I was given a JD LX172 by my brother in law. It ran when I got it but it had a vicious oil leak. And it only started with the blades engaged. I took the engine off the frame and pulled the bottom cover. Put in a new crank seal and gasket. While trying to put lower cover back on, it pulled the cam out of place. Barely came out, so it wasn't hard to realign the timing marks. Got the engine back in place and hooked everything back up. Well, now it won't start. The engine turns over but will not fire. Had spark at the spark plug, but put in a new one anyhow. It is getting gas. Even if I shoot in some ether, it turns over but will not fire. I reset the air gap at the coil to factory spec, no difference. Fuses on ignition module are good. Started checking switches. Here is the down low. Neutral switch works. If you put it in gear, it kills the ignition. Seat switch : not made, has 12v on one wire, made 12v on both wires. Engine cranks the same made and not made. PTO switch : with key on and switch is pushed in, unplug connecter, 12v on one wire, other goes to ground. Plug in connecter and PTO engages. 12v to one wire and engine cranks. If you pull the PTO switch, no voltage to both wires and no engine cranking at all. I can usually tinker with anything and get it to run, but this is kicking my butt.