re: Piston erosion

8hygro said:

Remember...doubling the oil in a given amount of fuel is leaning out your jetting. Fuel helps cool the piston crown. No need to go beyond 32:1-50:1.

8hygro

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Mercury, Johnson/Evinrude and Yamaha all recomend doubling oil after rebuilding a two-stroke, carbureted powerhead. That's 50:1 in the tank plus regular oil injection or 25:1 in the tank if oil injection has been disabled.

Pieces of piston appearing on the plugs sounds like detonation to me. Therfore I would lean more towards the timing.

What are the compression readings on all cylinders? Is #3 exceptionally high or low? Have you got the coil wires going to the correct cylinders?

EDIT: Also are you certain the block and intake were absolutely clean of debris? Is it possible that there was already metal debris behind the reed valves and is being sucked into that cylinder causing the damage to the piston?

Just throwing out ideas.

Cheers and good luck