Whether you’re just cleaned or rebuilt your ATV carburetor and need to adjust it, the idle speed is out of spec, or you’re struggling with poor engine performance you suspect is caused by improper carburetor adjustment, this simple guide has you covered.
Topics covered in this post:
- How ATV carburetor adjustment work.
- When to adjust the carburetor and when not to.
- How to adjust the idle speed.
- How to adjust the air/fuel mixture with the pilot screw.
How Does ATV Carburetor Adjustment Work?
Carburetor adjustments are often seen as a complete headache, but luckily it is more straightforward than you might think.
Most ATV carburetors have two adjustment screws to ensure it provides the proper mix and amount of air and fuel for optimal engine performance.
- Idle-speed screw
- Air/fuel mixture screw or pilot screw
It’s not always easy to tell the two adjustment screws apart.
The Idle speed screw is usually more readily available for easy adjustment, whereas the pilot screw is often slightly recessed into the carb to prevent unintentional adjustments.
The Idle Speed Screw
As the name suggests, the idle speed screw adjusts the engine’s idle speed (RPM).
This screw usually has a toolless or a brass gold-colored flat-head screwdriver design.
What the idle screw does is it adjusts the position of the throttle platet or throttle slide depending on carburetor design to adjust how much air and fuel passes by at idle.
The Pilot Screw (Air/Fuel Mixture)
This screw typically has a brass gold-colored flat-head screwdriver design and adjusts the pilot jet.
The air/fuel mixture screw determines how much air relative to fuel the carburetor puts in the mixture, and this screw is either a fuel adjustment screw or an air adjustment screw.
The easiest way to determine whether your ATV has a fuel adjustment screw or an air adjustment screw is by its location on the carburetor.
- Most of the time, when the air/fuel mix screw is located on the engine side of the carburetor, it meters the amount of fuel.
- Most of the time, when the air/fuel mix screw is located on the air filter side of the carburetor, it meters the amount of air.
Both screws influence the air/fuel mixture, but they operate in opposite ways:
How the pilot fuel screw works:
- Turning the screw IN makes the mix leaner.
- Turning the screw OUT makes the mix richer.
How the pilot air screw works:
- Turning the screw in makes the mix richer.
- Turning the screw OUT makes the mix leaner.
When to Adjust an ATV Carburetor & When Not To
There are times when adjusting the carburetor on your ATV is necessary, but there are also times when it’s better to leave it be.
When to Adjust Your ATV Carburetor
- When the idle speed is too high or too low: Most of the time, you’ll notice the idle speed is off by ear.
- After rebuilding the carburetor: If you have disassembled the carburetor for cleaning or replaced any of the jets or other internal components, you likely need to adjust it after putting it back together.
- After tuning or re-retting the carburetor: Changing jet sizes always require complete carburetor adjustment.
When Not to Adjust Your ATV Carburetor
- After the ATV has been sitting: Carburetors do not go out of adjustment over time. If your ATV won’t run properly after an extended period not in use, the problem is likely caused by something other than the carb being out of adjustment.
- When the ATV only runs on choke: The problem is likely caused by a clogged pilot jet and not improper carb adjustment.
- Other engine issues: Most of the time, when you’re having engine performance issues, it’s caused but other things like bad fuel, a faulty spark plug, or an ignition problem. Do yourself a favor and leave the carburetor as it is until you’ve identified improper adjustment as a likely cause through a process of elimination.
On our troubleshooting page, you’ll find troubleshooting guides for the most common ATV engine problems.
How to Adjust ATV Carburetor Idle Speed
Adjusting the ATV idle speed is a relatively straightforward procedure:
Identify the Proper Idle Speed
Please refer to the owner’s manual for the proper idle speed.
Alternatively, if you don’t have the manual, you can determine if the engine speed sounds too high or low by ear. Most ATVs should idle somewhere between 1100 to 1700 RPM.
Set the Idle Speed Screw Base Setting
If the idle screw was removed or is entirely out of adjustment, you should begin by turning it to its base setting.
The user manual should contain your particular carburetor’s proper idle speed screw base setting.
On most ATV carburetors, 1 to 1 1/2 turns out from lightly seated is a good place to start.
Start the Engine and Allow It to Warm Up
Idle speed adjustment should only be performed when the engine is thoroughly warm. Go for a 10-minute ride to ensure the engine has reached operating temperature.
Apply the parking brake, put the vehicle in park “P,” and leave the engine running.
Adjust the Idle Speed
Use a screwdriver to turn the idle screw or adjust it by hand, depending on its design.
- Turn the screw IN (clockwise) to increase engine speed (RPM).
- Turn the screw OUT (counterclockwise) to decrease engine speed (RPM).
Many ATVs with digital instrument panels have a selectable RPM mode that tells you the engine speed.
