One of the most annoying things that often happened on my first motorcycle, and old Yamaha XT500, was that it would stall when I give it gas. It would idle perfectly after starting the motor, but as soon as I cracked open the throttle the bike would die.
A motorcycle will stall when you give is gas when the air/fuel mixture is too rich or way too lean. A dirty air filter, water in the fuel tank, clogged carburetor jets, slack throttle cable, or torn carb slide diaphragm may also cause a motorcycle to die when the throttle is opened.
While it can be frustrating to diagnose a motorcycle that stalls when giving it gas, taking a deep breath and a methodical approach may help you solve the problem yourself. Let’s have a look at 10 possible reasons why your motorcycle stalls when you give it gas and how to fix each one.
#1 – Dirty Air Filter
If your bike was running fine and suddenly started stalling when you give it gas, the first thing you need to check is your air filter. A dirty air filter can become so clogged up that it barely allows clean air to pass through to the carburetor or intake manifold.
Your bike may start and idle fine because only a small amount of airflow is necessary at closed throttle. Then, as soon as you crack open the throttle, demanding more fuel and air, the bike dies.
If you have a paper air filter, a quick blow out with compressed air should help get you going again. If you have the foam type, you’ll have to thoroughly clean and re-oil it. It is important to regularly clean (or replace if necessary) your air filter, especially if you often ride in the dirt.
#2 – Clogged Jets
If dirt gets into your carburetor, it could easily clog up the very tiny holes of the pilot or main jets. If you’ve recently opened the carburetor or your air filter is damaged, this could very likely be your problem.
There are two jets, the pilot jet and the main jet. If either or both of these jets are partially clogged, the bike could still idle, but may stall when you demand more fuel by opening the throttle.
The pilot jet supplies fuel at small throttle openings and at idle (closed throttle). If the bike starts and idles, but dies as soon as you touch the throttle, it could be the pilot jet that is partially clogged. If the bike pulls away at a small throttle opening, but splutters or dies when you give it full throttle, the main jet is most likely your culprit.
Your only solution is to clean the carburetor properly. In a pinch, you could try spraying some carb cleaner into the air intake (by removing the air filter). A better solution is to drain the carb float bowl by removing the drain screw, and then filling the float bowl with carb cleaner by removing the fuel supply line. You can buy carb clearer for cheap on Amazon here.
Start the bike without touching the throttle and let it idle. This will suck in carb cleaner through the idle jet. After a minute or two, rev the bike to run some carb cleaner through the main jet as well.
If this helped, great. If not, you need to remove the carburetor from the bike, take off the float bowl, and unscrew the main and pilot jets. Blast each jet clear with carb cleaner and compressed air. Don’t thread any wire through the jet holes as you will easily damage them.
Make sure the inside of the carb is squeaky clean before putting it back on the bike.
#3 – Vacuum Leak
If air is leaking into the carburetor or anywhere between the carb and the engine, the air-fuel mixture will not be correct. The carburetor’s job is to mix the exact amount of fuel and air for combustion in the engine.
A vacuum leak will suck in air and mix it with the air-fuel mixture to create a much leaner mixture that won’t ignite properly. The more throttle you give, the more air will be sucked into the system.
Check for torn intake boots between the carb and the engine intake manifold and make sure the clamps are not loose. Ensure that all the vacuum hoses that are attached to the carburetor body is in good condition and not dislodged. If there is a vacuum connection hole on the carb without a hose going into it, you can plug this hole and test it again.
It is also possible for the carburetor to suck in air through a damaged gasket. If you’ve recently opened the carb, make sure the float bowl gasket is in good condition and properly in its place. Rather replace it if you are unsure.
Finally, make sure you tighten (but not too tight!) the float bowl screws. Yes, this is unlikely to be your issue, but I am mentioning it because I once forgot (yes, forgot!!) to tighten the screws after re-jetting the carb on my Honda XR650L. One screw rattled out and the other three were close behind. It is a miracle that the thing even start and ran (sort of).
#4 – Torn Diaphragm in a CV Carb
Motorcycles with constant velocity (CV) carburetors have a diaphragm inside the carb that lifts a needle that rides inside the needle jet. If this diaphragm is torn or damaged, air will leak through it and it won’t lift to expose the jet hole. As a result, fuel won’t be sucked into the intake manifold and the bike will run way too lean.
The diaphragm is quite sensitive and can easily be damaged when the carburetor is opened. To check the diaphragm, carefully remove the top of the carburetor and slowly lift out the diaphragm inside. Hold it up to the light and check for any holes or tears. If the diaphragm is good, make sure it sits neatly inside the groove around to top of the carb. If it is not properly seated, it could also get damaged.
