My Toro Timecutter Z5000 won't start so I changed the spark plug but the replacement won't tighten. It's the right size listed in the manual. When I used starting fluid the entire plug popped out.

SOURCE: toro 421 snowblower won't start

check to see if in line fuel shut off valve is open.If so,sounds like carb is gummed up.

SOURCE: Won't start, just clicks

I'm not familiar with this Toro Model, but that click sound probably is the starter solenoid engaging. Have you checked the wiring from the battery to the solenoid and from the solenoid to the starter? Also, check the ground connection on the mower frame.
You may have a bad starter motor. You can by-pass the solenoid by - placing a heavy gauge wire from the POSITIVE battery terminal directly to the STARTER terminal, where the heavy wire is attached. Be careful NOT to ground out the wire from the battery.
If the starter turns - it's a bad solenoid. If it does not turn - it probably is a bad starter motor.

SOURCE: TORO Z5000 zero turn mower. right side

it should hold hydralic fluid there should be a resirvour somewhere on the back along with the pump you should be able to check the level there and if you don't get the leak stopped it will lock up the whole drive system.

SOURCE: electric starter not working on Toro

I've had the same problem (I think) - the starter will not activate. First time, I found the fuse that's attached to the battery "tray" had corroded. For that I put a good coating of silicone dielectric grease on the fuse contacts and holder. This keeps moisture out. With that it was good for a few months. Now it's gone bad again. This time the fuse is good, but the battery is dead, and so dead that it won't recharge. Apparently the cause of the discharge was/is grass and moisture finding its' way into the battery area, and providing a conductive path between the battery terminals. I was recharging it regularly, following the owners manuals' instructions, so it's definitely not a negligence problem caused by me.
So, obviously what we have is a design flaw in the mower that allows grass clippings to get into the battery area. Time for a recall, but for a quick fix for your current pioblem, you might just find the fuse is corroded, and a replacement + non-conductive grease may be all you need to git'er going.

SOURCE: my toro mower won't start.

The Toros have a Tecumseh engine and are famous for this. I'd suggest you do a thorough cleaning of the carburetor, but there is a little known secret that I've learned about this particular carb. Pull the bolt from the bottom of the float bowl on the carb (1/2 inch wrench; the "main jet"). Be sure that all the little holes in that jet are clean and you can stick a wire thru them. There is a microscopic hole at the very top of the threads on that brass bolt that often gets clogged and your mower will not start. I use the wire from the needle valve inside the carburetor to stick thru it, as it's about the only thing that'll fit. (note: if you have 50 year old eyes like me, you can't even see this hole w/out specs). Hope this helps and best of luck to you. (also, be sure you put the float bowl back on the right way; there's a bevel in the back that needs to go toward the hinge in the float or the float won't work. Learned that one the hard way as well. )
James