applecroftfarm - if you look around closely, you should find an I.D. plate on your tractor. It may be under the tilting seat. Get the full model number, serial number, etc. off it and then go to the john deere website. There's a good chance that you can view the owners manual there or at least order one up.

Only the solenoid post that has the battery cable attached will show voltage without the key switch turned to start. When the key switch is turned to start is when the solenoid post with the starter cable attached becomes energized. If the key switch you installed was only "similar" to the one you removed, and you aren't sure of the function of each terminal on both key switches, it's possible that you sent 12 volt DC power to the magneto under the top cover of the engine, and that never has a happy ending. If you had a way to post photos showing the terminals of both switches and the terminal labeling, and tell us which wires you had connected to each terminal, then we might be able to tell what happened. It might also help if we knew the model number of the Murray, sometimes you can find a wiring diagram for them still. Otherwise, "similar" ain't exact enough for me. I'm sorta similar to my wife in some respects (but very different where it counts).

It could be external, but it doesn't sound promising. If they toss a rod, many times the block behind the starter is broken. Otherwise look for loose flywheel magnets, a starter that may have stayed engaged and locked. Do some digging and maybe things will be alright.

Did you have the "anti-backfire" solenoid out of the carburetor and check it to see if it is working as designed? With the solenoid out of carb, can you push the plunger (part sticking out of carb end of solenoid) with your finger? Does it pop back out when you remove your finger? If the answer to either question is no, put some spray carb cleaner (such as Gumout or Carb Medic brand for example) in a small container (such as the cap from an aerosol can) and stand the solenoid in this liquid so that only the plunger and barrel are immersed. Allow it to soak for an hour or so and try pushing in on the plunger. If you get the plunger freed up to where it pops back out by itself, go on the the electrical tests. If the answer to both questions is yes, proceed to test the electrical function of solenoid. (you may need a helper to sit in the tractor seat to perform these next tests). Unplug the solenoid wiring connector. Turn the ignition switch to the run position (just "run", not crank). Using a 12 volt test light or volt meter, check the wires for 12 volts (it will be one wire only, the other is a ground). If you DO NOT have 12 volts you will need to find out why (blown fuse or defective safety interlock switch). If you DO HAVE 12 volts, continue to test solenoid electrically. With the solenoid removed from carb, re-attach the solenoid wiring to the tractor wiring harness. Turn the ignition switch to the run position (just "run", not crank). The solenoid plunger should be drawn into the solenoid barrel with an audible click (if it does not act by itself, push in on the plunger with a finger tip, if it draws into the barrel after this, it should stay there until you turn off the key switch. If the plunger moved freely in & out during the intial test (finger only-no electricity) but will not move with electrical power applied, there is a good chance the solenoid is defective. You will need to do a couple more tests with an Ohm meter before condemning the solenoid for certain. Unplug the solenoid from the tractor harness, using the technique (from earlier, be sure the seat is occupied) for testing for 12 volts, identify the wire that shows voltage. Using your Ohm meter or a continuity tester, attach one test lead from meter to the "WIRE THAT DOES NOT SHOW 12 volts" and the other wire to the NEGATIVE post of battery. If this test shows that there is no continuity between the end of wire and the Negative battery post, this circuit is defective. You will need to find the cause, it could be a "ground side" acting safety switch if your machine uses these. If this test shows that there is continuity between the end of wire and the Negative battery post, this circuit is good and you now need to do the final electrical test on the solenoid itself. Using an Ohm meter, attach one test lead to each solenoid wire. If the solenoid windings (inner wire coils) are good, the Ohm meter should display some kind of reading from about 20 OHM to 300 or higher OHM (guessing as specs of your solenoid not known to me). If your solenoid reads "infinity", it is open. If your solenoid reads "zero OHM", it is shorted. Either case and you will need a new solenoid.