John Deere L120 Starting & Running Problems: Quick Fixes

Jessica Stone
by Jessica Stone

With the proper care and regular maintenance, your John Deere L120 will last you for several years with very little problems. However, if you can’t get your mower to start, it loses power before you can finish the job or smoke is escaping from underneath it, it’s important to diagnose, troubleshoot, and fix the issue as soon as possible.

These are just some of the possible problems that can occur with a John Deere L120 riding mower. Knowing how to accurately troubleshoot issues and find an explanation will help to reduce stress and give you a greater sense of independence.

Let’s explore a few most common problems associated with the John Deere L120 along with the possible solutions.

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John Deere L120 Won’t Start

Most often, when a John Deere 100 series is having issues, it has something to do with not being able to turn over in the first place. A riding mower that won’t start could be due to a number of issues including bad or no gas, a faulty battery, dirty fuel filter, problems with the spark plugs, and more.

Bad or No Gas

If your John Deere L120 won’t start, it’s possible that it is low on fuel. This is especially true if the mower has been kept in storage for longer than 30 days. When these machines sit for a while, it opens up the potential for more water to exist in the tank than fuel.

If you believe this to be the case, you can simply put fresh fuel in the tank and, after a few tries, the mower should start up as normal. However, if the mower has sat idle for three to six months, it has likely gone bad and will either need to be replaced or you can try adding a fuel stabilizer.

Quick Tip: For a John Deere L120, you should be using a fuel that has a minimum octane rating of 87. However, a higher rating is generally preferable and better for the machine.

Faulty Battery

To troubleshoot a riding mower that doesn’t start, turn the key and examine the reaction. If it cranks slowly, makes a clicking noise, or does not crank at all, the issue may lie with your battery. To remedy a faulty battery, check for any loose connections by wiggling the cables and use a wire brush to clean them.

Lift the seat to inspect the battery and if you have loose cables, tighten them using an open-end wrench. To clean, remove the negative cable first and use the brush to remove any built-up corrosion. Then, reattach the cable and attempt to start up the mower again.

If this doesn’t fix the problem, you can use a multimeter to test the battery’s voltage. If the voltage is any lower than 12.7 volts, the battery needs to be charged or replaced entirely. Another tell-tale sign of a battery that needs replacing is if your John Deere L120 needs a jump every time before mowing your lawn.

Spark Plugs Need Replacing

If the engine in your John Deere L120 at least tries to turn over, then you can check the spark plugs off your list. However, spark plugs are often overlooked and in order to maintain the proper function of your mower, they should be switched out every season or every 100 hours.

If you haven’t changed the spark plugs in a while, replace them and see if this corrects the issue. If you’ve recently installed new ones, examine them to see if they need to be cleaned or are loose.

Dirty Fuel Filter

The fuel filter on your John Deere L120 is essential for keeping dirt, debris, and other gunk from infiltrating into the carburetor. Over time, that gunk will start to collect and limit the flow of gas.

Take a look at your fuel filter and if it appears to be dirty or it hasn’t been changed in a couple years, the machine will likely benefit from installing a new one. Fuel filters are generally very inexpensive and simple to swap out, making this task an easy one to try for fixing the issues with your mower.

Dirty Air Filter

Similar to the fuel filter, the air filter’s job is to collect dust and debris that could potentially restrict airflow. It should be replaced with a new one regularly in order for your John Deere L120 to continue running at its full, best capacity.

Remove the filter and attempt to tap the debris loose or give it a good clean with a vacuum. You can also use a mixture of mild dish detergent and warm water to dislodge and rid the air filter of any debris. If the dust in the filter is extensive, you should replace it.

After cleaning, make sure that you allow enough time for the filter to dry before re-installing it. Once dry, reinstall the air filter and close the hood before attempting to start up the engine.

John Deere L120 Loses Power During Mowing

If your John Deere L120 riding mower decreases in power halfway through the mowing job, there could be a couple of reasons as to why this is occurring. The first thing you should consider is the grass that you are cutting, as you may be trying to cut too much.

If the type of grass you are mowing is tall or thick, you need to raise the mower deck up a bit to compensate. Also, make sure you do a couple of different passes over the area. On the other hand, if you’re cutting short grass, the issue may lie in the deck itself.

You could have grass clippings or other debris lodged underneath the mower deck. If this is the case, a good cleaning should resolve the issue. Additionally, if there are any issues with the spark plugs or air filter, this could be another reason why your mower loses power in the middle of a mowing job.

John Deere L120 Has Smoke Coming from Underneath

When you see smoke coming from the underside of your riding mower, your first response may be to panic. This is undeniably a stressful and frightening experience. If smoke is releasing from underneath your John Deere L120, you need to immediately turn the machine off, dismount, step away and allow a few minutes for the mower to cool down.

Most of the time, this is caused by debris in the mower deck that is blocking the discharge. Simply cleaning out the mower deck can often correct this issue and prevent the smoke from reoccurring. If it doesn’t, the problem may be with your blades. Consider replacing them or having a mechanic take a look and attempt to fix the issue.

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Wrapping It Up

Whether the problem with your John Deere L120 lies with the battery, spark plugs, fuel, air filter, or lodged clippings in your mower deck, knowing how to effectively troubleshoot will help you to fix the issue and have the machine working like new in no time. Just like anything, the appropriate maintenance and care will ensure that your John Deere L120 remains in good, working condition for many years to come.

For more riding mower tips, check out: “ How To Tell If A Riding Lawn Mower Engine Is Seized and “ How To Lift Riding Mower To Change The Blades.”

Jessica Stone
Jessica Stone

Jessica considers herself a home improvement and design enthusiast. She grew up surrounded by constant home improvement projects and owes most of what she knows to helping her dad renovate her childhood home. Being a Los Angeles resident, Jessica spends a lot of her time looking for her next DIY project and sharing her love for home design.

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