I have a friend that has a 2 cycle 25cc mtd engine that came off a trimmer. It's been on a airboat with a cut down 18x6 pusher propeller. The back plate on the engine is a cheap plastic type. It goes into the crank case and the last couple times running the boat it has ran terrible. After looking over the engine, we noticed the plastic back plate had a hole in it.
We removed the back plate and on the inside of the plate it has swirl marks and it appears that the connecting rod did the damage.

Something inside the engine rubbed a swirl hole in the back plate.

Anyone know what may have caused this and why? Looking for feedback on this one.

Thanks,
-Gary