Dirt Bike Won’t Kick Start (Mechanics advice)

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:

  1. Seized engine
  2. Worn piston rings
  3. Valve out of adjustment (4 stroke)
  4. Burnt valve
  5. Failing stator

In this post you’ll learn 5 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 kick start 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 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, lets 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)

Hydro-locked engine

Although this sounds serious, it’s not and if this is your problem, your lucky1.

So what is a hydro-locked engine? It’s when a liquid (usually gas) fills the cylinder so full, that it prevents the piston from moving. Liquid isn’t compressible, so the piston simply won’t move.

The solution is obvious, remove the fluid and problem solved. Yes, that usually does the trick until the next time it happens.

So how does the fluid get into the cylinder? The usual fluid to fill the cylinder is gas, and it gets in there because of a faulty carburettor 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 flows into the cylinder.

Fuel float out of adjustment will have the same effect and so too will laying your bike on its side.

Oil in the cylinder is possible too, usually when the bike is layed over on its side.

How to check and fix hydro-locking? Go ahead and turn off the gas and remove the spark plug. Turn over the engine slowly, if your engine is hydro-locked gas will spray from the plug hole.

Clean the plug, kick over the kike a few times to dry out the cylinder. Leave the plug out for twenty minutes or so to allow gas evaporate.

Refit plug, turn gas on and start the engine.

Be mindful, unless you had your bike on its side or overfilled with oil, you’re likely only dealing with the symptom 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.

In addition, a leaking carb needle, will dilute the engine oil, so’ll you’ll need to change oil and filter.

Too Much Oil

Too much oil in the engine will prevent the piston moving, as it 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 carburettor needle valve. 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.

Dropped Valve

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 there not, they collide with the piston and can lock the engine.

So why does that happen? Various reasons could cause, them 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 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 is understandable, but what’s compression? In very basic terms, it’s your engines ability to hold air pressure, an internal combustion engine is really just an air pump.

Your piston as you know is pushed downwards in the cylinder under the force of the exploding fuel mix. 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 Blow by 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 blow by equals broken or worn rings.

An engine with low or border line 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 tail pipe
  • Oily plug
  • Misfiring engine
  • Engine oil leaks

How to diagnose worn rings? To test the cylinders 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 as new factory spec.

As a ballpark, an engine will struggle to operate below 100 psi, but you can easily check your engines spec with a google search.

To run a compression test:

  1. Remove the spark plug
  2. Screw in the compression tester adaptor and connect the gauge
  3. 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, 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.

Wet Test

This test is identical to the compression test just performed, but before the adaptor is fitted to the plug hole, 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.

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 it’s actuator, the rocker. The lash changes as engines wear and should be checked and adjusted routinely but are often forgotten.

Valves can be come 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 simply 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 leak out.

Symptoms of damaged valves include:

  • Popping in carburettor
  • Misfiring
  • Whit plug
  • Erratic hanging idle
  • Backfiring

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 compression stroke, filling the cylinder with air and using twin gauges, measuring pressure loss over a given time frame.

Very often however, diagnoses is done with your ear. Air leaking out will make its presence known.

Air leaking from:

  • Carburettor suggests an intake valve issue
  • Exhaust/Muffler suggests exhaust valve issue
  • Cylinder head suggests a head gasket issue
  • Dip stick tube would suggest Blow by (worn or broken rings)

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 starting will start the engine. Bump starting, turns the engine over more quickly and for longer than kick starting.

The increase cranking speed and duration helps 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.

  1. Clean the plug and fit the plug wire
  2. Ground the threads of the plug by placing it firmly against the metal of the engine
  3. Kick start the engine and check spark strength. Using a spark tester works best because it creates high resistance and tests all ignition components

A faulty ignition system may be caused by faulty:

  • Ignition switch
  • Safety lockout switch
  • Stator
  • Pickup
  • Coil
  • Coil wire
  • Cap
  • Plug.

John Cunningham

John Cunningham is an certified mechanic and writer on ATVFixed.com. I’ve been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of ATV ownership, from maintenance, repair to troubleshooting.

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