Spark Plug Readings (Lean, Rich, Fouled, Burnt)

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.

How to Read a Spark Plug

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 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 Chart

Use the images below to help you diagnose your spark plug conditions.

Fouled or Rich Spark Plugs

Good Spark Plugs

Lean Spark Plugs

What is a 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 is the normal color of a good spark plug?

A good spark plug should be light tan or light grey in color at the insulator tip.

Which spark plug does your motorcycle need?

Before you start buying spark plugs, you’ve got to realize that they’re not a one-size-fits all part.

Your motorcycle’s service manual will tell you exactly which type of spark plug you’ll need.

Often times, this will include a specific brand. If that’s the case and you’re looking to switch to a new spark plug manufacturer, there are plenty of conversion/compatibility tables out there and each brand typically has some fitment/application specs.

There are a few reasons why you’ll need to get the right spark plug for your bike, including differences in:

• Size (thread size and length).
• Heat range.
• Plug gap.
• Materials.

If you install the wrong spark plugs in your motorcycle or have an improper plug gap, you may experience:

• Power loss.
• Misfires.
• Plug fouling.
• Bad gas mileage.
• Damage to the pistons or cylinder walls.

How Often to Replace Spark Plugs

If you keep your motorcycle well maintained, you don’t have to replace your spark plugs all that often.

The time to replace you spark plugs depends on:

• How often you ride.
• The type of riding you do.
• How well your bike is tuned.
• The quality of the spark plug you have.

If you’re experiencing performance issues with your bike, the spark plugs are a good first place to check before you get into any bigger repairs or maintenance work. Pull your plugs and take a look at their color and confirm the plug gap.

When you’re tuning your carbs and performing plug chops, it’s always best to start with some fresh spark plugs. If you’re doing a full rebuild, it definitely doesn’t hurt to have a few backups on hand.