That is the whole story. This tractor hasn't been "beat" or neglected. I have taken very good care of it. I change the oil every 50 hours and it is kept clean and greased. The only thing that I may have screwed up on it was moving the cam gear due to my misunderstandin of how the timing marks are supposed to align. When the tractor quit I was mowing in the middle of an open 1.5 acre patch of lawn with absolutely NOTHING to come in contact with the blades or any other part of the tractor what so ever. I am not pushing blame on anyone or anything. I just asked if the broken valve seat could have caused this and if so what would have caused the valve seat to become pushed out enough to prevent the exhaust valve from retracting fully enough to close properly.
Well, my first question would have to be, did this mower work great until???
If so, please give us an idea of the timeline.
How many times have you worked on/disassembled this engine prior to this problem.
Point being, give us a detailed history of this engine.
