Mowing jd 17hp backfire wont start

[bandrew]

i have a jd l100 17hp. it recently sucked the butterfly valve screws through the intake and into the engine. the head and piston was mangled up but not too bad. i then took it to a guy that put a new head on it. i mowed with it once and it ran fine. in the middle of mowing my neighbors yard i shut it off and went to start it up and it wouldnt start, just kept back firing. once i got it into the garage i pulled the spark plug out, it has extremely low compression. when i pulled the valve cover off the exhaust rocker arm will push the valve down, but comes up about 1/4 of an inch off of the valve. i adjusted the rocker arm down to the valve and it still did the same thing. any ideas of what the problem could be? any help at all would be apreciated

 

[tcartwri]

1/4" is about the thickness of the valve seat. It has probably popped out of the casting and is holding the valve open. Are both springs the same height when the valves are closed? They should be.

 

[5030]

and/or a burned valve. Time to yank the head and have a look-see.

 

[bandrew]

pulled the head off and ring that valve is seated in came down what caused this and is there a way to prevent it from happening in the future

 

[tcartwri]

If your plan is to re-insert the valve seat, you can lightly dimple the surface of the head around the radius of the seat with a centre punch. No guarantee that it won't happen again.

If the head was overheated, it could have caused the seat to pop out. Overheating could have been caused by numerous things, but likely pre-ignition or detonation. If the engine burns a little bit of oil, this will worsen the detonation. So, you should be running a higher grade of fuel.