John Deere 420 cranks, won't start

[Spanner]

Well you got air, fuel, spark. The last thing needed is compression. It is time to dig deeper. Compression check both cylinders (doing both a dry and wet (drop oil into the cylinders via spark plug). What is compression readings for the cylinders? Leak down test for possible valve problems (valve seats, valves, valve stem seals, valve springs or bad cam).

When rebuilt - What was done? Was the bore measured? or just honed and new rings? Was the electronic ignition module replaced along with the plastic flywheel magnet that goes on crank ( if it is black plastic nope, there were know to fail, new ones are grey plastic, tip from boomers_influence@yahoo.com)

The thing about "Spark" is that you need to have it at the correct "Time".

Old-school points & condensor systems needed the timing to be correct to get any spark at all (Weak or strong) The colour of the spark gave a clue as to the condition of the points.

Modern electronic systems will spark whenever the magnet on the flywheel passes the armature or 'pick-up' irregardless of the timing of it all. But sparking at the wrong time usually causes back-firing through the carb intake.

Also faulty spark-plugs can cause similar symptoms and spark out-of-time. You say they're new so...

Check the flywheel key next. Cheap/easy option. Then go forward from there,

 

[Egon]

Try a squirt of 90 weight oil in each cylinder and then crank it over with the plugs out. Replace the plugs and try to start it.