A spark plug is a part of your ignition system which also includes the coil pack that sits near the flywheel
and produces current when magnets in the flywheel move past them and also some wiring that connects it all.
Magnets are set in such a fashion that they meet the coil pack exactly in time when the combustion
should occur in a cylinder. A spark plug is a very important part of the system as its job is to ignite the fuel
and start the combustion.
It also bears the most load in this system due to insane operating temperatures and build-up processes that erode the electrodes over time, no wonder it is the weakest link in the ignition system.
Bad gasoline and engine problems that let oil inside the combustion chamber will contribute to premature wear of your spark plug, so it’s critical to take good care of your tractor all around.
Prepare for the spark plug changing procedure:
You will need some tools, a 5/8-inch deep six-point socket (it’s best if it is designed for spark plugs as
they have a special rubber boot inside to hold the spark plug), a 3/8-Inch or 1/2-inch ratchet (it’s great if you have a torque limit ratchet-wrench), and a spark plug gapper (you especially need it if the spark plug comes ungapped to OEM spec or when you want to recover old spark plugs for a little while).
Having a multi-meter, torque wrench and a fender clip remover tool wouldn’t hurt either, to test your old spark plugs, tighten them to spec and remove the spark plug boot properly.
Then make sure that your vehicle is parked on a nice and even surface, that your key is out of the
ignition switch and that the engine has completely cooled off.
The recommended spark plug is: John Deer M78543 (Champion RC12YC)
Changing your spark plugs:
(1) Spark Plug location
First open the hood and locate your spark plugs. You have a one-cylinder engine with the cylinder top
facing the headlights. The spark plug port is located at the top of the cylinder, near the valve cover, and is facing the front of your tractor.
You can also locate your spark plug by following the only thick wire, called the spark plug lead, coming from underneath the plastic air cooler housing, it should lead you directly to the spark plug boot.
You can remove your hood beforehand for easier access. For that, you’ll need to unplug the headlight
terminal and unhinge the hood from the plastic hinge (be careful not to damage the hinge). If you’re not sure how to do that, just leave the hood open.
Before unscrewing your spark plug, you will have to pull out the spark plug boot. It needs to be done very carefully to avoid damaging the metal tab (socket) in the spark plug boot and breaking the wire loose. Do not pull for the boot itself or GOD forbid the spark plug lead.
Pry the metal tab that sits inside the boot off of the spark plug using a fender clip remover tool or some other prying tool (like a luge head Phillips screwdriver).
Then unscrew and remove a spark plug with a 5/8-inch socket and screw a new one in, previously
checking it for a gap and adjusting it with a gapping tool if there is a need. Be careful not to over-tighten it and break the threading.
It’s best if you use a torque wrench or torque limit ratchet-wrench, that way you can tighten it to spec: 20 N•m (15 lb-ft).
You can test your old spark plug with a multi-meter set to Ohms. A good spark plug should be between 5 and 15 Ohms.
If the spark plugs are tested well, you can check the gap on them; when everything is fine, they can be good for some time.
What is a spark plug gap, and how to check and adjust it?
A spark plug has two electrodes, a central one (1) that runs longitudinally through the whole body of a
spark plug and a ground electrode (2) in the form of a hook that begins at the side of a spark plug tip and is bent towards its center.
The spark is created within the gap (3) between these two electrodes. The gap should be small enough for the spark to occur with available voltage and big enough to have an effective ignition. The manufacturer has calculated a precise gab size for proper ignition in your specific engine.
The gap size in John Deere D110 is 0.76 mm (0.030 in.)
A spark plug gap adjusts by bending the hook of the ground electrode until the gap between two
electrodes is up to spec.
For that you need a special instrument called a “spark plug gapper;” it’s usually a round piece of metal with varying thickness and thickness markings around its body, also a special bore for hooking and bending the ground electrode.
The gap should be adjusted on brand-new spark plugs that come not gapped to the OEM spec and older spark plugs, whiches can have eroded and unbent electrodes from prolonged use and frequent rapid temperature variations.
What to do when changing the spark plugs didn’t help?
If the purpose of the change was not regular maintenance but an attempt to troubleshoot the “not starting” issue bad engine performance. You should troubleshoot the rest of your ignition system, your fuel supply system, and your electrical system. It also can be the engine itself.