If you find yourself cranking over the engine several times before you manage to get your lawn mower started, then your mower has some type of fault. Luckily, with a quick bit of servicing, you should be able to resolve the issue and get your mower started with just one or two pulls of the cord or a quick turn of the key. So what’s going on? Let’s go over 7 reasons why your lawn mower is hard to cold-start and how to fix it.
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Why Does My Lawn Mower Have a Hard Time Starting?
If your lawn mower is hard to cold-start, it could be down to several potential mechanical issues. These issues can affect all types of lawn mowers, from walk-behinds to riding mowers and zero-turns. Here are seven items to check if your lawn mower is hard to start when cold.
- No Choke
- Damaged Primer
- Fouled Spark Plug
- Dirty Air Filter
- Clogged Fuel Filter
- Dirty Carburetor
- Bad Battery
What Isn’t the Problem With the Lawn Mower
So that you don’t waste your time checking the wrong potential issues, it’s important to remember that your lawn mower will eventually start. This means you can eliminate several issues. For example, if there were an issue with the starter solenoid, the mower wouldn’t start at all. Likewise, if there were a blown fuse in the ignition systems, again, your mower wouldn’t start. So the fact your mower does start (even if it takes some time) helps you eliminate several issues. To sum up, I’ve created the following list that details the issues you’d look at if your lawn mower refuses to start.
- Electrical Harness
- Starter Solenoid
- Starter Motor
- Throttle Systems
- Ignition
- Charging Systems
- Fuses
What Causes a Mower to Be Hard to Start? (A Closer Look)
In order to figure out why your lawn mower is hard to start when cold, you’ll have to do a process of elimination. By looking at each item one at a time, you can rule out each of them until you find the culprit. So, let’s take a closer look at each item and see how you can rule each of them out.
No Choke
Regardless of what type of gas-powered lawn mower you have, the mower has some type of choke. For example, the choke could need a manual operation type or an auto choke that the lawn mower does automatically. In either case, the choke is designed to restrict the air intake with a butterfly valve so that the cylinder head receives a rich fuel mix when first starting from cold.
With a manual choke, you need to pull the choke lever before starting the mower, then push the lever once the engine starts and is running. On the other hand, an auto choke is likely thermostatic and engages while the engine is cold and then disengages as the engine gets hot. In order to read the temperature, auto chokes have a small lever mechanism that sits on the exhaust pipe. So as the engine warms up, the lever automatically moves and disengages the choke.
However, the lever must be correctly positioned and undamaged for the auto choke to work successfully. Unfortunately, it is not uncommon for the small lever to become bent or dislodged when mowing the lawn. When this happens, your lawn mower can be hard to cold-start. To check it is working properly, look at the auto choke mechanism, and make sure it’s not damaged or bent.
Damaged Primer
Next, let’s look at the primer. In short, the primer is the little rubber button that you would press a few times to pull fuel through the fuel system and to the carburetor. If you want to learn more, check out my article on how to prime a lawn mower. Now if you don’t use the primer button, the mower relies on the fuel pump to pull the fuel through the fuel system. Unfortunately, this process can take either a long time or may not work at all, making it hard to cold-start your lawn mower.
Another thing to keep in mind is that these little rubber buttons are prone to splitting. Eventually, the fuel will break down the rubber, or the repetitive pressing motion will wear the button down. So what happens when you press a primer that is damaged? Well, the primer sucks air through the split instead of pulling fuel from the gas tank.
So check your lawn mower primer bulb and look for splits, and test to see if it is actually pulling fuel. If it’s not or you notice visible cracks, then you need to install a replacement primer.
Fouled Spark Plug
First, locate the spark plug on your mower. As the name suggests, spark plugs produce a spark to ignite the fuel, which powers the engine. Unfortunately, there are a few reasons why a spark plug can stop working, causing several problems, like your lawn mower being hard to cold-start. To explain, I will break down spark plugs into three common issues and go over how they can make a lawn mower engine hard to start.
