Incorrect idle adjustment on your ATV can lead to all sorts of issues. If the idle is set too low, the engine may stall and struggle to stay running. On the other hand, an idle set too high causes premature drive belt wear, shifting may become difficult, and you may even experience unwanted vehicle movement.
Keep reading to learn how to get your ATV to idle correctly for optimal vehicle performance.
An ATV is running idle when the engine is turning slowly, at no load, with no throttle applied, and usually out of gear.
Too high idle is when the engine turns faster at idle than recommended (higher RPMs).
Too low idle is when the engine turns slower at isle than recommended (lower RPMs).
Please note that improper adjustment is just one of several possible causes when your ATV is idling too high or too low:
Related: How to Know if the Gas in Your ATV Is Bad or Not
If you’re unable to get the idle to where you want it by adjustment only, these are the most common culprits. Troubleshooting and fixing these issues will be covered in a separate post.
The correct engine speed an ATV should idle at varies, but most ATVs should idle somewhere between 1100 to 1700 RPM.
Engine size, year of manufacturing, brand, carb style, and whether it’s a two-stroke or four-stroke are all factors that play a role when determining the correct idle speed.
Please refer to the owner’s manual for your specific ATV’s exact engine idle speed.
The idle adjustment is not the same in all ATVs. Some bikes, typically budget models or older ATVs, can be easily adjusted at home. Newer and more advanced models may require help from a trained mechanic with access to special tools.
Follow the steps below to adjust the idle on a carbureted ATV by adjusting the idle screw.
The engine needs to be at operating temperature to adjust the idle properly. Adjusting the idle when the idle is cold can lead to poor idling when the engine warms up.
Some ATVs are designed to run at a slightly higher idle when the engine is cold before it settles down automatically after a few minutes when the engine has had time to warm up. Adjusting the idle in the cold-start phase will cause the idle to drop too low when the engine heats up.
The idle adjustment screw is located on the carburetor. On most ATVs, the carb is readily accessible, but you may need to remove a plastic cover on some models.
The adjustment screw is usually on either side, but it can also be at the front of the carb. Look for a brass-colored screw designed to be adjusted with a Phillips or flat-head screwdriver.
Reaching the screw with the screwdriver may be a bit fiddly.
It helps to identify the best access angle and have the correct length screwdriver before you begin the actual adjustment.
To know if the idle needs adjusting, whether it needs to be changed up or down, or when you’ve achieved the correct idle, you need a way of determining the engine RPMs (Revolutions Per Minute).
Note that this is a job that requires mechanical skills slightly above average. Adjust the idle by turning the idle screw in or out depending on whether you’re looking to raise or lower the idle:
If your ATV has a tachometer, turn the screw slowly until the gauge displays the correct RPM specified by the manufacturer.
If you’re adjusting by ear, turn the screw slowly clockwise until you begin hearing signs of engine stalling, such as popping or stuttering. As soon as these symptoms occur, turn the idle adjustment screw half a turn clockwise to increase the RPM slightly.
The engine should now be idling smoothly with no signs of stopping.
This is how you adjust the idle on an ATV with an adjustable throttle cable. The same steps apply whether the ATV is carbureted or fuel injected.
Any time you’re making adjustments that affect idling, you need to ensure the engine is at operating temperature for the best results. Please refer to step 1 above.
Older ATVs, budget-friendly models, and most youth ATVs use a mechanical steel cable to transfer movement from the thumb throttle and down to the carburetor. The cable should move freely inside a protective sleeve that s held in place at both ends.
You’ll find an adjustment mechanism at either end of the throttle cable assembly. When adjusted, it will affect the bike’s idle.
On most ATVs with a throttle cable, the adjuster is located up by the thumb throttle, usually covered by a rubber grommet.
Pull back the rubber grommet by hand to reveal the adjustment mechanism. You may need to use a screwdriver to pry the rubber back in some cases.
A locking nut, also called a jam-nut, secures the adjustment screw. When tightened, this nut prevents the adjustment screw from unwanted movement.
Usually, the locking nut can be loosened by hand. Alternatively, you need to use the correct size wrench to get it loose.
Bring the locking nut a few rounds of thread back to allow free movement in the locking nut.
Just as when adjusting the idle by making adjustments directly to the idle screw located on the carburetor, you need to know whether the idle is too low or too high and when it’s adjusted properly.
