Kicking, kicking, and nothing, you’ve checked the gas – it’s on and fresh, the plug wire is on, and the air filter is clear.
Top 5 reasons a dirt bike won’t start on the kick-start include:
- Seized engine
- Worn piston rings
- Valve out of adjustment (4 stroke)
- Burnt valve
- Failing stator
In this post, you’ll learn five reasons your bike won’t kick start, how to diagnose them and how to fix them.
1 Seized Engine
If your kick start actually moves when you kick over the engine, this section won’t apply to you. This will only apply to you if the kickstart won’t actually move when you try and kick it over. If that is the case, a seized engine is, unfortunately, the most likely and worst possible outcome.
A seized engine occurs when the internal components get so hot they fuse together to become one. The cause is usually a lack of oil or oil quality. But this isn’t the only possible reason your engine won’t crank over. Before we jump to any conclusions, let’s check a few things.
Other possible causes of a kick-start that won’t move include:
- Hydro-locked engine
- Too much oil
- Dropped valve (4 stroke)
Symptoms of hydro locking include – the engine won’t crank over, smell of raw gas, overfull oil level, gas leaking from the carburetor and exhaust manifold.
Wow, sounds serious, right? If this sounds like your problem, you got lucky. Ok you say, but what is a hydro-locked engine? Hydrolocking is when a liquid (usually water) fills the cylinder so full that it prevents the piston from moving. The liquid isn’t compressible, so the piston can’t move back up the cylinder.
The solution is obvious, remove the fluid, and the problem is solved. And, that usually does the trick until the next time it happens.
So how does the fluid get into the cylinder, and what is the fluid? The usual fluid to fill the cylinder is as said water, and this is common in ATVs that use snorkels, it’s not however common in dirt bikes. The more common fluid to cause hydro locking in dirt bikes is gas, and the gas gets in there because of a faulty carburetor float needle.
The needle is 5 dollar part that helps maintain the fuel bowl gas level.
When it fails, it allows gas to overflow the bowl and flow into the cylinder.
Fuel float out of adjustment will have the same effect.
Laying your bike on its side is another common cause of cylinder flooding and oil can sometimes make its way in there also.
How to check and fix hydro-locking?
Go ahead and turn off the gas and remove the spark plug. Turn over the engine slowly with the kick start. If your engine is hydro-locked, gas will spray from the spark plug hole, so place a shop towel over the plug hole and disable the coil. (spark and raw gas = a crispy dirt bike)
While the plug is removed, go ahead clean, and gap it.
With the plug removed, kick over the engine slowly a few times to dry out the cylinder.
Leave the plug out for twenty minutes or so to allow gas to evaporate.
Refit plug, turn gas “On” but don’t start the engine just yet.
We’ll need to check the oil level and how it smells. An oil level that’s too full and stinks of gas means our oil has been contaminated and will need to be changed. (not applicable to 2-strokes)
Changing the oil is a great habit but bear in mind, if the gas flooded the cylinder because of a faulty carb valve, we’re only dealing with the symptoms here.
The faulty carb needle or float still needs attention. Turning the gas off when you shut off the engine is the short-term solution.
Too Much Oil
Too much oil in the engine will prevent the piston from moving, the crankshaft must displace the oil in the crankcase, except the oil, has no place to go. If your engine oil level is overfull and stinks of gas, it’s our old friend the leaking carburetor needle valve.
And you already know how to handle this one – you’ll need to remove the oil and replace it with fresh oil and filter, but you’ll also need to replace the leaking needle seal.
This isn’t an easy fix and only applies to four-stroke engines. The valves live in the top end of your motor and allow fuel in and exhaust gasses out. They operate sequentially and are timed off crankshaft/piston position.
The valves need to be closed when the piston reaches TDC (Top Dead Center). If they’re not, they collide with the piston and can lock the engine.
So why does that happen? Various reasons could cause them to collide, including:
- Valve tip breaks off allowing the valve fall into the cylinder
- Valve timing off
- Cam failure
To check for this condition, remove the spark plug and inspect the piston, looking for shiny impact witness marks. This type of repair will require new valves, seals, and head-gasket, but the piston usually survives with just a few scratches.
2 Worn Rings
For an engine to run, it must have three very important ingredients, Fuel mix, Spark, and Compression. Fuel and spark are understandable, but what’s compression? In very basic terms, it’s your engine’s ability to hold pressure. An internal combustion engine is just an air pump.
Your piston, as you know, is pushed downwards in the cylinder under the force of the exploding fuel mix. (Power stroke)
Metal compression rings around the piston help seal the wall of the cylinder and piston. If the rings don’t seal, much of the useful energy escapes past the rings into the crankcase. This escaped energy is known as Blowby and can be gauged by removing the dipstick from an idling engine and observing the pressure leaving the crankcase through the dipstick tube.
