Dirt Bike Hanging Idle [What Is It & How To Fix It]

A dirt bike hanging idle is not something you should avoid fixing. It can seize your engine, and it can also cause you to crash (speaking from experience here!)

In this article and video, I'm going to break down:

• What a hanging idle actually is
• What happens if you DON'T get it fixed
• What causes a hanging idle
• How to fix it based on specific problems your bike is having

What is a “hanging idle” on a dirt bike?

It's when you rev the engine by twisting the throttle, and when you let off the gas, the RPM stays high and takes longer to “settle down” to a normal idle RPM. 

Even when you completely let go of the throttle, it takes an extra 1-5 seconds more for the rpm to stabilize.

What will happen if you don't fix a hanging idle

Not only is it frustrating having to wait for the engine to settle down to a proper idle RPM, but it can also be dangerous for you and your bike. 

If you're riding and the RPM “hangs” when you let off the gas, it's going to keep accelerating, potentially causing you to fly off course or hit a tree in the woods. 

The engine will also run hotter and can potentially seize, depending on the cause of the hanging idle, which I'll cover next. 

What causes a hanging idle?

There are many possible things, such as an air leak, incorrect jetting, or throttle cable problems. The main cause of a hanging idle on a dirt bike is that there is too much air getting to the engine.

Think of it like the throttle being stuck open, making the engine rev higher, but then there's not enough fuel to keep the engine cool, which is why it will typically run hotter. 

Is a hanging idle rich or lean?

In virtually every case that I've had, a hanging idle has been caused by an air-fuel mixture that's too lean. 

That's not to say that the jetting isn't too rich, but being too rich typically doesn't make it have an idle that won't drop/settle down. 

How to fix a hanging idle

First, you need to troubleshoot if there's an air leak, a throttle issue, or a carb jetting problem. Always start with the easiest things first 

Below is a list of all the common causes for a dirt bike hanging idle that I've had to troubleshoot, diagnose and fix over the years. Many of them can happen on a 2 stroke or 4 stroke dirt bike, but I've found that it's more common on a 4 stroke motocross bike, such as a CRF250R because more things can cause it. 

Idle set too high

The easiest thing to check is the idle adjustment screw. The idle RPM changes as the engine warms up, so if it was set when the engine was cold/not fully warmed up, it may cause a hanging or high idle when the engine is hot. 

The idle screw is usually on the left side of the carburetor/throttle body and can be adjusted by finger or a screwdriver. Turning it out (CCW) will lower the idle RPM. 

Air leak around the carb

When the carburetor isn't properly sealed, air can leak around it, causing the air-fuel mixture to be too lean (too much air), resulting in a hanging idle. 

Make sure that the air boot and intake manifold are properly fitted all the way around the bells of the carb and that the clamps are tight. 

For carbs that are bolted to the intake manifold, make sure the nuts are snug, as well as the o-ring/gasket is in good shape and not cracked. 

Intake cracked

The intake manifold is the rubber piece that connects the carb to the engine. If it's cracked (common on bikes more than 10 years old because the rubber hardens and cracks over time), this will cause and air leak and a hanging idle condition. 

Replace the intake with a new one or a used one that's soft and flexible. 

Air/fuel screw too lean

The air/fuel screw adjusts the fuel mixture for the pilot jet circuit, which controls idle to roughly ¼ turn throttle openings. When it's not properly tuned, it can make your dirt bike engine run lean and have a hanging idle. 

¼ of a turn of the screw can make a fairly big difference in how it starts and runs. 

For a free guide on how to tune your air/fuel screw in just a few minutes, click the link below:

• 2-stroke air screw
• 4-stroke fuel screw

Air/fuel screw o-ring missing (if applicable)

The fuel screw on a 4 stroke carburetor should have a spring, washer, and a rubber o-ring installed on it in that order. If that o-ring is cracked or missing, it will cause an air leak that can make the idle hang because there's too much air getting through the carb. 

Simply remove the fuel screw and replace it. Pay careful attention when removing and installing because the o-ring can get caught or stuck very easily. 

The air screw on a 2 stroke carb may or may not have the rubber o-ring. If your bike is using the OEM carburetor, you can look up the parts fiche/diagram for your specific make/model/year to see if there should be one there. 

