My new to me CR 250 runs well, but will not idle. Even when the engine warms up or when I use the choke, it will not idle. I wanted to learn how to fix the problem.
How do you fix a rough idle or a dirt bike that will run but not idle? A dirt bike will idle rough or not run when there is a weak spark or the air to fuel ratio is not correct. These could be simple fixes like letting the dirt bike warm up or adjusting the idle screws.
As you can see from the quick answer, diagnosing the cause correctly takes a little time and investigating. Here is what I found in diagnosing my dirt bike and hope it helps you.
Weak Spark Causing Your Dirt Bike to Idle Rough
When the spark is weak, not all the fuel combusts making the engine idle rough. Many items on your dirt bike could cause a weak spark. Here are the most common items and how to fix them.
| Cause of Weak Spark | Fix |
| Fouled Spark Plug | Replace the spark plug. |
| Wrong Spark Plug | Study the current plug and get one that runs hotter or cooler. |
| Old Spark Plug | Replace the plug. You should replace 2-3x per year. |
| Bad Spark Plug Wire | Replace the wire. |
| Bad Coil | Replace the coil. |
Fouled Spark Plug
Normally with a fouled spark plug the dirt bike will not run at all. However, you can tell when the spark plug starts fouling out because the engine will misfire or it will “load up.” When you hear the term “loading up,” too much fuel remains in the combustion chamber and the spark plug gets soaked in gasoline.
At first you think a gasoline-soaked spark plug should fire and burn off the excess fuel. This does not happen because of the lower oxygen levels in the combustion chamber. When the fuel enters the chamber it should have the optimal gas and air mixture. The scenario will be like throwing a bunch of gas on a candle all at once. The flame may go out.
The same thing happens with the spark plug, too much gas soaks the plug and it eventually will not fire.
How to Fix the Fouled Plug:
- Replacing the plug is the best solution.
- If you do not have a spare spark plug, take it out and heat the tip with a cigarette lighter or small blow torch. The extra gas may burn off or evaporate from the heat. Do this at your own risk. The plug will get hot and you will deal with fire.
- The last way I have found to deal with a plug about to foul (you’ll notice it loading up) is stopping the motorcycle and turning the gas off. Open the throttle all the way and count to 60. Turn the gas back on and try starting the dirt bike. This allows air to move through the combustion chamber to let some gas evaporate. Keep in mind it will benefit you most when you have few or no tools and likely out in the middle of a trail somewhere. It does not work all the time.
Wrong Spark Plugs
I can remember going through several spark plugs on one particular motorcycle. At the time I had a new RM 125 and an older YZ 125. We had rebuilt the YZ’s engine, and the bored the cylinder. A bored cylinder increases the engine displacement. The YZ was more like a 140 or 150 cc engine.
The plug had fouled on the YZ and I replaced it with one I had for the RM. Needless to say, it and a few others kept fouling.
After some investing, I found the YZ needed a different spark plug. See spark plugs are not the same. Depending the octane, fuel (2 vs 4 stroke), and air/ fuel mixture, you will use different plugs. Some burn hotter some cooler.
I needed a hotter plug since the motor required more fuel and the air/ fuel mixture differed from the RM. The point is, make sure you put the correct plug into the dirt bike to start.
Old Spark Plug
Like all things, spark plugs wear out. The electrode at the end of an old plug does not conduct electricity the way a new plug does. The old one produces a weak fire as the spark moves across the gap.
Replace the old spark plug. Before you put the new plug into the cylinder, put it into the spark plug boot and turn the engine over. Look at the spark and see how strong it is. Through in the old plug so you have a reference for the new plug.
Pro-Tip: Do not touch the spark plug, wire, or rubber boot while turning the engine over. You will get shocked if you do touch any of these items.
Bad Spark Plug Wire
Maybe you do not have a fouled or bad spark plug, what do you check next? When an old dirt bike did not fire after completely checking all the points discussed, I checked the spark plug wire.
In my case, I found a break in the boot. I had been riding a lot in the rain and mud. Moisture made its way into the core part of the wire making the it go bad. Look for breaks and cracks in the wire.
If the wires are old and do not have cracks or breaks, replace them anyway, like spark plugs they eventually wear out.
Bad Coil
Let’s assume you have a new plug, new wire, and your dirt bike still will not idle.
You want to check the ignition coil next. The coil will be located at the opposite end of the plug wire from the spark plug. On some dirt bikes it is part of the wire and not on others.
For those riders who are not mechanically adept, the ignition coil turns a low voltage into a big voltage that helps create the spark for the spark plug. You may have guessed when the coil does not amplify the voltage enough a weak spark is the result making the dirt bike not idle or idle rough.
In most cases, when you replace the plug wire, you will also replace the coil.
Bad Timing
Even when you have strong spark, if the ignition does not happen at the right time, your dirt bike may not idle or it will idle rough.
Each dirt bike is a little different so check your owner’s manual on how to correct the timing.