Lots of blowby equals broken or worn rings. An engine with low or borderline low compression will be difficult to start or won’t start at all. A common symptom is a dirt bike that will bump start but won’t start on the kick starter.
And the reason for that is a bump start turns the engine more vigorously and for longer, which helps the engine build enough compression to ignite the fuel mix.
Other symptoms of worn rings include:
- Heavy oil consumption
- Blue/white smoke from tailpipe
- Oily plug
- Misfiring engine
- Engine oil leaks
How to diagnose worn rings? To test the cylinder’s ability to seal, you’ll need a compression test kit.
The compression valve of a cylinder is measured in PSI (Pounds per Square Inch), and each engine maker will have their own new factory spec. As a ballpark, an engine will struggle to operate below 100 psi, but you can easily check your engine’s spec with a google search.
To run a dry compression test:
- Remove the spark plug
- Screw in the compression tester adaptor and connect the gauge
- With throttle held wide open, kick the engine over and note the highest compression value
As said, less than 100 psi is a sign of a problem, (check against your bike’s factory spec) but importantly a low reading doesn’t mean we can blame worn rings just yet. For that, we’ll need to run one more test – a wet test.
This test is identical to the dry compression test just performed, but before the adaptor is fitted to the plughole, add an oil container cap full of engine oil to the cylinder.
Run the test again as before. If compression readings are more than 10% higher, your rings are the cause of the low compression.
But if the reading is pretty much unchanged, your problem is in the top end.
Leak down Test
Some high compression bikes are fitted with decompression valves that make for easier starting but also cause a low compression reading which is misleading, and so for many, a better test is the leakdown test but you will need shop air. I cover the leak down test below and you can jump to it here.
3 Valve Lash
Two-stroke engines don’t have valves. This section only applies to four-stroke motors. You already know the importance of compression, and you know how to test it. A lack of compression can be caused by a top-end problem too.
What is valve lash? It’s a very precise gap between the valve tip and its actuator, the rocker. The lash changes as engines wear and should be checked and adjusted routinely but are often forgotten.
Valves can become tight or loose. In our case, we’re interested in tight valves, as they could hold the valve open slightly, reduce compression and cause hard starting. Your valves open and close and control the breathing of your engine.
When closed, they should seal the combustion chamber. To check lash, remove the valve caps, and using a feeler gauge check the spec.
4 Worn Valves
Valves, like all components, wear out, but some conditions can shorten their life. A rich or lean running engine can cause a range of problems, like:
- Burnt valve
- Worn valve/seat
- Bent valve
- Tight valve
- Sticking valve
Any of these problems will allow compression to leak out.
Symptoms of damaged valves include:
- Popping in carburettor
- Whit plug
- Erratic hanging idle
How to check for a valve issue? The best tool for top-end diagnoses is a leak-down tester. The test involves setting the engine to TDC on the compression stroke, filling the cylinder with air, and using twin gauges, measuring pressure loss over a given time frame.
Very often, however, diagnosis is made with your ear. Air leaking out will make its presence known.
Air leaking from:
- Carburetor suggests an intake valve issue
- Exhaust/Muffler suggests exhaust valve issue
- Cylinder head suggests a head gasket issue
- Dipstick tube would suggest Blow by (worn or broken rings)
Listen for the air leak.
5 Failing Stator
A bike stator creates the voltage needed to fire the plug and charge the battery (If fitted). If your stator is failing, your spark may be weak. A weak spark may not start the engine. Sometimes, you may find bump start will start the engine. Bump starting, turns the engine over more quickly and for longer than kick-starting.
The increased cranking speed and duration help the engine generate a spark strong enough to ignite the mix.
How to check spark? Go ahead and remove the plug, examine the electrode and check the gap. A black plug indicates a rich engine, a white plug a lean condition. A black oily plug may indicate a mechanical fault or electrical issue.
- Clean the plug and fit the plug wire
- Ground the threads of the plug by placing it firmly against the metal of the engine
- Kick start the engine and check spark strength.
Using an inline spark tester works best because it creates high resistance and tests all ignition components.
A fault in the ignition system may be caused by:
- Poor spark plug gap
- Wrong spark plug
- Bad plug cap
- Faulty coil
- Bad coil wire
- Ignition switch/kill switch wiring fault
- Faulty stator
- Faulty pickup
- Faulty lockout switch
- CDI box/ECU
I’ve covered checking the ignition system components previously in this post “Dirt bike won’t start when hot”.
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