rcairboater

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

Tidnab

My Feedback: (51)

Was there a gasket behind the crankcase cover ? A simple mod done to engines is to reduce the crankcase volume by adding a piece of metal or material to the inside of the cover.Maybe someone tried to have the same effect by taking out the gasket but the crankshaft may have already been close to the inside of the cover,thus making it hit the cover.I had one that was too close to add any material to the inside of it at about .020" which would have been what I wanted if I had added material.I had a gasket behind it ,so taking it out would have caused the cover to hit the crank and probably wear a hole in it.If this is the case just put a stock gasket back behind it and patch the hole from the outside with something like JB Weld.

flyinwalenda

My Feedback: (5)

I think you may have a worn out case, worn front bearing  or worn crank causing excessive end play. The rear of the crank is obviously hitting the backplate causing the swirl marks. Grab the front of the crank and push in/pull out  and see if there is movement.
I've done the backplate mod to Ryobi engines and as long as the engine is in good shape the crank won't hit. Now I've taken apart used engines to convert and have seen marks on the backplate and the crank had excessive end play.