Please refer to step 3 above to learn how to read the bike’s RPM.
Adjusting the idle up or down is done by turning the adjustment screw or sleeve clockwise or anti-clockwise.
Keep turning the screw until you reach the desired engine idle speed.
Now that your idle is appropriately adjusted, you need to ensure the adjustment screw stays in place by tightening the locking nut back up.
Grab the adjustment screw with one hand and keep it from moving as you tighten the locking nut firmly with your other hand.
The idle on a fuel-injected ATV is set by the ECU module, automatically adjusting the fuel-air mixture. Unless you get an- aftermarket tuner box, there is no way of making adjustments to the idle.
Please note that manipulating the ECU with a tuner requires special mechanical knowledge and experience and is something you shouldn’t attempt unless you know what you are doing.
When the idle is not right on a fuel-injected ATV, most of the time, the problem is caused by a mechanical issue, as listed initially in this post.
When adjusting the idle on an ATV, the idle screw should be turned clockwise (in) to increase engine idle speed or anti-clockwise (out) to decrease engine idle speed.
Related: ATV Won’t Stay Running or Won’t Idle
Home - ATV & UTV - ATV & UTV Tech - Carburetor Adjustment & Troubleshooting Series- Part 1: dialing in the idle circuit
Carburetor adjustment is another one of those great mysteries of engine tuning that some perceive as being a black art. And true enough, it is a bit of an art if you lack the sophisticated equipment to do it scientifically such as CO analyzers and dynamometers.
And as an art form, it takes practice to get good at it.
Carburetor adjustment can be done by the garage tinkerer quite successfully, and has been for years. Unfortunately, it does take some trial and error and experience developed over time to do it well. The topic of carb tuning has been covered by just about every motorsport website, and has had countless books written on it. The intention with this installment in our carburetor series is to tackle the subject in the context of the asian mini-quad. Specifically, what are the pitfalls and troubleshooting techniques that are unique to these machines.
The most fundamental idea regarding carb adjustment is that there is an ideal air to fuel mixture for every engine operating condition. This is not only defined by the engine’s own unique requirements, but also by the air density (temperature, pressure /altitude, humidity). What this means is that not only must a tuner determine what the engine wants at a certain environmental condition, but that adjustments may need to be made to accommodate a colder day, a more humid climate, or a higher altitude.
| The idle mixture screw on the Asian mini-quad is located near the air-cleaner side of the carburetor (slotted screw on upper right hand of the photo). |
Before getting into the environmental adjustments, the first step is to get the adjustments and jetting correct for that engine configuration. Anything that would affect the engine’s breathing ability will affect carb adjustment. Obviously, installing an aftermarket carb will require tuning, but so will changing to a different air filter.
Cylinder porting and a pipe will not directly require a tuning adjustment since they only affect the engine’s ability to create airflow. The carb really doesn’t know what’s downstream. However, it might care what’s downstream – for example, if a pretty hot cylinder is installed with lots of compression and/or lowend porting, then the carb might want to be a little richer, or fatter, to decrease peak combustion temperatures.
Ideally, tuning should be done at the “near normal” conditions you expect to run so as to minimize the need for fine tuning when conditions change.
Whether stock or aftermarket, the first step in carb adjustment is to dial in the idle circuit. Recall that a relatively high vacuum exists downstream of the throttle slide when it’s closed, or at idle. A passage in this area is connected to the float bowl that allows fuel to be sucked into the engine while the throttle is closed. As the throttle is opened, this vacuum drops and the amount of fuel drawn into the engine through this passage decreases accordingly.
We can adjust this amount of fuel by turning the idle mixture screw. On these carburetors the idle mixture screw actually controls the amount of ‘bleed’ air that’s T’ed into this passage. The more the screw is turned out, the more the bleed passage is opened, and therefore more air instead of fuel that’s being sucked into the engine via the primary or idle circuit.
The idle circuit air supply passage can become blocked with dirt and cause a rich running condition. This is the hole at about 7 o'clock on the carb inlet. |
The question is just how do we know when the adjustment is correct. The simple answer is when you’ve obtained the highest idle rpm.
