Starting your dirt bike with the choke on is a regular practice, particularly in cold weather. But have you had the problem where once the bike is warm, and you try to turn the choke off that your bike stalls? Then if you start it up again with the choke on, it starts fine but won’t idle without the choke in the “on” position.
This can be a frustrating problem, and the cause may not be immediately obvious, particularly if you are a new dirt bike owner. Some thought about the fuel system and how it works will, however, reveal the most likely culprit.
The essential reason your dirt bike won’t idle without the choke is that the engine is not getting the right air-fuel mixture. The fuel component in the mix is being restricted, the most likely reasons being the carburetor needs tuning, or a clogged pilot jet in the carburetor.
The best philosophy to follow when troubleshooting a problem is to start with the possible cause that is easiest to check or resolve and then work through the list to the more complicated cases. With this philosophy in mind, we will start with the potential cause that can be fixed quickly and easily, with minimal stripping of the bike and then move on from there.
The choke on a dirt bike enriches the fuel mixture entering the combustion chamber via the carburetor. In other words, an increases the amount of fuel in the fuel-air ratio. This extra boost of fuel helps the engine to start, especially on cold days. If, however, once the engine has warmed up, but still won’t continue to run without the choke on, then we have a problem!
Since the choke increases the amount of fuel, we know the problem has to be somewhere in the supply of fuel to the carb or from the carb to the engine.
Bad Carburetor Tuning
The place you want to start your troubleshooting the root cause of the problem should begin with an investigation of the carb tuning. When last was your bike serviced, or the carb tuned?
The carburetor is tuned by adjusting the fuel-air mixture that is processed in the carb. The fuel flow adjustment screw will increase or diminish the amount of fuel in relation to the airflow. If the carb is tuned to make the fuel mixture too lean, then your dirt bike may not have sufficient fuel in the mixture to idle without the choke on.
Fortunately, this problem is not difficult to remedy and does not require a major dismantling of your bike or any components. Most carburetors can be tuned by simply adjusting the fuel mix screw with a screwdriver.
The simplicity of this fix dictates that it should be your first point of investigation to cure the problem of your dirt bike not being able to idle without the choke on!
Dirty Fuel Filter
The fuel filter is usually a device that is in the fuel line between your bikes fuel tank and the carburetor. The easy accessibility of the fuel filter makes it the next item to check to determine the lack of fuel getting to the motor.
The fuel filter is there to filter out dust particles and other debris that may contaminate the fuel and cause a problem in the carburetor or engine. The filter will, however, as time goes by, becomes clogged with the debris that it is preventing from going further down the fuel line.
When the fuel filter becomes clogged, it will restrict the amount of fuel that can pass through the filter, and this may starve the carburetor and engine of sufficient fuel. This had the potential to cause the bike not to idle correctly unless the choke is on.
Some fuel filters can be opened and cleaned, while others are sealed units that cannot be opened. The easiest way to test the flow is to take the pipe off the engine side of the filter. Then open the petcock valve on the tank to allow the fuel to flow.
Examine the strength of the flow of fuel passing through the fuel filter. If the stream of fuel that comes out of the opposite end of the fuel filter looks strong and unhindered, then it is unlikely that the fuel filter is part of the problem. If, on the other hand, the flow of the fuel is weak, then the fuel filter is restricting the passage of the fuel.
This is another problem on your dirt bike that is fixed easily and cheaply. Fuel filters are not expensive and are easy to replace. Buy yourself a new filter and install it in place of the old one. Take the old filter with you when you go to purchase the new filter so the size of the connectors can be matched to your old one to make sure the fuel line pipe will fit.
Clogged Carburetor Jets
Carburetors have very fine jets, through which the fuel-air mixture is introduced into the engine in a fine mist! These jets are pinhole-sized openings that can easily become clogged or blocked up for several reasons.
The first reason is once again dust and other debris that can contaminate the fuel, and if it manages to bypass the fuel filter and enter the carburetor, the pilot jet can become blocked with these contaminants. The minuscule size of the opening of the jets means it does not take much for them to become blocked.
The second potential cause for blocked carb jets is old fuel that has started to degrade. Gasoline has a limited shelf-life; if it stands for a length of time, it starts to lose its volatility as the vapors evaporate. As the fuel also starts to break down chemically, it starts to thicken and become sludgy.
This thickening of the fuel will make it impossible for it to pass through the tiny openings of the jets in the carburetor. The sludge that becomes associated with this old fuel also becomes a contaminant that blocks the jets.
Whether the cause is contaminated fuel or old fuel, the process to rectify the problem is going to be similar. The carburetor will have to be removed, cleaned, and serviced. The needle valves and the pilot jets will all need to be inspected to determine if they can just be cleaned or should be replaced with new components.
If you are new to this kind of repair to a dirt bike, then it may be a job for a professional, or enlist the help of an experienced buddy to give you a hand. Either way, this job is going to take a bit of time to remove the carb, dismantle it, check the components, clean or replace them, set the needle valve correctly, reassemble and re-fit.
The job is not over once that part is complete. You will probably also need to empty out the fuel tank, clean it out, replace the fuel filter, and then fill the tank will fresh fuel.
The Problem Of Too Much Air
This potential cause of the problem I have left until last because it is a little more obscure and less likely to occur, but is a potential problem nonetheless! It is also a problem that is difficult to diagnose and would probably require the services of a professional to resolve.
Since the problem of your dirt bike not being able to idle without the choke is a problem related to the fuel-air mixture, what if the problem was not a restriction of the fuel, but too much air? Broken seals or cracked pipes in the air delivery system to the carb or from the carb to the engine could contribute to the cause.
This type of problem can result in additional air being sucked into the engine combustion chamber. This will affect the fuel-air ratio, and with too much air now being in the system, the choke is needed to supply additional fuel to compensate for the extra air!
As I have said, although this problem is rare, it has been known to occur. If you have tried all the other methods to resolve the idling issue and it has not been resolved, then this may be an alternative you can explore.
Take your dirt bike to a professional and get them to perform a vacuum test on the air supply system and see if they can locate an air leak!
If your dirt bike is experiencing the problem of idling only while the choke is on, the positive outlook is that the problem should be pretty straight forward to diagnose and repair.
While the problem is not necessarily going to do damage to your bike, you will want to get it sorted as soon as possible. The problem, if left unchecked, will cause an increase in fuel usage and a drop in the performance of your bike!
Happy dirt biking!