If a new or recently fitted engine will not start, check the installation before replacing ignition or engine components. Many first-start problems are caused by fuel supply, intake installation, wiring connections or initial setup.

Fuel Supply

• Use fresh fuel. Fuel left in the original bike or fuel container for a long period may have degraded or become contaminated.
• Check the fuel tap is on and the fuel pipe is not kinked, trapped or restricted.
• Confirm fuel is actually reaching the carburettor.
• If the original carburettor has been reused, do not assume it is suitable or correctly set for the replacement engine.

Carburettor and Intake Installation

• Check the carburettor, intake manifold and any intake spacer are correctly fitted and securely tightened.
• Ensure all required intake gaskets and O-rings are fitted.
• Check the gaskets and spacers are fitted in the correct orientation and that their openings align correctly with the intake ports. Some gaskets can physically be fitted incorrectly and partially obstruct the intake.
• Inspect the intake manifold for cracks, damage or poor sealing.
• On slide-type carburettors, check the throttle slide is installed in the correct orientation and correctly located inside the carburettor body.
• Check the slide returns fully when the throttle is released and that the throttle cable is not holding it partially open.

Check for Spark

• Use a known-good spark plug rather than assuming the existing plug is working correctly.
• Fit the plug into the cap, earth the threaded metal body securely against clean engine metal and kick or crank the engine while checking for a consistent spark.
• Check the spark plug cap is properly screwed onto the HT lead and contacting the copper conductor inside the cable.
• If the end of the HT lead is damaged or corroded, trim a small amount from the end to expose clean conductor before refitting the cap.

Engine and Wiring Connections

• Unplug and inspect the engine wiring connector rather than only looking at it from the outside.
• Check for bent pins, pushed-back terminals, corrosion or terminals that are not correctly locked into the connector housing.
• Gently check each wire is securely attached to its terminal. A wire can appear connected while being loose or broken at the crimp.
• Check the ignition coil connections and earth are clean and secure.
• If a different engine has been fitted to the bike’s existing wiring loom, confirm that the stator, CDI, loom and ignition components are compatible. Similar-looking connectors do not always use the same pin arrangement or ignition system.

Kill Switch and Key Switch

• A faulty kill switch, ignition switch or their wiring can prevent an otherwise good ignition system from producing a spark.
• Where suitable for the wiring system, temporarily disconnect the kill-switch circuit to rule it out.
• If the bike has a key switch, inspect its wiring and connector and isolate or test the switch if required.

Valve Clearances and Compression

• Valve clearances should be checked on a newly fitted engine rather than assuming they are correctly set.
• For many common pit bike engines, a typical valve clearance is IN 0.10mm and EX 0.15mm. Check the specification for the particular engine where available.
• Valve clearances that are too tight can cause low compression, difficult starting and problems that become worse as the engine warms up.
• If spark, fuel supply and valve clearances appear correct but the engine still will not start, check engine compression.
• A leak-down test can help identify leaking or damaged valves, piston-ring leakage or other compression problems.

What to Check Next

• No or weak spark: continue to Electrical & No Spark.
• Fuel, carburettor or intake problem: continue to Fuel & Carburettor.
• Good spark and fuel but still will not start: continue to Mechanical Problems.

Basic Starting Procedure

• Make sure the fuel tap is on.
• Turn the ignition or key switch on where fitted.
• Ensure the kill switch is in the run position.
• For a cold engine, use the choke as required. Reduce and turn the choke off as the engine warms.
• Make sure the bike is in neutral before starting.

Using the Kick Start

• On larger or higher-compression engines, bring the engine up to the compression point before kicking. See our Top Dead Centre (TDC) starting guide.
• Allow the kickstart lever to return fully before each attempt.
• Kick firmly and smoothly through the full travel of the lever.
• Avoid repeated short, aggressive or partial kicks, as these can damage the kick-start mechanism.
• Avoid excessive throttle while kickstarting, as this can flood the engine and increase the chance of kickback.

Engine Turns Over but Will Not Start

An engine normally requires three basic things to run: a suitable ignition spark, the correct fuel and air mixture, and sufficient compression.

• Check for a consistent ignition spark using a known-good spark plug.
• Confirm fuel is reaching the carburettor.
• Check the choke is operating correctly.
• If spark and fuel appear to be present, check valve clearances and engine compression.

