Hitting a hidden obstacle while mowing the lawn is a common frustration for homeowners. That sickening thud often precedes a bigger problem: a lawnmower that refuses to start. Understanding the potential damage and knowing how to troubleshoot the issue can save you time, money, and a whole lot of aggravation. This guide provides a comprehensive look at why your lawnmower might be refusing to cooperate after an unexpected collision.
Understanding the Immediate Aftermath
The immediate impact of hitting a rock, tree root, or other solid object can cause a variety of problems, ranging from minor inconveniences to major mechanical failures. The severity of the impact and the nature of the object struck will determine the extent of the damage.
Initially, take a deep breath. Assess the situation calmly. Don’t try repeatedly to start the mower if it’s making strange noises or exhibiting unusual behavior. This could exacerbate the problem. Turn off the mower completely and disconnect the spark plug wire to prevent accidental starting while you inspect the damage.
Potential Damage to the Blade
The lawnmower blade is usually the first point of contact in these situations. A bent, chipped, or severely damaged blade can prevent the mower from starting.
Blade Damage Symptoms
If the blade is visibly bent, even slightly, it can throw the mower out of balance. This imbalance can prevent the engine from starting, or cause it to run roughly and vibrate excessively. Look closely for cracks or chips along the blade’s cutting edge. A damaged blade not only hinders the mower’s performance but also poses a significant safety hazard.
Pay attention to any unusual noises emanating from the blade area. Grinding, scraping, or rattling sounds are indicative of potential issues. A bent blade might be rubbing against the mower deck, causing friction and noise.
Testing and Repairing the Blade
Visually inspect the blade. If you notice a bend, try to straighten it carefully using a vise and hammer. However, if the bend is severe or if the blade is cracked or chipped, it’s best to replace it entirely. Replacing the blade is generally safer than attempting to repair a severely damaged one.
Before removing the blade, make sure the mower is turned off and the spark plug is disconnected. Use work gloves to protect your hands. Typically, you’ll need a wrench to loosen the blade bolt. Remember the orientation of the blade so you can reinstall the new one correctly. Always use the correct replacement blade for your lawnmower model.
Once the new blade is installed, tighten the blade bolt securely. Check that the blade is balanced and spins freely without any obstructions.
The Role of the Crankshaft
The crankshaft is a critical component of the lawnmower engine. It converts the reciprocating motion of the piston into rotational motion that drives the blade. Unfortunately, a hard impact can bend the crankshaft, leading to significant problems.
Recognizing Crankshaft Damage
A bent crankshaft is a serious issue. It can prevent the engine from starting or cause it to run erratically. Common symptoms include difficulty starting the engine, excessive vibration during operation, unusual noises coming from the engine, and a feeling of “resistance” when trying to pull the starter cord.
In severe cases, the crankshaft can be bent to the point where the piston cannot move freely within the cylinder. This will make it impossible to start the mower.
Checking and Addressing Crankshaft Issues
Checking for a bent crankshaft typically requires specialized tools and expertise. One method involves using a dial indicator to measure the crankshaft’s runout. Runout refers to the amount of deviation from a perfectly straight axis of rotation. Excessive runout indicates a bent crankshaft.
If you suspect crankshaft damage, it’s highly recommended to take your lawnmower to a qualified repair technician. Repairing or replacing a crankshaft can be a complex and time-consuming process. It often involves disassembling the entire engine.
Depending on the severity of the damage, the technician may be able to straighten the crankshaft. However, in many cases, replacement is the only viable option. Crankshaft replacement is a costly repair, and it might be more economical to replace the entire lawnmower, especially if it’s an older model.
Shear Pin or Key Failure
Some lawnmowers, particularly those with larger blades, use a shear pin or key to connect the blade to the engine’s crankshaft. This small, inexpensive part is designed to break under stress, protecting the engine from serious damage if the blade hits a solid object.
Understanding Shear Pins and Keys
The shear pin or key is a sacrificial component. When the blade encounters a sudden obstacle, the shear pin is designed to break, disconnecting the blade from the engine. This prevents the force of the impact from being transmitted directly to the crankshaft, which could cause it to bend or break.
