John Deere 757

[deeresucks]

I have a John Deere 757 mower that popped back fired and sputtered while mowing. I have 117 hrs. on it. It has the Kawasaki 25 HP engine. It ran fine for about 15 min. then the noise and complete discharge in power. Long story short I removed the 2 push rods on the right side with out loosening the rocker arms. That's how bent they were. My question is. The valves push in freely. So I'm hoping that they aren't bent. If there not stuck does that mean that they aren't bent? Also what would cause the push rods to bend so badly?

 

[EngineMan]

One cause could be hydro-lock. i.e. a cylinder full of fuel, which would be caused by a plug not firing repeatedly (filling it with unburned liquid fuel). this would be a bad coil, plug, plug wire.

Another cause could be pre-ignition. a cylinder firing when it is in the wrong part of its stroke, thereby causing the explosions "up-force" to fight either the valves attempt to open, or forcing a not fully seated valve closed. this would be caused by the timing being off its mark, though your damage would be more wide spread.

Also, it could be caused by a stuck lifter, though that would ultimately lead to one of the 2 above causes.
there could be other causes, but that's the most likely 3 (short of a bad cam, and timing).


Replace them making sure they are set right, check the timing and renew the oil, there maybe fuel in it..!

 

[Rivets]

Just had the same problem last week. Found the the valve guides in both heads had moved. Before replacing just the push rods check these first. Another cause could have been that the valve clearances were out of adjustment. This adjustment should be checked every 100 hours.