Check the Spark Plug After Starting Attempts

• A spark plug that remains completely dry after repeated starting attempts can indicate that fuel is not reaching the combustion chamber.
• A plug that is very wet with fuel can indicate flooding or an ignition problem preventing the mixture from burning.
• Spark plug condition is only one diagnostic clue and should be considered together with the other checks.

Common Running Symptoms

• Only runs with choke: check the pilot jet, fuel supply and intake system for air leaks.
• High or hanging idle: check the throttle slide, throttle cable, intake manifold and carburettor adjustment.
• Flooding or repeated wet spark plug: check the choke, float valve, float height and ignition spark.
• Bogs or loses power when the throttle is opened: check fuel flow, carburettor jetting and the main fuel circuit. An ignition fault under load can produce similar symptoms.
• Cuts out after several minutes: check the fuel-cap breather and fuel flow. If these are correct, check for an ignition component that fails as it becomes hot.
• Difficult to restart when hot: check valve clearances, compression, carburettor condition and ignition components.
• Backfiring through the intake or exhaust: check carburettor setup, intake air leaks, ignition condition, valve clearance and cam timing where applicable.

Basic Maintenance Checks

• Replace a worn, fouled or questionable spark plug.
• Check the air filter is clean and correctly fitted.
• Check the intake system and exhaust are correctly fitted and secure.
• Check engine oil level and condition.
• If the problem began immediately after maintenance or fitting a replacement part, recheck that work before continuing.

Electric Starter Does Not Turn the Engine

• Check the battery terminals are clean and tight and that the main earth connection is secure.
• Check the main fuse where fitted.
• A battery can show around 12v with no load but still be unable to supply enough current to operate the starter motor.
• Rapid clicking from the starter relay commonly indicates a weak battery or poor electrical connection.
• A single heavy click can indicate a weak battery, poor connection, faulty starter relay or starter motor problem.
• If nothing happens when the starter button is pressed, check the starter button, fuse, wiring and any neutral or clutch safety switches fitted to the bike.
• If the starter motor spins but does not turn the engine, inspect the starter drive or one-way starter mechanism.

No Spark or Weak Spark

• Start with a known-good spark plug.
• Fit the plug into the plug cap and earth the threaded metal body securely against clean engine metal while kicking or cranking the engine.
• Do not carry out a spark test where fuel is leaking or fuel vapour is present.
• Look for a consistent spark rather than a single occasional spark.
• Replace the spark plug if it is worn, fouled, damaged or its condition is uncertain.

Spark Plug Cap and HT Lead

• Check the spark plug cap is properly screwed onto the HT lead and contacting the copper conductor inside the cable.
• If the end of the HT lead is corroded or damaged, trim a small amount from the end to expose clean conductor before refitting the cap.
• Check both ends of the spark plug cap for water, corrosion or contamination.
• Inspect the HT lead for cuts, deterioration or damage.

Kill Switch

• A faulty kill switch or damaged kill-switch wiring can permanently earth the ignition circuit and cause a no-spark condition.
• Inspect the switch and wiring for trapped, damaged or shorted wires.
• Where suitable for the wiring system, temporarily disconnect the kill-switch circuit to rule it out.

Ignition / Key Switch

• A faulty ignition or key switch can also prevent the ignition system from producing a spark.
• Inspect the switch connector for loose terminals, corrosion, bent pins or pushed-back terminals.
• Where appropriate, isolate the switch or test it with a multimeter for the correct continuity in the ON and OFF positions.

CDI and Ignition Coil

• If available, substitution with a known-good compatible CDI or ignition coil can help isolate a fault.
• Check CDI plugs for water, corrosion, bent pins and pushed-back terminals.
• Check the ignition coil connections are clean and secure.
• Ensure the ignition coil and engine have a good earth. Paint, corrosion or loose mounting points can prevent a reliable electrical connection where the mounting itself forms part of the earth path.
• Ignition coil resistance can be checked with a multimeter, but readings should be compared with the correct specification for that particular coil.