The highest rpm is achieved when the air-fuel ratio (AFR) is optimized. Remember that while this adjustment is being made the throttle is essentially fixed, that is, held at idle. We are not changing the amount of air being drawn into the engine. If the engine rpms increase, that would imply that the engine is doing a more effective job converting the fuel and the given amount of air to useful mechanical power. Too little fuel, and the revs are too low. Too much fuel, and the revs are also too low. Somewhere in between, and combustion efficiency and the revs are maximized.
With the engine off, turn the idle mixture screw in (CW) until the screw seats and count the number of turns. Note this number as a “fall-back” if you decide that your adjustments are not working and you want to get back to the baseline, or ground zero point.
Next, turn the screw back to the baseline adjustment and start the engine and warm it up thoroughly, maybe even ride the quad a bit before starting the adjustment.
With the engine running at idle turn the screw in (CW). This has the effect of closing the ‘air bleed’ passage to the idle circuit, thereby allowing more fuel to be drawn up from the float bowl, richening the fuel mixture. Make 1/4 turn increments until you notice a drop in engine speed. Then turn the screw out (CCW) in 1/4 turn increments. You should notice the engine revs going back up, then dropping again. Take note of how many turns it took to reach this point. Divide this number in half to determine where the theoretical optimized point is. Turn the screw in to reach this point. You may need to experiment with this procedure a few times to satisfy yourself that you’ve found the sweet spot.
If by turning the screw you were never able to find the point where the revs peaked, i.e, the revs keep climbing/dropping until you bottomed the screw, or the screw came out, then you may need a different size pilot jet.
As noted above, the idle circuit is fed by a passage that connects a port in the carb to the float bowl.
The amount of fuel drawn by this circuit is not only controlled by the idle mixture screw, but also by the size of the orifice in the pilot jet at the float bowl. If you’ve turned the screw in completely (air bleed totally closed) and the revs seemed to keep increasing up to that point, it would indicate that the engine is wanting an even richer mixture. Install a bigger pilot jet. If you’ve backed the screw completely out and the revs still haven’t peaked, it would indicate a need for even more bleed air, or less fuel. Install a smaller pilot jet.
If adjustment of the idle mixture screw doesn’t seem to have any affect, then one of two things is going on. The first most likely possibility is the air bleed passage may be blocked. This can occur if the air-filter has passed too much dirt which has gotten into the passage at the mouth of the carb. If this happens the engine will be running very rich, but can still run.
The second possibility is that the pilot jet itself is blocked from dirt getting into the float bowl. If this happens, very little or no fuel will enter the carb.
It’s quite likely that engine will not even run since the primary ingredient in the recipe for combustion is missing – fuel. Engine’s generally have a higher tolerance for too rich than for too lean. If it seems you need quite a bit of throttle to keep the engine idling (no longer using the idle circuit for fuel supply) then this could be your problem.
Next step -->>> high speed, or main jet adjustment.
In order for the ATV to work like a clock, the rider needs to regularly carry out a complete maintenance of the equipment. And a key element of maintenance is ATV carburetor adjustment. Over time, the system starts to work incorrectly, which increases fuel consumption, worsens dynamics and other problems appear.
So how do you adjust an ATV carburetor? nine0003
The operation and life of the engine directly depends on the quality of the fuel mixture. The carburetor is responsible for the preparation of the composition, but after prolonged use of the equipment, the settings “go astray”. This results in:
An improperly prepared mixture leads to the fact that a large amount of soot appears in the combustion chamber, due to which the engine overheats. Naturally, this reduces the life of the engine and leads to its breakdown. nine0010 Many riders who decide to buy an inexpensive ATV simply do not spare the technique and ignore this procedure. But even if you plan to change your device in 1.5-2 years, adjustment is needed. The reason for this is increased fuel consumption. A quad with an untuned carburetor "eats" 30-40% more gasoline, and these are tangible costs.
So how do you adjust the carburetor on a Chinese ATV? For this you need:
However, it is important to remember that all adjustments are made only with a "clean" carburettor.
This means that the rider needs to remove all the dirt from the node and flush the system to remove condensation and sediment from it.
Note: Just before adjusting, the engine must be warmed up to operating temperature.
