help!jd l100 backfiring and 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

 

[Just Bill]

Bent valve or something holding valve open.
It doesn't sound like the valve is seating.
A piece of the piston might have came off and gotten under the valve.
The engine might have jumped timing and the piston hit the valve.
When both valves are up are they at the same hieght?
The last repair might not have found all of the damage.
Good Luck

 

[Fixemall]

Check that the valves are operating correctly then start with a new plug

 

[RPW]

Could have had a crack in the intake valve that wasn't noticed and now it's broken. If it's a two cylinder engine then you might be able to pull one of the plug wires off. If it's a single cylinder then you need to take the head off again.