Stator and Pickup Coil

• Inspect the stator wiring and engine connectors for damage, corrosion or loose terminals.
• Check the stator and pickup coil resistance against the correct specification for the ignition system.
• Where required, check ignition-source output while the engine is being kicked or cranked.
• Inspect the pickup and flywheel area if there is reason to suspect physical damage, incorrect positioning or previous engine work.

Wiring, Connectors and Terminals

• Unplug suspect connectors and inspect both sides closely rather than only checking that the plugs appear connected.
• Look for bent, damaged, corroded or pushed-back terminals.
• Check each terminal is fully locked into its connector housing.
• Gently check individual wires are securely crimped into their terminals. A wire can appear connected while being loose or broken immediately behind the terminal.
• Replace twisted, taped or damaged electrical joints with suitable properly terminated connections.
• Check earth connections are clean, tight and making good electrical contact.

Component Compatibility

• If an engine, stator, CDI, ignition coil or wiring loom has been changed, confirm the components are compatible with each other.
• A connector physically fitting does not guarantee that the pin arrangement, CDI type or ignition system is compatible.

Quick Ignition Test Using a Simple Wiring Loom

A simple off-road ignition loom can bypass the bike’s original wiring, switches and other circuits. This can help establish whether a no-spark fault is within the bike wiring or the engine-side ignition system.

1. Prepare the test loom:
   • Assemble a Kick Start Wiring Loom Kit.
   • Connect the compatible CDI and ignition coil.
   • Use a known-good spark plug such as our NGK Spark Plug 50cc-160cc where suitable.
2. Disconnect the original ignition loom:
   • Disconnect the engine ignition wiring from the bike’s main loom.
   • Connect the engine directly to the test loom.
   • Depending on the engine connection, an Engine Loom Adaptor may be required.
3. Check for spark:
   • Fit the spark plug into the cap and earth the threaded plug body securely against clean engine metal.
   • Kick the engine over and check for a consistent spark.
   • Spark with the test loom but not with the original loom: investigate the bike’s original wiring, switches, connectors and earths.
   • No spark with either loom: continue testing the stator, pickup coil, CDI, ignition coil, plug cap, connections and earths.
4. Attempt to start the engine:
   • If the engine runs correctly using the test loom, the problem is likely within the original bike wiring or switches.
   • If a good spark is present but the engine still does not start, continue with the fuel and mechanical checks rather than replacing more ignition components.

Faults That Appear When Hot

• An ignition coil, CDI, pickup, stator or wiring fault can sometimes appear only after the component becomes warm.
• If the bike runs correctly when cold but loses spark once hot, repeat the spark test while the fault is actually present where possible.

Fuel Quality

• Old or contaminated fuel is a common cause of difficult starting and poor running.
• If fuel has been standing for a long period or its condition is uncertain, drain the tank and carburettor and refill with fresh suitable petrol.
• Water contamination can cause intermittent running problems and may collect in the bottom of the carburettor float bowl.

Fuel Flow

• Check the fuel pipe for kinks, damage or blockage.
• Inspect the fuel filter where fitted.
• With suitable precautions, disconnect the fuel pipe at the carburettor and operate the fuel tap to confirm fuel flows freely from the tank.
• If flow is poor, check the fuel tap, filter, tank outlet and pipework for restrictions.
• A blocked fuel-cap vent or breather can create a vacuum inside the tank and gradually stop the fuel supply.
• If the engine runs normally for several minutes and then cuts out, check the fuel-cap breather and fuel flow before changing carburettor settings.

Correct Carburettor

• Make sure the carburettor is suitable for the engine and intake being used.
• Do not assume a new carburettor is automatically correctly set or fault-free. New carburettors can still have blocked passages, incorrect adjustment, sticking floats or other faults.
• If the problem appeared immediately after changing the carburettor, compare the new carburettor with the previous setup before altering unrelated engine components.

Throttle Slide and Cable

• On slide-type carburettors, ensure the throttle slide is installed in the correct orientation and engages correctly with the locating pin inside the carburettor body.
• The slide should return fully when the throttle is released.
• An incorrectly installed slide can remain partially open, making the engine difficult to start or causing it to immediately run at an excessively high speed.
• Check the throttle cable has suitable free play and is not being pulled tight by its routing or handlebar position.
• After removing or refitting a carburettor, always check throttle and slide operation before starting the engine.

