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How To Tell If Your Dirt Bike Is Running Rich Or Lean

Is your dirt bike running rough but you can’t tell whether it’s running rich or lean? Jetting is important, and it can cause engine damage sooner if it’s too far off. 

This article will explain the difference between rich and lean, as well as how to tell how your dirt bike is running. This works for 2 stroke and 4 stroke bikes.

What Does Running Rich vs Lean Mean?

Running rich vs lean is regarding how the carburetor or fueling system is controlling the air and fuel supply.

If you’re running rich then there’s more fuel/gas than there should be compared to the amount of air that’s going through the engine. Running lean means there’s too much air and not enough fuel for the engine to burn properly.

how to jet a dirt bike

What Is Worse Running Rich or Lean?

Neither is ideal, but in the long run, running lean is generally worse because it can do more damage. Running lean will cause the engine to run hotter. Under extreme conditions, this can cause the piston to melt down or seize.

The “perfect” air to fuel mixture for a gas engine, also called the stoichiometric mixture is 14.7:1.

This means that for every 1 gram of fuel, there’s 14.7 grams of air.

Getting this perfect mixture is the most efficient, but it’s not always ideal because there’s so many changing variables, such as air temperature and humidity that change this mixture.

So, your goal is just to get as close to possible without spending too much time.

Running Rich Symptoms

Whether your dirt bike is running rich or lean, there’s key traits that will make it easier to identify which way.

A dirt bike that runs rich will have symptoms like:

Running Lean Symptoms

Lean jetting symptoms aren’t always the opposite of jetting that’s too rich, so you must pay careful attention. 

The most common symptoms of lean jetting in a dirt bike are:

Rich Bog vs Lean Bog

Is your dirt bike bogging when accelerating? There’s a good chance the jetting is off.

But how do you know if it’s a rich bog or a lean bog? Here’s a quick way to tell:

A rich bog will:

  • Sputter and hesitate
  • May stop accelerating
  • Sounds like it’s hitting a rev limiter

A lean bog will:

  • Sound and feel like the engine is dying
  • If you get past the bog, it will feel like a surge of power
  • Usually happens when quickly opening the throttle

Can Running Rich Damage The Engine?

Running rich will very rarely damage an engine. However, running too rich for too long can cause problems long-term that will affect performance and reliability. 

Rich jetting will foul a spark plug more easily. This can strand you in the middle of nowhere if you don’t have a spare plug and tool to change it. 

Rich jetting causes spark plug to foul
Turn Up The Heat!

Rich jetting means that the air fuel mixture is not properly burning. The combustion chamber and exhaust can get unburnt carbon built up in it due to this inefficiency. 

Built up carbon can slowly affect performance over time. 

Also, poor combustion from too much gas can build up on the piston rings or grooves. This will create a lack of seal, lowering the compression and possibly cause oil blow-by.  

Rich vs lean spark plug

It’s hard to tell if your jetting is good or not simply by looking at the spark plug these days. This is because pump-gas is not very consistent and can give you inaccurate results.

With that said, a spark plug is usually darker or black when the air fuel mixture is too rich. It’s usually lighter or white in color when running too lean. The goal is to get a nice “tan brown color” for proper jetting, but use it as more of a guideline.

You can run a hotter or colder spark plug as a “band-aid” if your jetting is too rich or lean, but I advise against it as a long-term fix. The lower the “number” on a spark plug, the hotter the spark. BR8 = hotter than BR9.

Is It Bad For An Engine To Run Lean?

It’s worse to run your dirt bike lean. Lean means there’s not enough gas, and gas cools the engine. Running lean is bad because it can overheat and damage the engine. 

Jetting on the lean side is good for crops throttle response and maximum horsepower, but under hot and extreme conditions it will create too much heat.

Heavy loads (accelerating in sand or uphill is a heavy load) bring out the worst of jetting problems, but are great for tuning. 

The greater the load, the more heat the engine creates. Running lean will only make it worse. 

What Happens When You Run A 2 Stroke Rich?

Running a 2 stroke rich will most commonly result in:

  • Fouled spark plugs
  • Smokes a lot
  • Poor throttle response
  • Low power
  • Spooge out the exhaust
  • Lower fuel economy
  • Floods more easily
  • Harder to start when hot

As you can see, there’s a lot of potential problems if you run your 2 stroke dirt bike too rich. This is why jetting is so critical. 

Pilot jet rich vs lean

The pilot jet is the most important jet circuit for most riders because its controls roughly 0-1/4 turn throttle position openings. This means that anything from idling to revving it up to 1/4 throttle is primarily controlled by the pilot jet and air/fuel screw.

Common rich pilot jet symptoms are:

  • Hard to start when hot
  • Won’t stay running/won’t idle
  • Poor low-end throttle response – sputtering
  • Bad gas mileage – less miles before needing to refill
  • Excessive smoke out the exhaust

Common lean pilot jet symptoms are:

  • Hard to start when cold
  • Hanging idle/high idle
  • Bogs on small throttle openings
  • Power surges/too responsive at bottom-end power
  • Runs hot/overheats/leaks coolant out radiator overflow faster than other bikes

Changing the is fairly easy on most dirt bikes, but still takes a few minutes – but you should try something easier first!
The air screw on a 2 stroke or fuel screw on a 4 stroke should be adjusted first because it’s the “fine-tune” adjustment for the pilot jet. It only takes a few seconds with a flat blade screwdriver and tuning it will tell you if you need to go richer or leaner on the pilot/slow jet.

How Do You Fix A Rich Running Dirt Bike?

To fix a rich running bike you must correct the air fuel mixture by leaning it out. The question is, where does it need to be leaned out? 

A carb has three main circuits: the pilot jet, the needle, and the main jet

As a rough rule, the pilot jet is for tuning 0-¼ turn throttle openings, the needle is for ¼-¾ turn throttle openings, and the main jet is for ¾-full throttle openings. 

Lean pilot jet symptoms aren’t the same as lean main jet symptoms, so you can treat them the same. 

Want to learn how to easily start tuning your jetting in just a few minutes? Click Here to learn more.