Your motorcycle’s spark plugs can be very useful to diagnose running conditions in your engine. You can use the burn color of your plugs and reference the plug color charts below to determine if your engine is running rich or lean.
Here’s some quick references to reading a motorcycle spark plug:
- Light tan spark plug: Good mixture.
- Dark brown or Black Spark Plug: Too rich
- Black, oily Spark Plug: Oil fouled (see valves or piston rings.)
- White, no color on spark plug: Too lean.
- Good running conditions: If everything is good, the spark plug should have a tan/light brown color.
- Rich running conditions: If your engine is running too rich, the spark plug will be black and sooty.
- Lean running conditions: If your engine is running too lean, the spark plug will be white.
- If the spark plug is black and oily, they are oil fouled.
If your motorcycle does not seem to be running right, take a look at the spark plugs. Whenever you are rejetting or tuning your carburetors, a plug chop can take away a lot of the guess work. A plug chop will tell you how your engine is running a certain throttle positions.
If your spark plugs are white, you’ll want to fix the problem as soon as possible. White plugs are caused by engine overheating. If you run it too long like this, you can ruin your engine and burn holes through the tops of your pistons. Common causes for white spark plugs include:
- Too hot a heat range spark plug for your engine
- Low octane fuel
- Incorrect timing
- Cooling problems – dirty fins, no/low water for water cooled engine, low engine oil
- Air/fuel mixture is too lean
The heat range of a spark plug determines when it will fire. If you have a plug that gets too hot and fires are the wrong time consistently, it can burn a hole in the top of your piston. If it’s too cold, the plug will get fouled and stop firing.
Hotter plugs have longer electrodes and cooler plugs have shorter electrodes.
Use this spark plug chart from www.venturers.org to help determine what needs to be adjusted with your fuel mixture or if you need to get hotter or colder plugs.
Spark Plug Color Charts
Spark Plug Chop
A plug chop is when you check the condition of your spark plugs at different throttle conditions. This can help you track down where you may need to make adjustments if you are rejetting your carbs.
To preform a plug chop:
- Find a road with little to no traffic.
- Ride for a little while at a constant throttle position.
- Pull the clutch, kill the engine, and coast to a stop.
- Remove and inspect the spark plugs.
- Repeat for throttle positions as necessary.
Before you do a plug chop, you should ensure that:
- The bike is properly tuned.
- The battery is good.
- Your carbs are signed.
- The spark plugs are new.
- There are no vacuum leaks.
What color should my spark plug be?
A good spark plug should be light tan or light grey in color at the insulator tip.