Choke Operation

• Use the choke where required for cold starting and turn it fully off once the engine is sufficiently warm.
• Check the choke mechanism moves fully between its ON and OFF positions.
• If a warm engine only continues running with the choke applied, check the pilot circuit, fuel supply and intake for air leaks.
• A choke that remains partially applied can cause rich running, poor throttle response, plug fouling and excessive fuel consumption.

Pilot Jet and Low-Speed Running

• The pilot circuit controls much of the starting, idle and low-throttle operation.
• If the engine will not idle, only runs with choke or stalls as the throttle closes, inspect the pilot jet and pilot passages for contamination or blockage.
• Clean jets carefully without enlarging the calibrated hole.

Main Jet and Higher-Throttle Running

• The main jet becomes increasingly important as the throttle is opened.
• If the bike runs correctly at low throttle but develops a problem under heavier load or larger throttle openings, check the main jet, fuel supply and carburettor setup.
• Do not assume a larger jet will automatically improve performance. Jet size must suit the engine, carburettor, intake, exhaust and operating conditions.

Float and Fuel Level

• Check the float moves freely and the float valve opens and closes correctly.
• Fuel leaking from the overflow can be caused by dirt in the float valve, a worn valve, a sticking float or incorrect float height.
• A float valve stuck closed or an excessively low fuel level can prevent enough fuel entering the carburettor.
• Incorrect fuel level can cause rich running, lean running or inconsistent performance.

Mixture and Idle Adjustment

• Carry out final carburettor adjustment with the engine fully warmed to normal operating temperature.
• Adjust idle speed and mixture gradually, allowing the engine to respond between adjustments.
• The function and adjustment direction of the mixture screw varies between carburettor designs, so use the correct procedure for the carburettor fitted.
• If the engine requires an extreme mixture-screw position to run correctly, look for another problem such as incorrect jetting, a blocked passage or an intake air leak.

Intake Manifold and Air Leaks

• Inspect the carburettor, manifold, intake spacer and cylinder-head connection.
• Check all required gaskets and O-rings are present and undamaged.
• Make sure gaskets and spacers are fitted in the correct orientation and that their openings line up correctly with the intake ports.
• Inspect rubber intake manifolds for cracking, splitting or deterioration.
• Ensure carburettor clamps and manifold fasteners are secure.
• An intake air leak can cause difficult starting, a high or hanging idle, poor throttle response and lean running.

Common Fuel and Carburettor Symptoms

• Only runs with choke when warm: check the pilot circuit, fuel flow and intake system for air leaks.
• High or hanging idle: check the throttle slide, cable, manifold and intake for air leaks.
• Fuel leaking from the overflow: inspect the float, float valve and fuel level.
• Black or repeatedly fouled spark plug: check for an excessively rich mixture, choke problem or excessive fuel level, while also ruling out a weak ignition spark.
• Cuts out after sustained running: check tank breathing and fuel flow before changing jets.
• Runs at idle but bogs under load: check main jetting and fuel supply, but remember an ignition fault can sometimes produce similar symptoms.

Valve Clearances

• Incorrect valve clearance can cause difficult starting, poor running, reduced compression, loss of performance or problems that become worse when the engine is hot.
• For many common pit bike engines, a typical valve clearance is IN 0.10mm and EX 0.15mm.
• Check the specification for the particular engine where available, as some engines require different settings.
• Valve clearances are normally checked with the engine cold and correctly positioned at Top Dead Centre on the compression stroke, unless the engine manufacturer specifies otherwise.
• Clearances that are too tight can prevent a valve from fully closing, reducing compression and making hot starting particularly difficult.
• Valve clearances should also be checked after cylinder-head or valve-train work and periodically as part of maintenance.
• A Valve Clearance Adjustment Tool Kit can be used where suitable.

Compression Problems

• If the engine has a good ignition spark and correct fuel supply but remains difficult or impossible to start, check the mechanical condition of the engine.
• Incorrect valve clearance, incorrect cam timing, leaking valves, damaged valves, worn piston rings or cylinder damage can all reduce compression.
• Before dismantling the engine, check valve clearances and cam timing where applicable.

