If your motorcycle only runs when the choke is on, don’t despair. Although there are various reasons why a motorcycle will only run on the choke, it is not that hard to diagnose and fix it yourself.
If your motorcycle only runs with the choke on, the air-fuel mixture is too lean. A vacuum leak on the carb body or between the carb and the engine may result in a lean condition causing the bike to only run with the choke on. A dirty pilot jet from old fuel can also cause the bike to die when you switch off the choke.
Like any motorcycle troubleshooting exercise, you need to first take a deep breath and then follow a methodical approach. We’ll get your motorcycling running again without the choke in no time.
Why Does Your Motorcycle Only Run With the Choke on?
If you motorcycle only runs with the choke on, but stalls or runs uneven without the choke, it is due to the air-fuel mixture in the carburetor being too lean.
Your motorcycle engine needs an exact air-fuel mixture (the stoichiometric mixture of 14.7 grams of air for every 1 gram of gas) to run properly. This cocktail is mixed by your motorcycle’s carburetor. During cold starting, the engine requires a bit more fuel (i.e. a richer air-fuel mix) than usual and that is the job of the choke.
On some motorcycles, the function of the choke is to choke (haha) the air supply and thereby increasing the relative amount fuel compared to air going to the engine intake manifold, thereby richening the air-fuel mixture. On most dirt bikes, however, the choke operates by squirting more gas into the mix.
Either way, with the choke in operation (or in the ‘on’ position), the air-fuel mixture will be richer than normal (i.e. when it is ‘off’).
Okay, so why does my bike only run on the choke???
Now that we know what the choke does, it should be easier to understand that when the motorcycle only runs with the choke on, the normal air-fuel mixture is too lean. So, by running with the choke on, you are making the air-fuel mixture richer and in line with the required normal air-fuel ratio.
In short, your motorcycle only runs when the choke is on because the carburetor is too lean. This may cause your bike to backfire when you decelerate. To find out why, check out this post I wrote.
If, on the other hand, your bike idles fine but then stalls as soon as you give it gas, read this post to solve the issue.
Why Does my Motorcycle Carb Run Too Lean?
If your bike ran fine and now all of a sudden it will only run with the choke on, something happened that caused your carb to go out of adjustment and become too lean. The two most obvious reasons for a carb to start running lean are clogged carb jets or a vacuum leak (or both!).
If your motorcycle carburetor’s jets are clogged (particularly the pilot jet), enough gas won’t be able pass through the tiny jet hole to achieve the correct air-fuel ratio. The reduced flow of fuel will cause the air-fuel mixture to be too lean. By using the choke in this condition, more fuel is added to correct the air-fuel mixture.
If there is an air leak somewhere on the carburetor body (like the float bowl) or between the carb and the engine intake manifold, air will be sucked in and mixed with the air-fuel mix supplied by the carburetor. This will lean out the air-fuel mixture resulting in the situation where the bike will run better with the choke on instead of off.
If your bike ran fine before and suddenly won’t run without the choke on, then it is most likely due to a dirty jet or a vacuum leak.
If, on the other hand, you just bought a second-hand bike and it won’t run without the choke on, there could be another reason (in addition the ones already discussed). The carburetor may be incorrectly tuned or jetted.
The sizes of the small holes in your carb’s pilot and main jets will determine the air-fuel mixture at a certain altitude. The altitude matters, as there is more air (or oxygen specifically) at the coast compared to higher altitudes because the air is denser. This means that at sea level, you need bigger pilot and main jets to compensate for the larger volume of air sucked into the carb.
At higher altitudes, your motorcycle will run rich if you don’t change out the jets because there is less air the higher up you go. Less air, means less fuel is required to obtain the correct air-fuel mixture.
On fuel injected motorcycles, the air-fuel mixture is adjusted automatically as the air density changes. On a motorcycle with a carburetor, you need to re-jet it whenever you intend to ride at a much higher (or lower) altitude for an extended period of time.