Adjust Throttle Cable Free Play
Adjusting the idle speed affects throttle cable free play and can also affect the Throttle Release Switch, TRS (if equipped).
The throttle free play on most ATVs should be between 1/16″ and 1/8″ (1.6mm – 3.2mm) before the engine RPM increases.
If adjustment is required, this is how you do it:
- Peel back the rubber boot protecting the throttle cable adjuster mechanism.
- Loosen the lock nut.
- Turn the adjustment nut in or out until the free play falls within the acceptable limit.
- Tighten the lock nut and slide the rubber boot back in place.
- If the ATV has a TRS switch, please verify that the switch plunger gets pushed in when the throttle lever is in the idle position.
- Turn the handlebars to the far left and then to the far right. The engine speed should remain stable throughout the entire handlebar movement. If the idle speed increases, ensure the cable is routed correctly before repeating the adjustment procedure.
How to Adjust ATV Air/Fuel Mixture (Pilot Screw)
There is usually no need to adjust the air/fuel mixture throughout the ATV’s life span, and most manufacturers advise that you leave it alone.
However, in some cases, as outlined above, if the screw is altered from the factory setting, you might need to bring it back within spec.
Identify the Pilot Screw Base Setting
Whether your ATV has a fuel pilot screw or an air pilot screw, the owner’s manual or service manual has information about the screw’s base setting.
Ideally, you should look up the proper base setting for your ATV as it can vary from 1,5 to 2,5 turns out from its inner position.
If you cannot find the specific base setting, 2 two turns out should work fine for most ATVs.
Turn the Air/Fuel Screw in Until It Lightly Seats
Turn the screw clockwise all the way in until it lightly seats.
Do not overtighten, as it will damage the tip. Fuel screws have a more pointy tip which breaks more easily, but pilot screws (air) can also become damaged if overtightened.
Turn the Screw Out Two Turns
Turn the screw out counterclockwise according to factory specs or two full turns if you don’t have the factory specs. This is the base setting for further adjustment. The engine should now start and idle.
From here, your aim is to fine-tune the adjustment screw until you find the point where the engine idles the highest without hesitating when you flick the throttle.
Start the Engine and Allow It to Warm Up
As with the idle speed, the air/fuel mixture should only be adjusted when the engine has reached operating temperature. This will ensure that the automatic enricher (if available) does not affect the adjustments.
Also, the engine should be idling low. If the engine idles too high, adjust the idle speed as outlined above before adjusting the air/fuel mixture.
Turn the Screw IN at ¼ Turn Increments
Turn the screw ¼ turn in from the base setting and let the engine settle for a couple of seconds. Notice if the engine idle speed increases, decreases, or stays the same.
If the engine idle speed increases or stays the same: Keep turning the screw in ¼ turn and listen again.
If the engine idle speed decreases, stop and turn the screw in the opposite direction, as described in the next step.
Turn the Screw OUT at ¼ Turn Increments
Turn the screw ¼ turn out from where the engine speed began to decrease, and let the engine settle for a couple of seconds and listen. The engine speed should begin to increase.
Continue turning the screw out in ¼ turn steps until the idle speed no longer increases. As you’ve reached this point, turn the screw ¼ turn back in.
Flick the Throttle and Listen
At this stage, you must listen carefully to how the engine behaves when you flick the throttle.
If the screw you’re adjusting is an air screw and the engine hesitates or stumbles when you flick the throttle, this tells you that the engine is running slightly lean at idle.
However, if the screw you’re adjusting is a fuel screw and the engine stumbles, sputter, or cough when you flick the throttle, this tells you that the engine is running slightly rich at idle.
In both cases, turn the screw ¼ turn in and flick the throttle to see if the hesitation or stumble went away.
Continue until the engine accelerates from idle without hesitation or stumbling, and you should be done.
Choke Cable Adjustment
The cable between the throttle lever and carburetor can be adjusted for free play. On most ATVs, the cable is supposed to have 1/16″ to 3/16″ (1.6 to 4.76 mm) of free play.
- To adjust the choke cable free play, loosen the adjustment locking nut and adjust in or out to achieve the desired free play.
- Also, ensure the cable moves smoothly. If the cable drags, inspect the cable for kinks and sharp bends in the routing. If the cable still does not operate smoothly, consider lubricating it using cable lube or engine oil, but steer clear of solvents like WD-40 and penetrating oil.
- If there are signs of damage, it is not serviceable and needs to be replaced.
The Bottom Line
Navigating the intricacies of ATV maintenance, particularly carburetor adjustments, can seem daunting. However, it becomes a manageable and even empowering task with the proper guidance.
By following the steps in this guide, you’re not only prolonging the life of your vehicle but also ensuring safer and smoother rides.