Identifying a Broken Shear Pin or Key
If your lawnmower suddenly stops after hitting something, and the engine still runs, but the blade doesn’t turn, a broken shear pin or key is a likely culprit. You might also notice that the blade spins freely without engaging the engine.
To check the shear pin, you’ll need to remove the blade. The shear pin is typically located within the blade adapter, which connects the blade to the crankshaft. Examine the shear pin for any signs of breakage. If it’s broken, replace it with a new one. Always use the correct replacement shear pin for your lawnmower model.
Replacing the Shear Pin or Key
Replacing a shear pin or key is a relatively simple task. First, disconnect the spark plug wire to prevent accidental starting. Then, remove the blade. Locate the broken shear pin or key within the blade adapter. Remove the broken pieces and insert the new shear pin or key. Reinstall the blade and tighten the blade bolt securely.
Engine Problems: A Deeper Dive
Beyond the immediate damage to the blade and crankshaft, hitting an object can sometimes cause internal engine problems that prevent the mower from starting.
Flooding
A sudden stop can sometimes cause the carburetor to flood, meaning that too much fuel enters the engine cylinder. This can make it difficult or impossible to start the mower.
If you suspect flooding, try removing the spark plug and pulling the starter cord several times to clear excess fuel from the cylinder. Let the spark plug dry before reinstalling it.
Fuel System Issues
The impact could dislodge fuel lines, filters, or other components of the fuel system, preventing fuel from reaching the engine. Check all fuel lines for cracks or leaks. Make sure the fuel filter is clean and not clogged. The carburetor might also need cleaning or adjustment.
Ignition System Malfunctions
Although less common, the impact could potentially damage the ignition system, preventing the spark plug from firing. Check the spark plug for proper spark. You can do this by removing the spark plug, connecting it to the spark plug wire, and grounding the spark plug against the engine block. Then, pull the starter cord. You should see a spark jump across the spark plug gap. If there’s no spark, the spark plug might be faulty, or there could be a problem with the ignition coil or other components of the ignition system.
Safety Interlock Switches
Lawnmowers are equipped with safety interlock switches to prevent accidental starting. These switches are typically located on the handle, seat (for riding mowers), and blade engagement lever. If one of these switches is faulty or dislodged, it can prevent the mower from starting.
Check all safety interlock switches to make sure they are properly engaged. Make sure the bail lever on the handle is fully engaged, and that the blade engagement lever is in the correct position. Inspect the wiring connections to the safety interlock switches for any signs of damage or disconnection.
The Importance of Regular Maintenance
Regular maintenance is crucial for keeping your lawnmower running smoothly and preventing problems. This includes changing the oil regularly, cleaning or replacing the air filter, sharpening the blade, and inspecting all components for wear and tear. A well-maintained lawnmower is less likely to suffer serious damage from hitting an object.
Preventative Measures
While it’s impossible to completely eliminate the risk of hitting an object while mowing, there are several steps you can take to minimize the chances. Before mowing, walk the lawn and remove any visible rocks, branches, or other debris. Be especially careful when mowing near fences, trees, and other obstacles. Avoid mowing in areas where you know there are hidden hazards, such as underground pipes or tree roots.
When to Call a Professional
If you’ve tried troubleshooting the problem yourself and your lawnmower still won’t start, it’s time to call a qualified repair technician. Complex engine problems, such as a bent crankshaft or a malfunctioning ignition system, often require specialized tools and expertise to diagnose and repair. A professional technician can accurately assess the damage and recommend the best course of action. Trying to fix complex problems yourself without the proper knowledge and tools can potentially cause further damage and void your warranty.
Ultimately, understanding the potential consequences of hitting an object with your lawnmower and knowing how to troubleshoot the issue can save you time, money, and frustration. Regular maintenance and preventative measures are also crucial for keeping your lawnmower running smoothly and preventing future problems.
Why won’t my lawnmower start after hitting something in the yard?
Hitting a solid object with your lawnmower blade can cause several issues that prevent starting. The most common is a sheared flywheel key. This key is a small, inexpensive part that connects the engine crankshaft to the flywheel. Its purpose is to protect the engine from severe damage by breaking if the blade suddenly stops, misaligning the ignition timing.