Spark Plug Gap
To start, the gap between the spark plug ground electrode and the spark plug center electrode is set to a specific distance to achieve peak performance. The gap is designed to produce a strong spark that creates enough heat to ignite the fuel vapor generated by the carburetor.
For example, if the gap is too small, less heat will be produced, which may not be enough to ignite the fuel. On the other hand, if the gap is too big, the spark produced will be too weak and won’t produce enough heat to ignite the fuel vapor. Additionally, if the gap isn’t gapped correctly, the spark plug may not ground properly, which means the spark plug may not even spark with every cycle of the engine. Typically this will lead to misfiring, hard starting, and engine flooding. The best way to resolve this issue is re-gap the spark plug.
Dirty Spark Plug
A dirty spark plug is when carbon from burned gasoline begins to build up on the spark plug. This is the black sooty substance you’ll find covering the end of the spark plug that sits inside the cylinder head. So the carbon blocks the path of the spark and prevents the plug from grounding, which means you are not going to get a good strong spark to fire up your engine. So, you’ll want to give your spark plug a good clean.
Burned Out Spark Plug
Every time the spark plug fires, the center electrode burns down a little. As a result, the spark plug gap gets progressively worse every time you use your lawn mower. The typical solution would be to re-gap your spark plug. However, there comes a time when there isn’t enough center electrode left to re-gap the plug. In other words, it’s time for a new plug.
Dirty Air Filter
A dirty air filter will cause a few issues when cold starting your lawn mower. Above all, the dirt restricts airflow, starving the carburetor and preventing the creation of the necessary fuel-to-air mix.
So, remove the air filter from your lawn mower and inspect its condition. Now, if your lawn mower has both an air filter and a pre-air filter, you’ll want to remove both filters and inspect them. If they appear dirty, you’ll want to clean your mower’s air filter. However, if you can’t seem to get the filter/s clean enough, you’ll need to replace them.
Clogged Fuel Filter
Next on the list of components that could make starting your lawn mower hard is the fuel filter. In short, the fuel filter is the little filter that removes imperfections in the fuel and any junk that has worked its way into the fuel tank. So, if you haven’t changed your filter in a while or there is debris in the fuel tank, like grass clippings, you’ll likely need to clean or replace the fuel filter.
First, you could try cleaning the fuel filter, but as it only costs a few bucks, I recommend just installing a replacement filter. Once you do this, it will restore the fuel flow from the gas tank to the carburetor to its original flow rate, and you should have no problem cold-starting your lawn mower.
Dirty Carburetor
A carburetor needs to be able to achieve two main tasks. First, it needs to provide the engine with enough gas. Second, the carburetor needs to be able to create the fuel-to-air mixture. Therefore, components such as the fuel float and the carburetor jet need to be in good working order and not defective.
First, locate the carburetor on your lawn mower. Then, remove the fuel cup from the bottom of the carburetor to make sure that the float is operating correctly. Next, remove the jet to ensure it is free of blockages or debris. If you find that debris, then I suggest you clean your mower’s carburetor. Both of these issues can restrict the fuel flow to the engine, making starting your lawn mower difficult.
Bad Battery
The last item worth mentioning when it comes to a lawn mower that is hard to start when cold is the battery. Generally, if your lawn mower has an electric starter, it will rely on the battery to operate the starter motor. However, the starting process can take a while if the battery isn’t fully charged, so you’ll find it hard to cold-start your mower. In some cases, your lawn mower may not even start.
So, grab a battery tester and test the volts produced and the amount of amp generated. If either is low, your lawn mower will likely struggle to fire up the first time. The best way to resolve this is to fully charge your lawn mower battery.
How Do You Fix a Lawn Mower that is Hard to Cold-Start?
Now that I’ve gone over the potential causes of why your lawn mower is hard to cold-start, let’s take a look at the fixes. Here are all the steps, tools, and materials you’ll need to get your lawn mower started more quickly and easily.