#5 – Water in the Float Bowl
I am not 100% sure why water in the float bowl causes your bike to stall when you give it gas, but it does. I remember my first bike, a 1981 Yamaha XT500 that was fitted with an aftermarket 30 liter gas tank used to die upon take-off after sitting in the rain for a day.
Water somehow got into the gas tank through the filler neck which didn’t seal properly. Since water is heavier than fuel, it is the first to leave the tank and settle in the carb float bowl where it sinks to the bottom again. The bike would start okay and idle, but as soon as I opened the throttle, it spluttered and stalled.
All I can think of is that the idle circuit does not demand enough gas to suck up any of the water at the bottom, but as soon as the more fuel is required, there’s not enough of it because of the water.
It was easy to fix. Simply remove the drain screw at the bottom of the float bowl and drain the water at the bottom. It worked every time!
#6 – Stuck Choke
If your choke is stuck in the closed position, the air-fuel mixture will be too rich and the motorcycle may stall when you roll on the throttle. Check that you’ve opened the choke (switched it off) completely. If moving the choke lever does not make a difference to your engine speed at idle, the cable might be broken or completely out of adjustment.
If your bike idles fine and only stalls once you are trying to pull away, it is less likely to be a stuck choke. If the choke is stuck, it will likely also not idle if the bike is already at operating temperate. It is worth a check, however.
Generally, if the bike runs better when you use the choke, the air-fuel mixture is too lean. And if it dies when closing (switching on) the choke, it is too rich.
#7 – Dirty fuel filter
If your fuel filter is clogged up enough to restrict the flow of gas, there may be enough to let the engine idle fine, but too little to pull away. A fuel filter can get clogged due to dirty fuel or rust in the gas tank. Replace the fuel filter with a new one and you should be good to go.
Many motorcycles with carburetors do not have inline fuel filters. Instead, they have mesh strainers in the gas tank petcock. You can clean out the strainer by unscrewing the petcock. It is a good idea to install an inline fuel filter between the gas tank and the carburetor if your bike doesn’t have one. It is always the first thing I do when I buy a bike.
Fuel injected motorcycles usually do not have an inline fuel filter. Instead, the fuel filter sits inside the gas tank or in the fuel pump. If you want to install an extra inline filter, make sure it is rated for the higher pressure of fuel injected systems.
#8 – Gas cap vent clogged
If the vent in your gas tank filler cap is clogged, it will create a vacuum in the fuel system which will prevent gas to freely flow out of the tank. The air vent allows air from outside to enter the tank and equalize the pressure as gas is sucked to the carburetor (or pumped to the fuel injectors).
Visually inspect the vent hole in the filler cap and unclog it with a piece of wire if possible. If this doesn’t work, you’ll need to replace the gas tank cap.
#9 – Slack throttle cable?
If your throttle cable has way too much slack in it, it won’t properly open the butterfly valve in the carb throat and therefore won’t supply enough fuel and air when pulling away. While this won’t necessarily stall the engine, it will feel like your bike is losing power when you accelerate.
The bike should still start and idle fine because the pilot jet is supplying fuel while the throttle is closed. If you let out the clutch too quickly, the bike may stall due to not enough fuel being delivered to overcome the weight of the bike.
Adjust the throttle cable to remove the excess slack. There should be only about 1-2 mm of free play at the throttle tube. Some play is necessary. If the throttle cable has no slack at all it will pull on the butterfly valve all the time, thereby supplying more fuel than is necessary and you won’t be able to set the idle speed properly.
#10 – Timing advance not working?
If the timing of your motorcycle’s ignition system is out the spark plugs won’t fire at the right time. This will cause your bike to not run properly. If the timing advance is not working as it should, you may lose power or stall the engine when opening the throttle.
Most motorcycles since the 1970’s have electronic ignition timing that is regulated by a CDI (Capacitor Discharge Ignition). If the CDI fails it could cause a whole series of symptoms from intermittent starting problems to running poorly at high revs. If you’ve checked all the other issues above and suspect your CDI is at fault, you’ll have to take it to a mechanic to test it.
The best way to rule out a faulty CDI is to replace it with a known good one. If that fixes the problems, you need a new CDI.
A bike that idles good but stalls when you open the throttle can be very annoying but it is usually something simple to correct. Don’t get discouraged. Take a deep breath, take a methodical approach to rule out the culprit, and fix the problem yourself.