How To Tell If Your Dirt Bike is Running Too Rich Or Too Lean

There are a few noticeable signs that will help you identify if your dirt bike is running too rich or too lean. This guide will help you identify and fix your jetting problems

If your dirt bike feels like it’s running at a sub optimal level there’s a good chance that you might be running too rich or too lean. Running too rich or lean for an extended period of time can cause major engine damage. This guide will teach you how to tell the difference and the correct jetting setup for 2 Stroke and 4 Stroke dirt bikes.

What Does It Mean To Run Rich or Run Lean

Running rich or lean refers to the amount of air and fuel mixture that is flowing into the carburetor. If your bike is running rich, it means that there is an oversupply of fuel relative to the amount of air running through your bike’s engine. Running lean is the opposite. There is too much air relative to the amount of fuel which inhibits the engines ability to effectively burn the air-fuel mixture.

dirt bike running too rich

Is It Better To Run Rich or Run Lean

With an ideal bike setup, you should not be running rich or lean. Both of these can have a negative impact on your bike’s engine. Most experts will tell you that running lean can have a more severe impact than running rich. This is because when your dirt bike is running lean, it will cause the engine to run much hotter which could result in overheating and even engine seizure in the form of a destroyed piston. Read our replacement dirt bike piston guide.

What Is The Right Air To Fuel Mixture For A Dirt Bike

There is no “one size fits all” ratio of air to fuel for all dirt bikes. This is because there are many variables that need to be taken into account. Riders need to consider factors like air temperature, humidity, spark plugs, riding terrain, and more. As a general rule of thumb, you should aim for a ratio of 14.7:1. This means that there should be 14.7 grams of air for each gram of fuel flowing into your engine. It’s not always possible to reach this “perfect ratio” but getting an air to fuel ratio close to this will be the best option for your dirt bikes engine.

How Do I Know If My Dirt Bike Is Running Rich

There are a few noticeable signs that will help you identify if your dirt bike is running rich or lean. If your dirt bike is running too rich, look at for the following signs;

  • Your dirt bike struggles to start
  • Very smoky exhaust – especially when starting
  • Your dirt bike is not laying down enough power
  • Dirt bike lacks power at higher levels of elevation
  • Your fuel tank empties faster than other bikes – poor fuel economy
  • Your Dirt bike does not start easily when the engine is hot
  • You can cold start your dirt bike without using the choke
  • Low amount of throttle response or lacking acceleration.
  • Dirt bike engine keeps flooding
  • Your bike keeps fouling spark plugs.
  • If 2 stroke – Gummy substance seeping from the exhaust 

How Do I Know If My Dirt Bike Is Running Lean

When your dirt bike is running lean the symptoms may be different to those when running rich, although not all symptoms will be apparent. You should pay close attention to the way your bike responds under different scenarios. If your dirt bike jetting is running lean you should be on the lookout for the following signs;

  • Engine bogs down at low elevation
  • Engine struggles to turn over when cold
  • Dirt bike engine is idling to high
  • Dirt bike has very sudden response to closing the throttle (no smooth transition)
  • Engine tends to overheat quickly
  • Low response or bogging when you accelerate
  • Engine idle hangs (Could be fuel screw or air leak)
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Why Is My Dirt Bike Bogging Down

If your bike feels unresponsive when you open the throttle, or you can hear the revs and power drop when you accelerate your dirt bike is bogging. This is often due to incorrect jetting setup. This can happen in either scenario when your jetting is running lean or when your jetting is running rich.

Dirt Bike Is Bogging Due To Running Rich (Rich Bog)

The signs that your bike is bogging due to running too rich;

  • Engine is not responsive and sputters on acceleration
  • Engine is not responsive and very slow to accelerate (if at all)
  • Engine sounds like it’s at maximum output and hitting the rev limiter

Dirt Bike Is Bogging Due To Running Lean (Lean Bog)

The signs that your bike is bogging due to running too lean;

  • Normally occurs when opening the throttle quickly
  • Engine sounds like it’s cutting out and dying
  • Once the bog clears the engine sparks into life with a lot of power.

Will Running Too Rich Damage The Engine

New dirt bike spark plug

Although running a rich setup can have adverse side effects in the long term, running rich usually will not cause too much damage to an engine. The long term impact can come in the form of engine performance and reliability. The most common problem one can expect when running rich is burnt out or fouled spark plugs.

While fouling a spark plug isn’t necessarily bad for the bike, it could result in a dangerous scenario where a rider could become stranded out in the middle of nowhere without a spark plug or a way to get help. Always carry spare spark plugs like these ones. You can also read our review of the best motorcycle spark plugs.

If your jetting is too rich it means that your engine is not able to efficiently burn the air-fuel mixture. This can lead to carbon build up that will affect the engine’s performance over time. Inefficient combustion can cause carbon build up on the piston rings and/or grooves which will cause lower compression and potentially oil blow-by due to a poor seal.

Will Running Too Lean Damage The Engine

Running too lean is much worse than running too rich. If your dirt bike is running too lean it means that there is not enough fuel in the air-fuel mixture. Fuel is required to keep the engine running at a lower temperature. Running too lean can result in overheating and severe engine damage.

Although riding a dirt bike with a lean jetting can be beneficial for better throttle response and horsepower, it can cause the engine to overheat in hot riding conditions. A good way to test and tune lean jetting is to monitor your bike’s performance when accelerating up steep uphills, thick sand, or large technical passes. The harder the engine has to work, the more heat it creates which will be amplified by lean jetting.

Side Effects Of Running a 2 Stroke Too Rich

Running a 2 stroke with rich jetting can cause a number of serious problems. It is for this reason that it is important to ensure you have the correct jetting setup. The most noticeable symptoms that your 2 stroke dirt bike is running too rich include;

  • Frequently fouled spark plugs
  • Excessive amount of smoke from the exhaust.
  • Low throttle response when accelerating
  • Lack of power when opening the throttle
  • Gunk oozing out of the exhaust
  • Poor fuel economy (Burning too much fuel)
  • Engine seems to get flooded easily
  • Difficulty starting the engine when hot

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What To Do If Your Dirt Bike Is Running Rich

In order to make sure that your dirt bike is not running too rich, you must ensure that you have the correct air fuel ratio. If your bike is running too rich, you need to lean it out by allowing more air or less fuel into the carb. This can be done by adjusting the pilot jet, the needle, and the main jet.

Generally speaking, the pilot jet should be used for tuning 0-¼ turn throttle opening. The needle is used for ¼-¾ throttle opening, and the main jet is used to tune ¾ to full open throttle. Check out our guide for a more detailed explanation of how to fine tune your 2 stroke jetting.


  1. Whit Miller says:

    I have a 2021 klx110 and it was cutting out when I gave it full throttle. I increased the pilot and main jets, and now the spark plug looks too rich (all black carbon). If I move the pilot or main jet down one size, it starts cutting out again. Not sure what to do.

    1. Dirk Diggler says:

      Well, the pilot jet has absolutely nothing to do with how the main jet works. So, you’ve created having to troubleshoot 2 things. Always make one change at a time. At wide open throttle, your bike is solely using the main. At closed throttle to around 1/4 turn…your bike is operating on its pilot circuit.

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