Dirty carb

When gas sits in a carburetor (especially pump gas with ethanol), it breaks down and can clog or “gum up” the jet circuits. The first one to stop working properly is usually the pilot jet because it's the smallest, and that's when you get that hanging idle problem. It's usually hard to start as well because it's not getting enough fuel. 

The easiest solution is to just replace the pilot jet with a new one (it only takes a tiny amount of crud to affect the jetting). 

I prefer to try and clean the jets to save money.
I use an ultrasonic cleaner, which I show you how to use to clean a carb step by step here. 

Pro Tips: How to prevent your dirt bike carburetor from getting dirty again:

• Use gas with 0% ethanol (gas without ethanol lasts longer because it doesn't absorb water and break down as quickly)
• Ride your bike at least once per month (to keep fresh fuel running through it)
• Don't use carb cleaner sprays (they ruin the rubber gaskets/o-rings)

Incorrect Pilot jet

If the pilot jet is clean and you've already tried richening the mixture screw (once you get past 2.5 turns out, it's a sign that you need a different size pilot jet), then you might just need a larger pilot jet. 

Hot start

Most liquid cooler 4 stroke dirt bikes have a hot start knob on the carb or lever on the left side of the handlebars that makes it easier to kick start when the engine is hot. 

A hot start problem arises when the hot start:

• Hot start lever doesn't have any slack (cable adjustment on the clutch perch)
• Threads get stripped on the screw going into the carb body (they're made of plastic)
• The o-ring/gasket cracked or is missing
• The plunger going into the carb corrodes and gets stuck open

Float height

The carburetor float controls the fuel level for all of the jet circuits. If the float height is set too low, the jetting will be lean all across the board, which can cause a hanging idle and a number of other problems, such as hard starting when cold and bogging. 

It doesn't take much to bump the float level off of the proper level, so check out this article on how to check and adjust the float. 

Throttle sticking

Sometimes it's not an air leak or jetting problem and it's just your throttle sticking open. A dirt bike throttle can be sticky from:

• The throttle tube is dirty from tipping over and getting dirt stuck on the bar, 
• The cable is old and corroded
• The slide/plate in the carb being worn or moisture on it (the chrome plating wears off after so many hours)

To fix these problems, you must:

• Clean the handlebar and throttle tube (or replace if broken) and replace the torn grip so dirt doesn't get in there again
• Lube and/or replace the throttle cable
• Clean or replace the throttle slide 

FCR mid-body gasket

The Keihin FCR MX carburetor is a good performing carb when properly tuned, but old age and bad gas or harsh carb cleaner chemicals can wreak havoc when trying to troubleshoot a “jetting” problem on a 4 stroke MX dirt bike. 

If you've tried every jetting change, or the pilot jet needs to be way bigger than stock to start and not have a hanging idle, there's a good chance the mid-body gasket needs to be replaced. 

A bad mid-body gasket on an FCR carb can cause a hanging idle, hard starting, bogging, and the accelerator pump not working. It's fairly easy to replace if you know how to take the float bowl and jets out. Just be aware that “no name” brand kits often don't fit properly, and it's a very specialized gasket. 

4 stroke valves worn/need adjustment

Intake and exhaust valves that are worn and/or out of adjustment can cause a hanging idle. 

The valve clearances typically get tighter as the valves wear, and eventually, they will leave the valves open at all times, causing an air leak and lack of compression. If your 4 stroke dirt bike has been getting harder and harder to start over time, it's likely the valves that need to be adjusted or replaced. 

Exhaust leak

When the exhaust isn't properly sealed at the mating points, it will cause a leak that can result in a hanging idle as well as a dirt bike that backfires.

These are common causes of an exhaust leak:

• Head pipe gasket damaged or missing
• Exhaust pipe is bent and not properly seated
• Pipe or muffler is cracked or has a hole 
• Head pipe to mid pipe or muffler 

Bottom Line

Start with the easiest things first so you don't waste your time. After you get it fixed, let us know what your problem was to help others like you get their bike running well!

Just to recap, here are the most common causes of a dirt bike hanging idle:

• Idle too high
• Air leak
• Intake cracked
• Air/fuel screw too lean
• Air/fuel screw o-ring missing
• Dirty carb
• Incorrect Pilot jet
• Hot start not working properly
• Incorrect float height
• Throttle sticking
• Mid-body gasket (FCR carb)
• Valves worn/out of adjustment (4-strokes)
• Exhaust leak