Having a pull start rip out of your hand can not only be a painful experience but can also make getting your lawn mower started difficult. Unfortunately, this is actually a pretty common problem with most lawn mowers. It usually comes down to either poor adjustment, a disconnected part, or a damaged part. So, let’s look closely at what could be going on with your lawn mower engine and why your pull start rips out of your hand.
Why Does My Mower’s Pull Start Rip Out of My Hand? (The Short Answer)
When a pull start rips out of your hand, or a lawn mower pull cord jerks back, it usually means fuel is combusting too early, or compression in the cylinder is not able to escape at the right time. Here are 5 potential problems that can cause these symptoms.
- Valve Incorrectly Adjusted
- Push Rod Disconnected from the Rocker Arm
- Push Rod Damaged
- Flywheel Misalignment
- Slipped Magnet in the Flywheel
What Causes My Lawn Mower’s Pull Cord to Jerk Back (5 Possibilities)
So, let’s look at these 5 potential issues and see how they affect your mower. This should help you diagnose what is causing your pull start to rip out of your hand.
Valve Incorrectly Adjusted
There are two valves inside your lawn mower’s engine: the inlet and exhaust valves. These valves are designed to open and close at specific times to allow fuel and air into the cylinder, compress these two elements, and finally exhaust them as combustion gasses.
So, if this series of events can’t happen at the right time, you’ll be fighting the compressed air in the cylinder head. So, you’ll need to inspect the valves to make sure they are operating correctly.
Here are the steps to inspect the valves.
- Remove the spark plug from the engine.
- Remove the valve cover from the engine.
- Turn the engine so that the piston is at top-dead-center (TDC).
- Place a feeler gauge between the rocker arm and the valve cap on the inlet valve.
- Determine the gap distance.
- Remove the feeler gauge.
- Place the feeler gauge between the rocker arm and the valve cap on the exhaust valve.
- Determine the gap distance.
- Remove the feeler gauge.
Usually, the gap for the inlet and the exhaust valves should be set to approximately 0.04″ when the engine is positioned at TDC. Now this will vary between different lawn mowers. Therefore, you’ll need to check your service manual for specific measurements. So, if the valves are not opening and closing correctly, you can experience kickback.
Push Rod Disconnected from the Rocker Arm
Two metal rods sit against the crankshaft and the underside of the rocker arms, causing the valves to move at the right time. Now, as the crankshaft turns, it forces the push rod to extend and push against the rocker arms in alternating motions. As a result, one valve opens while the other is closed, and vice versa.
So, if you look under the rocker arms, you’ll see the valve on one side and a thin metal rod on the other. This is a push rod. If the push rod isn’t firmly positioned against the underside of the rocker arm, then the valve will stay closed. This is what is causing the lawn mower pull cord to jerk back.
Push Rod Damaged
The push rod needs to be a specific length for the rocker arm to move the valve effectively. If there is any excessive wear or bending, the push rod won’t achieve the required travel. So, you’ll need to loosen off the rocker and remove the push rod.
To do this, use a wrench and loosen the lock nut, then adjust the rocker arm enough so that you can turn the arm. With the rocker arm turned out of the way, you should be able to pull the push rod from the engine and inspect its condition. You’ll want to repeat this for both push rods.
Flywheel Misalignment
The flywheel is located on top of the engine and is responsible for various jobs. The one you are concerned with is its timing. The flywheel is locked into place on the drive shaft with a small metal key. This is so that the magnet on the inside of the flywheel can initiate a spark from the coil to the spark plug at a specific time. If the engine sparks at the wrong time, then it will force the piston to move backward.
Now, if this happens while you are trying to start your lawn mower’s engine, then it will rip the pull cord out of your hand. So, to confirm if the timing is out and the key is broken, you’ll need to be able to get to the top of the flywheel. Here are the steps to inspect the flywheel key.
- Remove the spark plug cable.
- Remove the engine cover.
- Remove the recoil starter assembly.
- Brace the blade.
- Remove the top nut of the drive shaft.
- Lift off the flywheel cup.
- Inspect the flywheel key.
Slipped Magnet in the Flywheel
Another cause for your lawn mower pull cord to jerk back is a slipped flywheel magnet. The result is just the same as if you had a broken flywheel key because the engine is trying to fire at the wrong time and forcing the piston to move backward.
Usually, when this happens, you won’t be able to start your mower because the air and fuel won’t be inside the cylinder head at the same time as the spark, and you won’t get a kickback. But if the magnet has only slipped a little bit, say a few degrees in the flywheel, then the spark can happen when fuel is beginning to be compressed, and you’ll get a kickback.
So, you need to remove the flywheel and inspect the flywheel magnet. Here are the steps to access the flywheel magnet to check its position.
- Remove the spark plug cable.
- Remove the engine cover.
- Remove the recoil starter assembly.
- Brace the blade.
- Remove the top nut of the drive shaft.
- Lift off the flywheel cup.
- Lift off the flywheel.
- Remove the flywheel key.
- Inspect the underside of the flywheel and check the magnet position.