So… if you are a surfer that live on the beach (at sea level), and you just bought a motorcycle from a mountain goat (at 8 000 ft above sea level), the bike will most likely run lean. The only way to fix this is to re-jet it with a bigger pilot and main jet.
Check out the video below for more on my own jetting woes on my Honda XR 650 L that was too lean when I got it.
How to Diagnose and Fix a Dirty Motorcycle Carburetor
A very common cause of a lean condition that will result in your motorcycle only running with the choke on is dirt in the carburetor. If the pilot jet clogs up due to gunk in the carb float bowl, your bike may not be getting enough fuel through the pilot jet hole and die when you switch off the choke.
Another indicator that a blocked jet may be your problem is when there is no rise or fall in the engine rpms when you adjust the air-fuel mixture screw. On most carburetors, there are two screws visible on the side. The larger one is usually the idle speed adjustment screw. Leave that one alone. The other (generally smaller) screw is the mixture screw.
Turn the mixture screw in or out half a turn at a time. Keep count so that you can reset it afterward. If it makes no notable difference after a full turn, a clogged jet is more than likely the issue.
Dirt can enter the carburetor in various ways. If your bike stood unused for a long period of time (e.g. over winter) and you didn’t drain the carb, old fuel can gum up the carburetor float bowl on the inside. When you fill it up with new fuel, the dirty film can dislodge and be sucked into the pilot jet blocking it.
A tear in the air filter is a serious concern. Dust and dirt will enter the carb through the air intake and quickly block all the carb jets and passages. Dirty fuel is another problem. On many older motorcycles (the ones that run on carbs), there are no inline fuel filters. Instead, the gas tank petcock valve has a mesh strainer. This can still allow smaller particles to get through and block the tiny jet holes.
The first thing I always do on a ‘new-to-me’ bike is install an inline fuel filter between the gas tank and the carburetor.
How to clean your carburetor yourself
There are more than enough videos on YouTube on how to clean a carburetor yourself. My only advice is to make sure your working surface and tools are 100% dust free and clean. Any small particle that enters the float bowl has the potential to block a tiny pilot jet.
Remove the float bowl (bottom side of the carb) and be careful to not strip the screws. I’ve replaced mine with hex head screws to be sure. Carefully pry off the float bowl cover without tearing the gasket. If you do damage the gasket, you’ll need a new one. You can usually easily find a complete carb kit for your motorcycle model.
If you don’t replace a torn float bowl gasket, you’ll definitely have an air leak afterward.
Inside, you’ll see two jets. A main jet in the middle with a bigger hole and a pilot jet to the side (maybe under a plastic baffle) with a much smaller hole. Unscrew both and generously spray the holes in the carb body and the rest of the inside clear with carb cleaner.
Next, spray the holes in the jets with carb cleaner and compressed air until they are clean. You should be able to see the light shining through them when they are clean.
Carefully (without getting them dirty again) replace the jets and close up the carb. If you worked carefully, your carb should run much better now.
The correct way to clean a carb is to disassemble the whole carb and remove all the little parts, and then put the whole lot in an ultra sonic cleaning machine. But who has one of those standing around? The method I described above works well enough.
It may seem daunting, and at first it is, but it is not rocket science. You can do it!
How to Diagnose and Fix a Vacuum Leak on Your Motorcycle
If you suspect your carburetor may have a vacuum leak somewhere, here’s how you can go about finding it. Visually inspect the carb to engine intake boot for cracks. My Yamaha AG200 had a tear in the intake boot after I tried to remove the carb and it would not idle.
Also check that all the little vacuum hoses that go into the carb body are in place. If one has come loose, replace it. If there is a hole in the carb where you suspect a hose is missing from, plug the hole to prevent air from being sucked in.
If you can’t find any obvious damage, take a can of carb cleaner (like this off Amazon) and use the little plastic straw on the nozzle to spray around the carb boot and body while the engine is running. If there is a hole where air is being sucked in, the carb cleaner will cause the engine speed to rise slightly as the carb cleaner is sucked in and burnt inside the combustion chamber.