Leak-Down Test

A leak-down test can help identify where compression is escaping from the combustion chamber. It is particularly useful when an engine has spark and fuel but still will not start, or when compression becomes poor as the engine warms.

• Air heard through the carburettor or intake: the intake valve is not sealing correctly. Possible causes include incorrect valve clearance, contamination on the valve seat, a damaged or bent valve, or valve-seat damage.
• Air heard through the exhaust: the exhaust valve is not sealing correctly. Possible causes include incorrect valve clearance, a damaged or bent valve, or valve-seat damage.
• Air heard through the crankcase breather or oil filler: possible leakage past the piston rings, piston or cylinder.

See this Leak Down Test Video for an example of the procedure.

Cam Timing

• If the cylinder head, camshaft, timing chain or top end has recently been worked on, confirm the cam timing is correct before further diagnosis.
• Incorrect cam timing can cause difficult starting, low compression, poor performance and backfiring.

Engine Will Not Turn Over

• If the engine or kick starter suddenly locks, do not continue forcing the kickstart lever.
• Check that the gearbox is actually in neutral and rule out obvious external problems before assuming the engine has seized.
• If the kick-start mechanism appears jammed, slipping or not engaging correctly, inspect the mechanism before repeatedly attempting to start the engine.
• For relevant YX engines, refer to the dedicated YX140/YX160 kick-start fault information elsewhere on this page.

Clutch Adjustment

• Check the clutch cable moves smoothly and has suitable free play at the lever.
• Too little free play can prevent the clutch from fully engaging and cause slipping.
• Too much free play can prevent the clutch from fully disengaging and cause dragging or difficult gear selection.
• If correct cable adjustment does not resolve the problem, the clutch mechanism or internal clutch components may require further inspection.

Chain Alignment and Tension

• Check the rear wheel and sprockets are correctly aligned and the chain runs freely.
• Adjust chain tension to the correct specification for the bike.
• A chain that is excessively loose or tight can cause accelerated wear, poor operation or component damage.
• Where necessary, check chain tension through the suspension movement because the tension changes as the rear suspension moves.
• Inspect the chain and sprockets for stiff links, excessive wear or damaged teeth.

Unusual Mechanical Noise

• Do not continue running an engine that develops severe knocking, grinding or another abnormal mechanical noise.
• Check the engine oil level and inspect for obvious loose external components before restarting the engine.
• If the source cannot be identified externally, further mechanical inspection may be required.

General Wear and Maintenance

• Inspect the air filter, intake system, fuel pipes, control cables and other service components regularly.
• Check engine mounting bolts, exhaust fixings and other components that can loosen through vibration.
• Where a problem starts immediately after maintenance or a component has been replaced, recheck that work before assuming another part has failed.

Recheck Your Work

• After troubleshooting or carrying out adjustments, recheck any wiring, fasteners, fuel pipes, hoses, cables and components that have been disturbed.
• Make sure electrical connectors are fully seated and that no wires have been trapped during reassembly.
• Check for tools, loose fasteners or other items that may have been left around the engine or chassis.

Final Safety Check

• Make sure the brakes, wheels, handlebars, suspension, throttle, clutch and other controls are correctly fitted and operating normally before riding the bike.
• Check for fuel, oil or other fluid leaks.
• If anything appears unsafe or the engine is making an abnormal mechanical noise, do not test ride the bike.

Test Ride

• If the bike is safe to ride, carry out a short test ride in a suitable area.
• Begin gently and confirm that the engine, brakes, steering, clutch, gears and throttle operate correctly.
• Where relevant, allow the engine to reach normal operating temperature and make sure the original fault does not return once warm.
• If the fault remains or the bike does not feel safe, stop riding and investigate further.

Ongoing Maintenance

• Regular inspection and maintenance can prevent many starting, running and mechanical problems.
• Follow the service information for the particular bike or engine where available, paying attention to engine oil, valve clearances, air filter, chain adjustment, brakes and fasteners.

If You Still Cannot Find the Fault

If the checks in this guide have not identified the problem, further diagnosis may require specialist equipment or experience.

At this stage, we recommend taking the bike to a competent motorcycle or pit bike mechanic. Tell them what checks, tests and replacement parts have already been tried, as this can save diagnostic time and prevent the same work being repeated.