Dirt bikes live hard and die young, but let’s not assume the worst just yet.
Top 3 reasons a dirt bike kick start won’t move include:
- Seized engine
- Hydro-locked engine
- Dropped valve
In this post, you’ll learn the most common reasons your kick start won’t move, how to diagnose it and what you can do to fix it.
1 Seized Engine
What is a seized engine? When your internal engine components become so hot, they fuse and become one.
What causes a seized engine? Usually caused by inadequate oil quantity or quality. Higher revving two-stroke engines are prone to overheating and seizing, as too are four strokes when they run low on coolant.
A lean running engine (lacks sufficient gas) will rev harder and run hotter than a well-tuned carburetor. Your carburetor may be out of tune or incorrectly jetted. Wrong gas, blended gas can cause engines to run leaner and hotter too. Mods like fast flow exhausts, bigger air filters, re-bores without adjusting the AFR will cause a lean condition and risk seizure.
How to diagnose a seized engine? Check the oil level, remove the spark plug and try to kick start or crank over the engine. If the kick start doesn’t move, not even a half-inch, you can bet it’s seized. The usual failure – piston and rings fuse with the cylinder wall.
Replacing the top end is doable, but you’ll need to check the lower end bearings. If a lack of oil caused the seizure, then the bottom end is toast too. There is no quick fix for this, and it’s sickening. I’ve been there, and I know the feeling.
What is hydro-locking? It’s when your engine cylinder fills with a liquid, and since a liquid isn’t compressible, the piston can’t move.
Why does it happen? It can happen by simply laying the bike on its side or a mechanical issue like head gasket failure. A leaking float needle is also a common cause of hydro-locking.
How to diagnose the problem? Remove the spark plug and turn over the engine slowly.
The liquid contents of the cylinder will spray out the plug hole, best to cover the plug hole with a shop towel.
There are four liquids that could be in the cylinder, Gas, Oil, Coolant, and Water. Checking which you have will help you determine what has failed.
Common reasons for gas to fill the cylinder include a worn-out leaking carburetor needle seal and seat. This is easy to diagnose, remove the float bowl and set the float to the off position. If fuel flows, you found the source of the problem. Replacing is simple too, but always replace the needle, seat, and seat o-ring seal together.
We’re not done with gas yet. There are another couple of common reasons gas may enter the cylinder, simply laying the bike on the ground can cause gas to flow into and fill the cylinder.
And a faulty carburetor float or a float that’s set too high will fill the cylinder.
This doesn’t apply to fuel injected bikes.
Failed crankcase breather and tilting bike over on its side can allow oil into the cylinder. The fix here is to drain the cylinder, replace the spark plug, top up the oil and run the engine until the smoke clears.
The coolant inside the cylinder suggests a failed cylinder head gasket. Replacing the gasket is a pretty straightforward job, but it’s advisable to understand why the gasket failed. Mostly it’s because of old age, but an overheating condition caused by a sticking thermostat, blocked rad, failed water pump, or faulty fan/switch will still be there.
I’ve been a mechanic for over twenty years, and if I’ve learned anything, it’s “assume nothing.” I’ve found water, diesel, paint thinner, and other undetectable concoctions inside gas tanks that obviously won’t burn and instead flood the cylinder.
Riding a bike into water will hydro-lock and possibly grenade an engine.
3 Dropped Valve
What is a valve? Valves allow fuel into the cylinder and spend the gases out. They are fitted to four-stroke engines, at least two per cylinder.
The valves open and close sequentially and are timed to perfection with the movement of the piston. As the piston rises in the cylinder, the valves close, as the piston retreats, the valves open, and so on. If the piston and valves overlap (use the same space at different times), the engine is an interference engine.
A timing chain, sprocket, and cams are employed to keep the valves in time and prevent contact.
Any failure in the valve train will cause the valves and piston to meet. This can cause the engine to lock.
A valve and piston could still meet in a non-interference motor. Keepers at the tip of the valve keep the valve from dropping into the cylinder.
As the valve ages, the stem stretches and can break off, allowing the valve to drop into the motor, hitting the piston.
To determine if this is the cause, remove the spark plug and, using a small light or your phone camera, examine the piston for shiny impact witness marks. A valve repair may require a full top-end build, depending on how much damage occurred.
Alternatively, run a leakdown test.
You’ll find all the tools needed here on the Dirt bike tools page.
OIl level too full – Simply overfilling the oil level may also cause the engine to become difficult to kickstart.
Decompression valve fault – the decompression valve is part of the valve train assembly whos job it is to open the valves early on crank thereby easing the effort required to crank the motor over. A faulty decompression assembly will therefore increase the effort required to move the kickstarter.
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- About the Author
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John Cunningham is a technical writer here at ATVfixed.com. He’s a Red Seal Qualified Service Technician with over twenty-five years experience. He’s worked on all types of mechanical equipment, from cars and trucks to ATVs and Dirt bikes.