When your lawn mower kicks back, it not only damages the engine but can hurt your arm as well. This is because when the engine kicks back, it usually rips the pull cord handle out of your hand with force. So, if you are holding the starter handle tightly, then the force of the kickback will pull you toward the engine, which is not a nice feeling.
Now, as far as your engine is concerned, it will face some pretty extreme forces, which it’s definitely not going to like. So let’s take a look at the main reasons your lawn mower engine is kicking back.
Why Does My Lawn Mower Engine Kickback When Starting? (The Short Answer)
If your lawn mower engine kicks back when starting, then this is normally caused by one of two issues. The first is a misaligned flywheel that affects the timing, and the second is a build-up of pressure in the engine due to incorrect adjustment of either the intake or the exhaust valves.
2 Possible Explanations for Lawn Mower Kickback
An engine kicking back when starting, or should I say trying to start, is a pretty common problem with lawn mowers. Several scenarios can cause your lawn mower engine to either break or lose its correct adjustment. So, let’s look at what’s going on.
Incorrect Engine Timing
The flywheel and ignition coil are located on the top of your lawn mower engine. With each revolution of the flywheel, a magnet passes over the coil, generates a charge, and sends it to the spark plug. As soon as the spark plug sparks, the fuel vapor in the top of the cylinder ignites.
Now, this is fine if the fuel is igniting at the right time. If it’s not, then the piston and valve will be in the incorrect position. So, the only way the engine can release the pressure is to reverse the engine’s motion. As a result, the engine moves backward and rips the starter cord out of your hand.
For example, if you put your finger on the end of a bicycle pump and try pumping, you’ll feel the pressure build and then try to push its way back out the same way it went in. It’s basically the pump trying to work backward, the same as your engine.
So, what happened, and how can you figure out why your engine kicks back when starting? Well, a tiny metal key locks the flywheel and crankshaft together in a specific position and ensures your lawn mower engine has the correct ignition timing.
Unfortunately, this key can break if the cutting blade hits something hard. Even if the flywheel is a few degrees out of line, the timing will be off, and your engine will kick back. So, let’s go over the steps to diagnose a damaged flywheel key.
Steps to Diagnose a Damaged Flywheel Key
First, remove the cable from the spark plug to prevent your mower from firing up unexpectedly. Then, use a wrench or a screwdriver to remove the engine cover and the recoil starter mechanism. You should now be able to see the top of the flywheel.
Next, find the flywheel key located between the crankshaft and the flywheel. You should see a groove cut-out between the crankshaft and the flywheel with a tiny piece of metal holding them in line. If the flywheel key is damaged, then you’ll find it either completely sheared in half, and the two cut-outs are totally out of alignment, or the key is bent, and the two grooves are just a few degrees out of alignment.
Either way, the key will need to be replaced and the timing corrected.
Incorrect Valve Adjustment
One part of the mower’s combustion process is to draw air into the engine and then exhaust air out of the engine at specific times in relation to the piston’s position. In a four-stroke engine, the position of the piston is broken down into four strokes: induction, compression, power, and exhaust. Here’s what’s happening during these four strokes and where the cause of the engine kickback lies.
The Four Strokes of a Four-Stroke Engine
- Induction Stroke – The piston travels downward in the cylinder, drawing in air and fuel through the inlet valve.
- Compression Stroke – The piston travels up the cylinder closing the inlet valve, sealing the cylinder, and compressing the air and fuel.
- Power Stroke – The piston reaches the top of the cylinder (Top Dead Center – TDC) and then starts returning down the cylinder. As soon as the piston passes below TDC, the spark plug ignites the fuel vapor and forces the piston down the cylinder through the power stroke.
- Exhaust Stroke – When the power stroke is complete, the piston begins to head back up the cylinder opening the exhaust outlet to release the exhaust gasses. Once complete, the process starts over with the induction stroke.
The engine will kick back if the exhaust gasses can’t escape the cylinder. This is because the engine will try to position itself where there is the least amount of pressure. For example, a hard-to-pull cord could be the inlet valve, and kickback will be the exhaust valve.
So, how do you tell if you have an exhaust valve that isn’t correctly adjusted, causing your lawn mower engine to kick back? Let’s take a look.
