Help locating what caused #6 rod to break (detonate)
 

Let me start off by saying I know it's an outboard but there is a lot of knowledge on this site and same principles apply.

I have a mercury 250efi 2 stroke which detonated #6 piston. The rod broke in half. All the other pistons and cylinders look good. The shop rebuilding the motor says that cylinder definitely starved for fuel which caused the failure.

I have taken the injectors to get tested, I was present and they were all firing great. The guy doing the test bench tested them at full load, pulsing and other tests and they were all the same. I took the fuel line coming out of the tank and it is clean with no blockage anywhere. I'm starting to think something else caused this? Am I missing something? That cylinder was definitely getting fuel. I'm thinking the rod broke in half from fatigue, causing top of piston to hit head which caused piston to disintegrate and look white?

Any help would be appreciated, thanks

Have you put this up on Scream & Fly?? They are the Outboard Guru's. Could be the wire going to the Injector that has the Problem.

Dam that dose look like it leaned out failure there!!! Now I dont know 2 stroke marine engines, but I know / see hundreds of blowen up snowmobile engines (their 2 strokes ;)).

When they tested the injectors had they removed the baskets or did they just slam them on the flow bench? Did they test the with an ohm meter? They also should be tested at engine temp as well as ambient. I have had a few injectors work well and go through a cleaning machine only to find under heat and load they start to fail electrically.
The bottom cylinders are the leanest. What type of fuel, oil and maintenance have been done. By all means your injectors may be fine but poor quality oil, poor quality fuel, and low fuel pressure can marginalize a fuel system so that your just past the edge of lean causing the weakest (leanest) cylinder to fail. One more thing would be to check water flow passages around the cylinder making sure you did not have a hot spot.

The shop's suggestion seems more likely than the scenario that you proposed. Definitely looks like that jug got hot and melted the piston. Could have been many things. I have no idea what the intake manifold / head arrangement looks like on that engine, but an (intake) air leak into that cylinder would cause a local fuel starve situation even if the injectors were firing correctly. In the air cooled 2 stroke motorcycle world the air leak generated by a leaking head gasket can burn a hole in a piston. It certainly looks like there were some cylinder head sealing issues with that cylinder on your engine, based on the corrosion pattern near the bottom. That could have played into your problems.

What does $hooter say?

Mrv8s comments make sense as well. It's hard to tell exactly from the one picture, but the little light colored freckles on the other pistons are most likely from pre ignition or detonation as well. Could have been a snow ball effect of several factors and then maybe you got into a tank of bad gas. It's early in the season up here still, was this on the fuel you winterized with? Sometimes pre ignition can damage a piston enough to cause a hot spot just like carbon buildup. If I were you is dump your gasoline before hooking up a new engine.

Thanks for the suggestions everyone. As far as the motor the crank and everything else is good. The rod literally broke in half the piston hit the head and damaged the head. This is on fresh 87 gas. Ive always used 87 and never had a problem on these 250's. My other motor is good thats what got me home. I was expecting the injector to be dirty but it wasnt.

the lack of carbon makes me wonder if water got in the cylinder while running.