Air Causing Your Rough Idle
Like you and I need clean air, so does your dirt bike. This is why it has an air filter to screen out particles from the air. Go without it and you will end up with scratched cylinder walls.
If your dirt bike does not get the proper amount of air, it chokes up and affects idling and performance. Here are common idle problems with air flow affecting dirt bike idle.
Dirty Air Filter
Start by locating the air filter. Most dirt bikes have it located under your seat or you can access by removing the side panel.
When you run your dirt bike with a dirty air filter, the engine gets deprived of oxygen which helps in combustion. Make sure it is not dirty.
Pull out the air filter and properly clean it. Then reinstall the filter. Restart the motorcycle to see if improves the idling.
Since we are talking about it, you may wonder how often should you clean your air filter? I have found for normal leisure riding, cleaning it every few weeks or monthly should keep the air flowing. When conditions get dusky, check it and clean it after every ride.
Blockage In the Air Box
If you cleaned your air filter, then you already know what the air-box is. On the chance you do not know, it is what the air filter sits in.
After pulling out the air filter, check the box for other obstructions. Do not check it with just a quick look. Pull out a flashlight and shine it down into the box and into the intake of the carburetor. It may surprise you what you find.
When I find something in the air-box, it is a leaf most of the time.
Similar to a dirty air filter, obstructions in your airbox choke off air to your engine and can stop airflow all together. Therefore, you need to shine the flashlight here.
Pro-Tip: When riding in heavy foliage or when you think debris has made its way into the air-box, suck out any loose material with a shop vac. This will help keep unwanted material from entering your engine.
Old Air Filter
Get rid of old air filters. They crumble when they get old and pieces get sucked into the carburetor as well as the engine. It is similar putting sand through your motor when this happens.
Depending on the filter and how you care of it, an air filter may last one ride (on a dusty day) or for a couple of years for foam filters with oil treatments.
Fuel As The Cause for your Rough Idle
When I was young and learning about dirt biking, I did not think gas could go bad. They pulled it from the ground where it had been for millions of years. Yes, gas goes bad. Here are ways bad fuel can affect your dirt bike’s idle.
| Rough Idle From Fuel | Fix |
| Bad Fuel | Replace fuel older than 60 days. |
| Dirty Fuel | Only get fuel from busy gas stations. |
| Air Fuel Mixture | If your plug tip is white, it is not getting enough fuel. Change jets or turns the air/fuel screw on the carburetor. |
Bad Fuel
How does fuel go bad? Without going into a long explanation, it breakdowns. Some experts estimate fuel breaks down after 60 days, other say 90. I try to use fuel within 30-60 days.
When the fuel breaks down it leaves a film behind clogging up carburetors and jets. If you have an older dirt bike, it will even breakdown your fuel line. Who wants pieces of fuel line moving through their engine?
If your carburetor or jets are partially clogged, you may not be getting enough or an even flow of fuel and air to run the engine.
To fix this problem, you may have to pull the carburetor off, take it apart and clean it. Take the jets out and clean those too. Check for build up around the float.
Pro-Tip: If it seems the film will not come off the jets, replace them and the float.
The second preventive measure is putting stable into tanks and gasoline you may not use for 30 days. Storing your dirt bike for the winter will be a great example of when you should use Sta-Bil.
Dirty Fuel
Has your car ever ran bad after you just filled it up? It likely had a rough idle. The same thing can happen to a dirt bike.
The fuel may have dirt in it but most of the time the fuel is not actually “dirty.” Water will be the problem. Condensation gets into the fuel and runs through your engine. The problem is water does not combust creating a rough idle or not allowing the engine to run at all.
Avoid the issue by getting your dirt bike fuel from stations that have frequent turnover at the pumps. The more gas moves through the storage tanks, the lower the chance of getting bad fuel (although it happens.)
Air Fuel Mixture
We talked about your air-fuel mixture earlier. When not in the right proportions, your dirt bike will run lean or rich. Too much in either direction and the dirt bike will not idle or idle rough.
Pro-Tip: When a dirt bike runs lean, it is not getting enough fuel. A sign of a lean dirt bike will be a white tip to the electrode of your spark plug.
When a dirt bike runs rich, you have too much fuel going into the engine. Over a long period, you may foul your plug if you give it too much fuel.
To fix the air-fuel mixture, you will look at two common adjustments:
- Adjust the fuel screw on the dirt bike to see of the situation may be corrected.
- If #1 does not work, you will need to look at changing the jets. There great videos on YouTube on how to do this.
Related Questions:
What causes a misfire at idle? There may be several causes related to the misfire such as spark plugs, plug wires, timing of the spark ignition, coils or wrong air-to-fuel mixtures.
What causes RPMs to fluctuate at idle? A vacuum leak maybe the cause of the fluctuation or it may be a dirty carburetor not mixing air and fuel properly. You should also check for a blockage in the air filter or air box.