John deere 285 lawn tractor only firing on one cylinder

Year, model, and engine model?

Typical symptom when one cylinder drops out.

OHV engines require periodic valve lash adjustment.

If a twin cylinder OHV engine, adjust valves and replace spark plugs with manufacture specified type at proper gap.

If the problem persists, could be failing ignition module or carburetor issue or other.

could be a spark plug. fuel line, filter, or carburetor maybe the coil.

I'm assuming the motor will crank over, but not start. Remove the air intake cover & filter off the carburetor air intake. Do you smell gas? Is gas dripping out of the carburetor? If not, spray a couple shots of starting fluid or carburetor cleaner into the carburetor air intake throat. Close the choke & crank the engine. If it starts, & stalls, repeat the process. If it won't stay running, your carburetor float jet is gummed up & needs to be cleaned. Check & clean the spark plug. Does the spark plug fire when the engine is cranked? If no, then you have an ignition problem.

a weak spark indicates a coil problem
that problem can come from a faulty condenser in the ignition system or incorrect timing for the system
if it has an electronic ignition system test / replace it
if it has a points system replace the condenser and set the points to the correct gap
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I had this kind of problem on my cub cadet. it was a weak coil. Replaced the coil and all was good.

Only option left.... 12 volts feeding coil is not there. The ignition key applys the 12 volts to the coil in the "Run" and "Start" positions. Verify 12 volts at input to coil and then (if needed) follow wire back to key and confirm voltage there....and (if needed) follow input wire to key from battery. One of those critters is open - since you have already confirmed the coil/spark plug wire is not at fault.

Will it restart right away or does it have to cool down? If it has to cool down, then run it until it quits. Next remove the plug, reattach the plug wire, and ground the plug to the block. Finally try to start it with the key. If you get no spark or very weak spark then the ignition coil needs to be replaced.

Let me know how it goes.

Weak spark can be caused by a bad ignition coil. The ignition coil is located under the engine fan/shroud, next to the flywheel. Check to make sure the coil lead and plug cap are not grounding against the engine robbing your spark as well.

If all of your tests are correct, the thing remaining is the ignition coil. A bit of a pain to get at and expensive too. Parts diagram here. Before you buy a new coil, see if there might be just a broken spark wire or loose connection at the spark plug contact end.

I just switched the coils around. The other cylinder went dead, so I knew I had a bad coil. Replaced the coil and mower runs great.