To set the carburetor idle speed, the float chamber must first be cleared of fuel. To do this, start the ATV, close the fuel cock and wait until the machine stalls (5-10 minutes). To speed up the process, you can take a little ride on the quadric. Of course, gasoline can be drained, but in any case you will have to warm up the device, so the first method of "removing" gasoline is more effective. nine0003
Then:
If at this stage the quadric stalls, then there are problems (the pipes are depressurized). Read the manual before adjusting the carburetor on a 125cc ATV to be sure. Remember that the location of the bolts is different in different devices, so read the documentation in advance. nine0003
To evaluate the quality of the fuel composition, make a test run. After that, unscrew the candle, and evaluate its condition. If it is:
This means that everything is in order. But if such a shade is only on one side, the composition is too poor.
Also look out for these “symptoms”: damaged tip (electrode failure), oil deposits (damage to rings and caps), chips and inclusions (sign of motor failure).
To adjust the mixture you need to twist the needle cover and pull it out. To get the needle, pull out the cable. After that, move the ring in the required direction (lower - more air, but less fuel and vice versa). Then start the engine and make sure it doesn't stall.
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The last step in adjusting the 125cc ATV carburetor is adjusting the petrol level. To do this, you need:
Note: Adjust as carefully as possible as system components are very fragile. Ideally, carburetor tuning should be done by a specialist.
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It often seems like an insurmountable task for beginners to tune and adjust the carburetor of an ATV, but in fact, it is far from being the case. Using our instructions, you can easily set up a carburetor with your own hands. nine0003
Due to improper carburetor settings, problems such as: failures during acceleration, jerks while driving, uncertain start of the engine, poor acceleration dynamics, overheating, loss of power, increased fuel consumption, formation of deposits in the combustion chamber.
So what exactly are we going to tune?
Attention! The adjustment is carried out under the condition of a warm engine and a clean carburetor.
First, make sure there is no condensation in the carburetor. First, you need to drain the gasoline from the float chamber, for this you need to close the fuel cock and partially unscrew the screw No. 1, after the gasoline has drained from the float chamber, tighten the screw No. 1.
Let's look at photos of candles with various deposits and defects. nine0122
A 100% indicator of a correctly adjusted combustible mixture is carbon deposits on a candle.
Consider the location of the screws for adjusting the quality of the mixture and idle speed of carburetors of different models.
#1 Float chamber plug; No. 2 Mixture quality screw;
#3 Idle adjustment screw.
1 - quality adjustment screw at idle; 2 - fuel inlet fitting; 3 - fitting through which you can sleep fuel from the float chamber; 4 - screw-plug of the float chamber. nine0003
1 - ventilation tube; 2 - idle speed adjustment screw; 3 - screw for adjusting the composition of the air-fuel mixture.
ATV carburetor speed and mixture adjustment.
1. Start and warm up the engine until it reaches operating temperature.
2. Adjust the idle screw to set the idle speed to normal. Completely, until it stops, tighten the fuel mixture screw, the engine should stall. If this does not happen, check the tightness of the air supply system from the air filter. nine0003
3. Loosen the fuel mixture screw 1 turn. (Turning the screw clockwise, we enrich the mixture, unscrewing it counterclockwise, we lean).
4. Start the engine and, by adjusting the idle screw, set the speed slightly higher than usual
5. Slowly unscrew the fuel mixture screw until the engine reaches maximum speed (we unscrew the screw no more than 2 turns, but depending range may fluctuate due to engine wear and other faults). nine0003
6. Re-adjust the idle screw to set the idle speed to normal.
7. Press the throttle trigger several times, check if the idle speed is stable.
Mixture quality adjustment with a needle.
Probably you have heard more than once about adjusting the quality of the mixture on the carburetor by manipulating the needle. I note that these are extreme measures, for the initial adjustment, use the mixture quality screw, and only if you do not get the desired result, unscrew the mount and remove the needle. What does she represent? This is the main element that regulates the amount of gasoline supplied to the combustion chamber. Connected directly to the throttle handle through a cable. When you turn the knob, the needle rises higher, opening up a channel for fuel, thereby delivering more mixture, which increases power, which translates into speed. nine0003
You will see 5 grooves on the needle at the attachment point. Initially, the retaining ring fixes it in the central position, this is the best option. But sometimes it is not possible to achieve the required quality of the mixture. Then we will need the remaining grooves, 2 on top and 2 on the bottom, this suggests that we can adjust the dose gradually.