Repairing the Auto Choke
To repair the auto thermostatic choke cut-off bar, you want to make adjustments when the lawn mower is both hot and cold. This is so that you know the bar is completing both operations correctly. So, when your lawn mower is cold, make sure that the auto-choke thermostatic bar isn’t making any adjustments to the choke. Then when the engine is hot, make sure that the bar is fully disengaging the choke. Now this is really just a case of straightening out the bar so that it can perform both operations. In the past, I used a pair of pliers to adjust the auto choke bar.
What You’ll Need to Adjust a Thermostatic Auto Choke Bar
Replacing a Primer Bulb
First of all, primer bulbs have many different designs, which can vary from one lawn mower to the next, So I’ll cover the steps for a common design used on many Brigg & Straton lawn mower engines.
Steps to Replace a Briggs & Straton Primer Bulb
- Remove the primer bulb’s outer ring.
- Unclip and remove the primer bulb mounting ring.
- Pull the old primer bulb from its housing.
- Place the new primer bulb into the primer bulb housing
- Install the mounting ring and clip it into position
- Install the primer bult outer ring.
What You’ll Need to Replace a Primer Bulb
- Screwdriver
- Replacement Primer Bulb (Mower Specific)
Fixing a Spark Plug
Depending on the issue with your lawn mower’s spark plug, you’ll need to carry out a few different repairs. Here are the three common repairs to fix problematic spark plugs.
Gapping a Spark Plug
To gap a spark plug correctly, you’ll first need to know the specific gap your spark plug requires. Usually, this can be found on the original box the spark plug came in, or you can look up your specific spark plug online. Typically the gap will be somewhere between 0.020 and 0.040 of an inch. So, once you know the specific gap, you can move on to gapping the plug. Now, I like to use a simple disc-style spark plug gapper as this tool includes measuring the gap, increasing the gap, and decreasing the gap. So, let’s go over the steps.
Steps to Gap a Spark Plug
- Place the gap tool between the center and ground electrode.
- Rotate the gap tool until you feel resistance between the center and ground electrode.
- Review the measurement.
- To decrease the gap, use the back of the tool to press the ground electrode.
- To increase the gap, use the hole in the tool to pry out the ground electrode.
- Remeasure the gap.
- Repeat the adjustment until the gap is within tolerance.
What You’ll Need to Gap a Spark Plug
Cleaning a Spark Plug
To clean the carbon buildup and dirt from a spark plug, all you need to do is clean the spark plug using a wire brush. Then, it’s just a case of using the wire brush to remove anything that could be causing an issue with the spark plug making a spark. Now if the dirt is a little stubborn, you can try using some WD-40, as this tends to soften up the dirt, making cleaning a bit easier. So, once you have cleaned the spark plug, it is well worth checking the gap to make sure it’s correct before reinstalling the spark plug back into your lawn mower.
What You’ll Need to Clean a Spark Plug
Replacing a Spark Plug
If you have diagnosed that your spark plug is bad, then you’ll need to install a replacement. Now, as you have already removed the spark plug, it’s just a case of reversing the process using a new spark plug. The only things you need to be careful of are that you install the correct spark plug for your lawn mower and that you don’t over-tighten the plug. It’s pretty easy to overtighten a spark plug and have it snap in half. Trust me; this isn’t ideal. So, here are the tools you’ll need to replace a lawn mower spark plug and fix a lawn mower that is hard to cold-start.
What You’ll Need to Replace a Lawn Mower Spark Plug
- Replacement Spark Plug (Engine Specific)
- Spark Plug Wrench
Cleaning an Air Filter
Most of the time, you should be able to quickly clean your lawn mower air filter to restore the carburetor’s airflow. Of course, this is as long as you don’t have an air filter covered in oil or a damaged air filter. In this situation, you’ll want to go ahead and install a replacement air filter instead. So, let’s look at the steps I use to clean my lawn mower air filter.
Steps to Clean a Lawn Mower Air Filter
- Remove the air filter housing cover.
- Remove the air filter from the housing.
- Using an air compressor, blast the dirt from the filter from inside to out.
- Clean down the inside of the filter housing using a shop cloth.
- Install the air filter.
- Install the air filter housing cover.
