How to Troubleshoot a Rough Idle in a Motorcycle

by Kyle McBride
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A broad range of problems can cause rough idling in a motorcycle engine. A problem with fuel delivery, spark plug operation or metering of the air-fuel charge can cause problems at idle that may not be obvious at higher rpm ranges. Motorcycle manufacturer recommendations vary, but most issues can be isolated using a basic set of general procedures and a few tools.

Spark Plugs

Step 1

Inspect the spark plug wires for signs of damage. Look into the boots at the ends of the wires and check for corrosion or burning of the terminals within the boots. Replace as necessary.

Step 2

Remove the spark plugs using a spark plug socket and ratchet. Inspect the spark plugs for mechanical damage and oil or carbon fouling. Replace as necessary.

Step 3

Insert a spark plug gapping tool between the electrodes on the spark plug and check the gap. Refer to the manufacturer’s specifications for gap information, and gap the spark plugs accordingly.

Inspect the spark plugs for mixture indicators. White deposits indicate a lean mixture, and excessive coking or carbonizing indicates a rich mixture. Adjust the carburetor accordingly.

Compression Check

Step 1

Install a compression tester into a spark plug hole with all other spark plugs removed.

Step 2

Turn off the fuel at the fuel tank petcock.

Step 3

Spin the engine over using the starter switch until the compression tester gauge needle stabilizes.

Step 4

Record the reading and move the tester to the next spark plug hole.

Repeat steps 4 and 5 until all of the cylinders have been tested. Refer to the manufacturer’s specifications to determine if the compression is within tolerance. Bad compression indicates worn rings, pistons and cylinders; the engine requires a rebuild to rectify this condition.

Fuel and Mixture Checks

Step 1

Inspect the fuel in the tank for signs of water, dirt or rust flakes in the fuel tank. Clean the tank and re-coat as necessary.

Step 2

Remove and inspect the fuel filter. Clean or replace as necessary.

Step 3

Remove the air filter. Inspect it for dirt or damage. Clean or replace as necessary.

Step 4

Inspect the intake manifold for cracks or deterioration that could be introducing air into the intake. Repair or replace as necessary.

Step 5

Check the carburetor idle circuit for blockage. Check the fuel injectors for damage or blockage if applicable.

Ensure the carburetors are synchronized if the motorcycle uses multiple carburetors.

Other Components

Step 1

Make certain the battery is fully charged and all battery cables are tight and free of corrosion and sulfate deposits.

Step 2

Check all wiring and terminals at the ignition coil and electronic control module, if applicable, for tightness and corrosion. Check for breaks in the insulation of all wires connected to the ignition components.

Check the exhaust pipes for damage or modifications. Modified exhaust pipes or baffles can change the back-pressure in the system and affect the mixture. Use stock exhaust pipes, and make certain the mixture is set for the back-pressure supplied by the exhaust system.


  • Refer to manufacturer’s specifications for year and model-specific information and procedures for these tests. Consult a qualified mechanic if you feel uncomfortable performing any of these procedures.


  • Do not spin the engine over for more than 10 seconds when performing the compression test to avoid damaging the starter, battery and solenoid.

Items you will need

  • Spark plug socket set
  • Ratchet
  • Compression tester

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