How to Fix a Pull Start That Rips Out of Your Hand ( 5 Things to Try)
As you can see, there are plenty of reasons why a pull start rips out of your hand. So, let’s go over the various repairs and check out the steps you’ll need to follow.
Setting the Valve Gap
To get started, you’ll first need to position the cylinder so that it’s at its highest position, top-dead-center (TDC). Then you’ll want to loosen off the lock nut holding the rocker arm at its specific adjustment. So, use a small wrench and back off the lock nut. Now you’ll be able to finely adjust the gap between the rocker arm and the valve cap. Next, position the feeler gauge between the rocker arm and the valve cap using the specific gauge thickness your manual states. Then, adjust the rocker arm, so the feeler gauge becomes pinched. You might need to use a screwdriver to do this.
You want the gauge to be pinched so that when you pull the gauge back and forth between the rocker arm and the valve cap, there’s some drag. This can be tricky, but as long as the gauge isn’t too loose or too tight, then you should be fine. Next, turn the engine and reposition the cylinder back to TDC. Then recheck the gap once again. Finally, once the valve is set correctly, you can tighten the lock nut and move on to the next valve.
Tools To Adjust a Lawn Mower Valve
- Feeler Gauge
- Small Wrench
- Screwdriver
Connecting the Push Rod
If your push rod has come loose from the rocker arm, you’ll need to push the rocker arm against the valve and reseat the push rod. Because the valve is spring-loaded, you can push the rocker arm against the valve, so the valve is pushed into the cylinder. Once you do this, there should be enough room to slide the push rod back into position.
Then, release the rocker arm and allow the valve spring to push the rocker arm right up to the push rod. If your lawn mower doesn’t have enough room to do this, then you’ll want to loosen the rocker arm to give you enough space. Finally, when you have the push rod back in place, you should follow the steps from above to set the valve gap.
Replacing a Push Rod
To replace a push rod, all you need to do is remove the old one and insert a new one. To remove the push rod, follow the steps from above to reseat the push rod. You shouldn’t need any tools at all if you can get enough clearance to swap the push rods over.
Finally, make sure that you set the valve gap. The new push rod won’t have any wear and will probably be slightly longer than the old one. Again, you can follow the steps above.
Part To Replace a Push Rod
- New Push Rod Matching Your Engine
Replacing the Flywheel Key
To replace a deformed or snapped flywheel key, you’ll need to remove the flywheel, remove the old key, and install a new one. This can be tricky because a broken key usually means the flywheel is probably wedged on the driveshaft.
So, I recommended that you get hold of a harmonic balanced puller. This is a tool that will both push and pull at the same time. So it will push against the driveshaft while pulling the flywheel. I definitely don’t recommend you use a hammer to bash the flywheel off. So, here are the steps to replace a flywheel key.
- Remove the spark plug cable and tuck it out of the way.
- Remove the engine cover.
- Remove the recoil starter.
- Brace the blade so that the engine can’t turn.
- Remove the top nut of the drive shaft.
- Lift off the flywheel cup.
- Attach the harmonic balanced puller.
- Tighten the harmonic balanced puller so that it pulls the flywheel from the driveshaft.
- Remove the flywheel from the lawn mower.
- Remove the damaged flywheel key, including any pieces stuck in both the flywheel and the driveshaft.
- Spray the top of the driveshaft with WD-40.
- Spray the inside of the flywheel where the driveshaft passes through with WD-40.
- Place the flywheel back onto the drive shaft.
- Ensure that the flywheel is fully pressed on the driveshaft
- Line the driveshaft and the flywheel so that both grooves line up.
- Insert the flywheel key into position to lock both the flywheel and driver shaft into position.
- Ensure the flywheel key is fully seated down into the groove.
- Install the flywheel cup.
- Install the driveshaft bolt and tighten to the required torque specification.
- Install the recoil starter.
- Reattach the engine cover.
- Reconnect the spark plug.
Tools & Parts to Replace a Flywheel Key
- Socket Wrench Set
- Blade Brace
- Harmonic Balanced Puller
- WD-40
- Replacement Flywheel Key
- Torque Wrench
Repositioning the Flywheel Magnet
This repair includes all the same steps as replacing a flywheel key other than actually installing a new key. The only difference is when you have the flywheel removed, you’ll need to reposition the magnet. So, if you look closely at where the magnet has moved, you should see the residue of the old glue.
This means you have a pretty good idea of where you need to re-stick the magnet. I try to use a small amount of very strong glue. A two-part epoxy such as Gorilla epoxy is great. Now all you need to do is stick the magnet back in the right place and allow the epoxy to cure overnight.
When it’s cured, you can reinstall the flywheel following the steps from above, and hopefully, you’ll no longer have a lawn mower pull cord that jerks back or a pull start that rips out of your hand.
Tools & Parts to Reposition a Flywheel Magnet
- Socket Wrench Set
- Blade Brace
- Harmonic Balanced Puller
- WD-40
- Replacement Flywheel Key
- Torque Wrench
- Epoxy Two-Part Glue
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