They didn't determine cause of failures. Obviously their just parts changers and not solid tech's. I'd take it somewhere else.

I will not throw them under the bus yet.. I want to see what happens when I talk to them monday.

What engine?

1999 150 hp Merc EFI and repowered to a 200 ..

So is it the 200 EFI that continues to fail?

Generally these are very hardy engines (I have one so I'm very interested in the outcome of this thread).

Or is it the 150? Your post is not clear.

it was a 150hp efi and repowered to a 200hp

The same mechanic worked on both, and both have blown, correct?

yes...

Is it the same cylinder that blows each time? If it is I would be looking at the injector for that cylinder as if it is intermittent or not working it would starve that cylinder of gas/oil premix. If it is a different cylinder each time I would be looking at the rail pressure as as you may have a bad pump not putting up enough pressure to feed all of the injectors. You also might check the filters so see if debris is getting into the fuel system plugging off individual injectors. Best wishes of finding the gremlin.

I suspect the issue lies in the fact that your mechanic tried to make a 200 out of a 150 block. The 200 has different port timing, different tuner, different ecu, etc. A 150 ecu with 200 porting and tuner would cause a lean condition which 2 strokes definitely dislike. I am always leery when I hear shops talk about rebuilding a 150 to 200+ HP. This is far more complicated than simply swapping a few parts and when Bubba picks up his porting tool disaster is sure to follow. On the chance that everything was done correctly, the first thing I'd look at would be the injectors. Another possibility could be the cylinders bored incorrectly to facilitate piston expansion at higher temps. Had a good friend lock up a 225 Pro Max twice before the shop (in New Orleans) figured out the issue. They accepted blame and did a 3rd rebuild, which is still screaming today.

I know he change the ECU to a 200hp.. Do not know about the rest. Will find out..

Dude.... ask questions.... you'll figure it out.

Sounds like he rebuilt the motor but never diagnosed what caused the failure in the first place. Without knowing what caused the failure new pistons, bearings, rings and such are only going to last so long. Depending on your particular motor it could have been a number of different things that blew the motor such as a bad fuel injector, low oil flow or bad oil pump just to name a few. Proper diagnosis is the key to being a good mechanic! I would not let him keep wasting my time because it is apparent he has not found the source of the failure. Just my opinion.

Number 2 piston locked up again.

Number 2 piston probably not getting fuel therefore no oil! Premix doesn’t fix everything.