When a pull start jerks back hard against your hand, the engine is firing at the wrong moment. Instead of pushing the piston down after it reaches the top of its stroke, combustion happens too early, forcing the piston backward and sending that violent snap right back through the starter rope. This is called kickback, and it can bruise your hand, sprain your wrist, or even break fingers. The most common cause is a sheared flywheel key, though several other problems can produce the same result.
How Kickback Actually Happens
In a normal start, the spark plug fires just as the piston reaches the top of its compression stroke. The explosion pushes the piston downward, the crankshaft spins, and the engine runs. Kickback occurs when the spark fires too early, while the piston is still traveling upward. The combustion pressure fights against the piston’s upward motion, reversing the crankshaft’s direction and whipping the pull cord back toward you.
The key detail: kickback only happens when ignition timing is advanced (firing too early). If timing shifts the other direction, firing too late, the engine will run poorly or not start, but it won’t jerk back.
The Most Common Cause: A Sheared Flywheel Key
The flywheel is a heavy metal disc bolted to the crankshaft. A small, soft metal key (usually made of aluminum) sits in a slot between the flywheel and the crankshaft, keeping them locked in the correct position relative to each other. That alignment determines exactly when the magnets on the flywheel pass the ignition coil and trigger the spark.
If the engine hits something hard, like a lawnmower blade striking a rock or a stump, the impact can partially shear that soft key. When the key deforms, the flywheel shifts slightly on the crankshaft. Even a small rotation changes when the spark fires. If it shifts in the direction that advances the timing, the spark fires while the piston is still coming up, and every pull of the rope kicks back.
A sheared flywheel key is by far the most frequent reason for sudden, aggressive kickback on a small engine that was previously starting fine. The key is designed to be a sacrificial part, absorbing impact to protect more expensive components.
How to Check and Replace the Key
You’ll need to remove the engine’s top shroud, disconnect the spark plug wire (for safety), and then remove the flywheel nut. A flywheel puller is the proper tool for pulling the flywheel off the crankshaft without damaging it. Don’t use a hammer and pry bar, which can crack the flywheel or damage the crankshaft.
Once the flywheel is off, the key is visible in a small slot on the crankshaft. If it’s partially twisted, sheared at an angle, or deformed in any way, replace it. New flywheel keys cost very little and are specific to your engine model. Reassemble everything, torque the flywheel nut to spec, and the kickback should be gone.
Valve Problems That Cause Resistance
Many small engines have an automatic compression release built into the camshaft. This mechanism slightly opens the exhaust valve during the compression stroke when you’re cranking the engine at low speed, releasing just enough pressure that you can pull the cord without fighting full compression. Once the engine starts and speeds up, the release disengages and the valve closes normally.
If the compression release wears out or sticks, the engine has full compression during cranking. The rope won’t necessarily jerk back the same way a timing issue does, but it can feel extremely hard to pull and snap back if you lose your grip partway through the stroke.
Valve clearance also matters. On overhead valve engines, the gap between the valve and the rocker arm typically ranges from .002 to .004 inches on one valve and .005 to .007 inches on the other. If the clearance is too tight, a valve may not fully close. This can cause hard starting, loss of compression, and inconsistent resistance on the pull cord. Checking clearance requires a feeler gauge, which is inexpensive and straightforward to use with a basic tutorial for your engine model.
Fluid in the Cylinder (Hydrolock)
If liquid gets into the combustion chamber, the piston can’t compress it. Liquids don’t compress the way air does, so the piston hits a wall of resistance and stops dead. When this happens during a pull start, the rope locks up suddenly or jerks back hard.
On small engines, the usual culprits are oil and fuel. Tipping a lawnmower the wrong way (carburetor side down) can let oil flow past the piston rings into the cylinder. A leaking carburetor float valve can flood the cylinder with gasoline. On engines stored for a long time, a stuck float is especially common.
To check for hydrolock, remove the spark plug and pull the rope. If fluid sprays out of the spark plug hole, that’s your answer. Let the remaining fluid drain out, reinstall the plug, and try again. If the problem was caused by tipping the mower, it’s a one-time fix. If fluid keeps accumulating, you likely have a leaking carburetor or, less commonly, a failing head gasket allowing coolant or oil into the cylinder.
A Broken or Stuck Starter Mechanism
Sometimes the problem isn’t the engine at all. The recoil starter itself has a spring, a pulley, and pawls (small hooks that engage the flywheel when you pull). If the rewind spring breaks or the pawls don’t retract properly, the rope can behave erratically. A spring that doesn’t rewind smoothly can cause the rope to snap back unevenly, mimicking kickback.
The difference is feel. Engine kickback is violent and sudden, like something inside the engine pushed back against you. A starter mechanism problem feels more like the rope got stuck or tangled and then released. If you remove the starter housing and inspect the spring and pawls, damage is usually visible.
Brake Band Still Engaged
Most lawnmowers and chainsaws have a flywheel brake, activated by the handle bail (the bar you squeeze against the handle) or a chain brake on a chainsaw. If the brake is still engaged when you pull the cord, you’re trying to spin the flywheel against a friction band that’s clamping it. The rope will pull hard and snap back because the flywheel can’t rotate freely.
This is the simplest fix on the list. Make sure the bail lever is fully squeezed or the chain brake is disengaged before pulling. If the brake cable is stretched or the mechanism is stuck, the brake may stay partially engaged even when you think it’s released.
Narrowing Down the Cause
Start with the easiest checks first. Confirm the flywheel brake is disengaged. Then remove the spark plug and pull the rope. If it pulls smoothly with the plug out, the problem is compression-related: likely a sheared flywheel key, stuck compression release, or hydrolock. If fluid comes out the plug hole, you’ve found it. If the rope is still hard to pull with the plug removed, the problem is mechanical binding in the starter or engine internals.
For the most common scenario, where the engine was running fine and suddenly started kicking back after hitting something, go straight to the flywheel key. It’s the right answer about 80% of the time for sudden-onset kickback on a small engine that previously started without issues.