Steps to Diagnose an Incorrectly Adjusted Lawn Mower Valve
First, use a screwdriver to remove the cover from the valves. Then take a spark plug wrench and remove the spark plug. Next, use a wrench to remove the engine cover so you can turn the engine by hand. The next job is to identify which valve is which. So, where the muffler connects to the valves is the exhaust valve, and the other is the inlet valve.
That was the easy part. Now it’s time to find your piston’s TDC. For this, I use a wooden dowel, thread it into the opening where the spark plug is installed and hold it lightly against the piston. Then I turn the flywheel by hand in a clockwise direction. This is important because you need to know the piston’s position in the four strokes.
If you turn the flywheel counterclockwise, the stroke will be backward, and this can get confusing. So, when you turn the flywheel, the piston will move up and down, pushing the dowel in and out. This is how you will find the TDC.
So, turn the flywheel until the dowel is pushed out of the engine at its furthest point. This is going to be the end of the compression stroke. Then, if you continue to turn the flywheel, the dowel will start to move back into the engine. This is the beginning of the power stroke.
The TDC is between these two points. The piston is at the point where it’s not moving in or out. Now, different engines fire at slightly different times. For example, some engines fire at 7 degrees before the TDC during the compression stroke.
Once I have found the TDC, I turn the flywheel a few degrees counterclockwise. You’ll have to check this in your service manual under the valve setting. And now that you have the piston set in the connect position, it’s time to check the valve clearance.
For this, you’ll need a feeler gauge and know the specific clearance of the valve. Usually, it’s around 0.04 of an inch. Again, this info should be in your service manual. So use the correct shim on the feeler gauge, and slide it between the rocker arm and the valve cap.
The shim needs to be able to move between the rocker arm and the valve cap with a bit of drag. If the shim is either very tight or loose, then your valve is incorrectly adjusted. Because both the intake and the exhaust valves are set to the same clearance, it’s best to repeat this test for both valves. If one is out, then usually both are.
How to Fix a Mower Engine That Kicks Back When Starting (2 Things to Try)
Well, that was definitely a pretty complicated diagnosis trying to work out what causes engine kickback. Thankfully the repairs are much easier. So, let’s go over how to fix the engine kickback by replacing the flywheel key and adjusting the valves.
Replacing the Flywheel Key
Replacing the flywheel key is actually a pretty easy process that will make a world of difference in stopping your lawn mower from kicking back. Here are all the steps you need to follow, along with all the tools and parts needed to complete the fix.
- Remove the engine cover
- Remove the recoil starter
- Brace the blade to stop the crankshaft from turning
- Remove the center nut on top of the crankshaft
- Remove the flywheel cup
- Attach a harmonic balanced puller or use a pry bar to remove the flywheel
- Remove the damaged flywheel key from the flywheel/crankshaft
- Coat the top of the crankshaft and the inside of the flywheel with WD-40
- Lower the flywheel back onto the crankshaft
- Align the two flywheel key grooves in the flywheel and the crankshaft
- Install the new flywheel key
- Ensure that the flywheel key, flywheel, and crankshaft are all seated together correctly
- Place the flywheel cup back onto the crankshaft
- Install the crankshaft top bolt
- Replace the recoil starter
- Attach the engine cover
- Remove the blade brace
Tools & Parts to Replace a Flywheel Key
- Socket Wrench Set
- Harmonic Balanced Puller or Pry Bar
- Replacement Flywheel Key
- Blade Brace
Adjusting the Valves
This fix is pretty straightforward as long as you have the correct valve clearance measurement and the right TDC offset (the number of degrees the flywheel needs to be set back from TDC). Oh, and if you were actually able to find the TDC in the first place. So, here are the steps you’ll need to follow to adjust your valves correctly.
- Remove the spark plug
- Remove the engine cover
- Remove the recoil starter
- Remove the valve cover
- Set the piston to TDC
- Adjust the TDC for the ignition spark. (e.g. back 7 degrees)
- Loosen the locking nut on the valve rocker arm mount
- Insert the feeler gauge between the valve rocker arm and the valve cap using the required feeler gauge shim
- Adjust the rocker arm adjustment bolt/screw until the feller gauge is lightly pinched
- Remove the feeler gauge
- Tighten the locknut
- Reinstall the spark plug, valve cover, recoil starter, and engine cover
- Test the engine to make sure that the kickback has stopped
Tools & Parts to Adjust the Intake & Exhaust Valves
- Socket Wrench Set
- Open-Ended Wrenches
- Screwdriver Set
- Feeler Gauge