NOTE: You need to be looking for air being sucked in somewhere between the carb and the engine. There is no use inspecting the area before the carb (on the air-cleaner side).
If you find that the carb boot is torn, the best is to replace it. You can try fixing it was gasket maker as a temporary fix (I did and it runs fine) while you are hunting for the correct replacement part.
Finally, and I am embarrassed to even tell you this, check that you tightened the screws on your carb’s float bowl after re-jetting or cleaning it. I somehow forgot this and my Honda XR 650 L would not idle. I am still amazed that it even started.
In short, if air is getting into the system between your carburetor and engine, your motorcycle will not run well.
How to Prevent Your Motorcycle From Running Lean
Prevention is always better than cure. If your bike is running well, here are a few things you may want to consider to keep it that way.
Clean carburetor and fuel system
It is very important that your carburetor stays squeaky clean inside. Any dirt will cause running problems. The best prevention is to install an inline fuel filter like this off Amazon between the gas tank and the carburetor.
Also, ensure that your air filter is regularly cleaned if it is a foam type, or replaced if it is a paper element. If your air filter is damaged in any way, replace it immediately.
Never run the bike without an air cleaner installed!
During winter months or any extended period of disuse, drain the carburetor of any fuel. Modern fuels do not age well and will soon gum up the inside of your clean carb. To drain the carb float bowl, close the petcock and run the engine until it dies by itself. Alternatively, close the petcock and unscrew the float bowl drain plug (mine is stripped!).
You can also add a fuel stabilizer to the gas tank before storage to ensure the fuel stays fresh.
Rubber boots and vacuum lines
The rubber carb to intake boots should last a very long time, but with age (and a lot of direct sunlight), they may become worn out. If you need to remove your carburetor for whatever reason, be very careful not to bend the intake boot too far.
I tore the intake boot on my Yamaha AG200 when I removed the carb after drowning the bike. We were nearly stranded in the middle of nowhere. Luckily, we managed to push the torn section shut by moving the airbox forward against the carb body.
Replace any old vacuum lines with new ones. They are cheap and easy to find.
Unless you change altitude by more than 3 000 feet, you should not need to re-jet your carb. If, however, you are fitting an aftermarket free flow exhaust pipe, you will need to increase the pilot and main jets. A free flowing exhaust will allow more air to flow through the intake and thereby lean out the air-fuel mixture.
If you don’t re-jet the carb after fitting a performance pipe, the bike will most likely run lean. Apart from not idling properly, a lean running engine may run hotter than normal which is even worse, especially on air-cooled motors.
Is it Okay to Run my Motorcycle With the Choke on?
Running your motorcycle with the choke on for a short period won’t hurt the engine but it may foul the spark plugs if the air-fuel mixture is too rich. Riding with the choke on for an extended period of time is a bad idea. The solvents from the resulting rich mixture may end up wearing out the cylinder walls which could cause the engine to burn oil.
This happened to my 1991 Toyota Corolla. The car had very low mileage (only 83 000 miles) but it burned a gallon of oil in only 3 000 miles. The cause? I didn’t notice, when I bought the car, that the choke was stuck in the ‘on’ position. I only realized this after a few months of owning the car.
My mechanic said the rich mixture most likely wore out the inside of the cylinders causing oil to seep past the piston rings. The only cure? An expensive engine rebuild!
Incidentally, how I learned that the choke was on all the time was because I noticed that there is no change in engine rpms when I pull out or push back the choke knob.
Lesson learned: Don’t ignore the choke! Make sure it is in good working condition. And when it is the only thing that makes your bike run good, fix the underlying condition.
While it may be annoying if your motorcycle only runs with the choke on, it will most probably be something that is relatively easy to fix.
Take a deep breath and a methodical approach to diagnose the problem, and then fix it yourself.