What You’ll Need to Clean an Air Filter
Mistakes to Avoid When Cleaning an Air Filter
There are a few mistakes you want to avoid when cleaning your lawn mower air filter. First, don’t blast the dirt into the filter; instead, blast it out. By this, I mean you want to clean from the inside of the filter, the side that sits against the carburetor. This is because if you clean from the outside to the inside, you’ll just be burying dirt deeper into the filter, which isn’t what you are trying to achieve. Second, don’t blast the air filter too hard to avoid damaging the filter material. Mainly, you need to keep the tiny openings in the filter to their original size. Therefore, if you blast too hard, you’ll open up the tiny holes, and more dirt will pass through the filter. As a result, the carburetor will clog, creating extra problems and work.
Replacing a Fuel Filter
Ideally, you should change your lawn mower fuel filter at least once a year, if not more often. Unfortunately, fuel filters are guaranteed to get dirty and will cause issues. So, for a few bucks, this easy maintenance task will avoid engine performance issues and problems starting your lawn mower. Here are the steps to replace an inline-style lawn mower fuel filter.
Steps to Replace a Lawn Mower Fuel Filter (Inline Design)
- Slide both spring clips down each of the fuel lines connected to the fuel filter.
- Remove both fuel lines from the fuel filter.
- Take your new filter and orientate it so that the arrow points to the fuel pump.
- Connect both fuel lines to the replacement fuel filter.
- Reposition the spring clips.
- Start your lawn mower.
- Check the fuel filter for leaks.
What You’ll Need To Replace a Fuel Filter
- Pliers
- Replacement Fuel Filter (Engine Specific)
Cleaning a Carburetor (Cleaning Spray)
There are two main ways to clean a lawn mower carburetor. First, you can use a carburetor cleaning spray. Second, you can remove the carburetor and carry out a thorough cleaning. So, let’s go over the first approach without removing the carb.
Here are the three stages of cleaning a carburetor without removing it: disassembly, cleaning, and assembly.
Disassembly
- Switch off the gas using your lawn mower fuel cut-off valve.
- Remove the spark plug ignition cable.
- Remove the air filter housing and air filter.
- Remove the fuel cup from the bottom of the carburetor.
- Remove the pin securing the fuel float to the carburetor.
- Remove the fuel float and float needle.
- Remove the fuel jet retaining screw.
- Remove the fuel jet.
Cleaning
- Spray all the removed parts and the inside of the carburetor.
- Clean down all the accessible parts of the carburetor.
- Pass a thin wire through the carburetor jet to clear any blockage and dirt.
- Make sure to wipe down all areas that were sprayed.
Assembly
- Insert the jet into the carburetor.
- Install the jet retaining screw.
- Install the float needle.
- Position the fuel float.
- Secure the fuel float with the fuel float pin.
- Install the fuel cup.
- Install the air filter and cover.
What You’ll Need to Clean a Carburetor
Charging a Lawn Mower Battery
Fortunately, most modern lawn mowers that use electric starters are pretty straightforward to charge. Usually, there’s no need to disconnect or remove the battery from the lawn mower. However, first make sure this is the case with your lawn mower by checking the owner’s manual. In the worst case, if you charge a battery incorrectly, the battery could explode. And then a lawn mower that is hard to cold-start will be the least of your worries.
If you do have a modern lawn mower, then all you need to do is connect a charger and wait for it to charge. I recommend that you use a smart battery charger as these types of chargers identify your type of battery and voltage requirements, and they know when to stop charging. So, this really takes out all the guesswork and reduces the risk of getting the settings wrong.
Steps to Charge a Lawn Mower Battery (Smart Chargers)
- Connect the negative charger cable to the battery’s negative terminal.
- Connect the positive charger cable to the battery’s positive terminal.
- Connect the charger to the power outlet.
- Set the battery to battery auto-detection.
- Set the battery charger to charge.
- Wait for the battery charger to tell you the battery is fully charged.
- Remove the charger from the power outlets.
- Remove both charger cables from your lawn mower’s battery.