Beyond the flywheel key, the impact could have bent the crankshaft. This is a more serious issue as a bent crankshaft can prevent the engine from turning over properly or cause internal damage. Also, check the blade itself for severe bends or damage. A heavily damaged blade can create excessive vibration and resistance, hindering the engine’s ability to start. It’s also essential to check for debris lodged around the blade and engine components, which can physically obstruct movement.
How can I tell if I sheared the flywheel key?
A primary symptom of a sheared flywheel key is that the engine pulls over very easily, feeling as if there’s little to no compression. Normally, you would feel resistance as the piston moves in the cylinder, compressing the air-fuel mixture. With a sheared key, the flywheel is no longer properly timed to the crankshaft, and the engine might not even attempt to fire.
To confirm a sheared flywheel key, you’ll need to remove the starter assembly and the flywheel cover. Once exposed, carefully inspect the key for any signs of damage, shearing, or being out of place. Replacement keys are readily available at most hardware or lawnmower repair stores and are relatively easy to replace with basic tools.
What damage could a bent crankshaft cause?
A bent crankshaft is a significant problem that can lead to numerous issues. The most immediate consequence is difficulty starting the lawnmower. The bend can create excessive friction within the engine, making it harder for the piston to move and reducing compression. You might also experience a strange knocking or vibration during operation if it does start, indicating the crankshaft isn’t rotating smoothly.
In the long term, a bent crankshaft can cause severe engine damage. The uneven forces can wear out bearings, connecting rods, and other internal components prematurely. If the bend is severe enough, it can even lead to engine failure. Repairing a bent crankshaft is often not cost-effective, and engine replacement is usually the recommended solution.
How do I check the lawnmower blade for damage?
Visually inspect the entire length of the blade, looking for any bends, cracks, or significant damage. Pay close attention to the tips and cutting edges, as these are the most vulnerable areas. A bent blade can cause imbalance and vibration, making the lawnmower difficult to control and potentially damaging the engine.
Even if the damage seems minor, it’s crucial to replace a damaged blade. A compromised blade can break during operation, posing a serious safety hazard. Always wear gloves and eye protection when handling a lawnmower blade, and disconnect the spark plug wire to prevent accidental starting during inspection.
What other parts of the lawnmower should I inspect after an impact?
Beyond the flywheel key, crankshaft, and blade, examine the blade adapter or spindle. This component connects the blade to the engine’s crankshaft. It’s susceptible to damage from sudden stops and can become bent or cracked, preventing the blade from spinning properly. Also check the mounting bolts that hold the engine to the mower deck, ensuring they are tight and haven’t been damaged in the impact.
Check the fuel lines and carburetor for any signs of damage or disconnection. The impact could have dislodged a fuel line, preventing fuel from reaching the engine. Similarly, inspect the air filter to ensure it’s clean and not obstructed. Finally, look for any loose wires or damaged electrical connections, as these could prevent the engine from getting the necessary spark to start.
How do I prevent this from happening again?
The best way to prevent damage from hitting objects is to thoroughly inspect your lawn before mowing. Remove any rocks, branches, toys, or other debris that could be struck by the blade. Clear the area you plan to mow, paying special attention to areas near fences, trees, and garden beds.
Raise the cutting height of your lawnmower, especially on uneven terrain. This will reduce the likelihood of the blade hitting hidden objects. Consider using a walk-behind trimmer (weed whacker) to clear areas with obstructions before using the lawnmower. Finally, mow in overlapping passes to ensure a consistent cut and reduce the chances of missing hidden objects.
When should I call a professional lawnmower repair technician?
If you suspect a bent crankshaft, or if you are uncomfortable disassembling the engine to check the flywheel key, it’s best to consult a professional. Diagnosing and repairing internal engine damage requires specialized tools and expertise. Attempting to fix a bent crankshaft yourself could lead to further damage or injury.
Also, if you’ve checked the obvious issues, such as the blade, flywheel key, and fuel lines, and the lawnmower still won’t start, a professional can perform a more thorough diagnosis. They have the experience and equipment to identify and repair complex engine problems, saving you time and potential frustration.