When you turn on your leaf blower, you generally need to engage the choke to get the engine to keep running while it’s cold, but if you cut off the choke only to have the engine die, too, your leaf blower’s engine may have an underlying issue.
Why does my leaf blower only run on choke? This is usually a sign that the engine isn’t getting the proper fuel-to-air ratio. Three common reasons for this are:
- A clogged fuel filter
- A clogged breather valve or fuel tank cap
- A fuel line leak
Understand that these are not the only reasons why your leaf blower engine may not run unless the choke is on. There can be a number of underlying issues but these are common causes and each of them has a fairly simple solution.
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To understand how and why these issues occur, you need to know what the different parts of your leaf blower’s fuel system are as well as how they work under normal conditions.
The Parts Of A Leaf Blower’s Fuel System
When fuel leaves the fuel tank of your leaf blower, it passes through the fuel filter before entering the carburetor and then eventually engine and its combustion chamber. Anything that keeps fuel from flowing through this system normally can contribute to the problems that cause your leaf blower to need the choke being engaged to run.
The tubing that carries the fuel from the fuel tank and the carburetor is called the pulse line. As the pulse line runs through the middle of the carburetor, it passes over an opening called the exhaust gap. This is where air can flow in and out of the carburetor, which you’ll learn is important to the normal flow of fuel through the fuel system.
The exhaust gap’s opening is controlled by the choke butterfly. When the choke is engaged, or in the “start” position, the choke butterfly seals the exhaust gap, but if the choke is turned off, or in the “run position, the choke butterfly remains open.
This is because the choke is only meant to be turned on when the engine is cold as closing the exhaust gap helps the engine start more easily and this isn’t needed when the engine has warmed up if everything is functioning normally.
How the Fuel System Normally Operates
When the leaf blower’s engine is started, the pistons within the engine start to lower and pressure is transferred from the engine compartment through the carburetor, which pushes down on the fuel pump diaphragm on the pulse line.
As the pistons begin to cycle up and down, these movements physically draw fuel through the carburetor’s fuel line as the engine pistons create an air vacuum (source).
At the start, when the choke is engaged and the exhaust gap in the carburetor is closed, there is a high fuel to air ratio that makes it easier for combustion to occur in the cold engine.
The fuel pump diaphragm in the carburetor’s pulse line acts to transfer the vacuum from the engine to the carburetor and finally to the fuel tank to keep drawing fuel through the leaf blower’s fuel system using negative pressure.
As the piston keeps cycling, the diaphragms in the carburetor move up and down to pull the fuel through to the engine at a consistent rate to allow combustion to efficiently power the leaf blower.
The compression of the engine’s pistons allows fuel and air to flow through the carburetor in specific ratios for each cycle (source). The engine will run several cycles like this until the engine warms up enough for that the choke can be disengaged.
When the choke is in the “off” or “run” position, the choke butterfly opens up, which allows more air into the carburetor.
Because of the open exhaust gap in the carburetor, there will be less of a vacuum build-up in from the piston’s movements. That means less fuel will be pulled into the carburetor.
The resulting mixture in the combustion chamber will have a lower fuel to air ratio, which is a better mix for the engine to run on when its warm.
To really get a good understanding of the combustion process of a 2-stroke yard tool and some great troubleshooting tips, watch this video from eReplacementParts:
When the Engine Only Runs on Choke
The reason it’s important to turn off the choke once the engine is warm is that a lower fuel to air ratio allows for more efficient use of gasoline. If anything disturbs the engine’s ability to get the right fuel to air ratio, then your leaf blower will need the choke
Because of how many working parts there are to the fuel system, the reason that the engine is not getting enough fuel can come from any point in the fuel’s journey from the fuel tank to the combustion chamber (source).
Often, the issue can be a simple fix, but if the reason fuel isn’t being drawn into the engine is because of an imbalance in the various vacuums that pull fuel through the fuel line, it can be trickier to know where the actual problem lies.
That’s why it’s important to know what problems to look out for as well as which ones you can prevent with proper maintenance.
Reason 1: A Clogged Fuel Filter
If the fuel filter is partially clogged with debris or dirt, the fuel filtration rate into the fuel line and carburetor will be limited. That will cause the amount of fuel in the air-to-fuel ratio to be too low.
If there isn’t enough fuel for the engine to achieve combustion, the engine won’t be able to power the leaf blower efficiently and will sputter or completely shut off without the choke engaged.
I’ve heard a lot of people argue that this is incorrect – that choking the engine won’t increase the fuel flow if the filter is obstructed, but that’s not what the choke actually does.
Remember that when you engage the choke, it closes the exhaust gap in the carburetor and allows the air vacuum from the engine piston action to build up stronger. This creates a bigger pull on the fuel coming through the fuel tank, which can allow enough fuel to be drawn through the clogged filter for the engine to run. That is, as long as you leave the choke on.
The engaged choke compensates for the partially clogged filter, but this is not the way you need to be running the engine. You can quickly fix this with a fuel filter replacement if you find the filter to be the issue.
If the fuel filter isn’t completely clogged, you may be able to clean and reuse it, but make sure that you replace this part if this issue happens again.
Reason 2: A Blocked Breather Valve Or Fuel Cap
At the top of the fuel tank, the opening where you pour in fuel has a cap that usually includes a breather valve that allows air to fill the tank as it empties of fuel. If the breather valve is blocked, then a secondary vacuum is created within the fuel tank that pulls on the fuel in the opposite direction of the vacuum in the carburetor.
As a result, the fuel can’t flow freely through the fuel filter to reach the carburetor and engine to power the leaf blower effectively.
Total blockage of the breather valve won’t allow the leaf blower to run, even is the choke is engaging, but a partial blockage will because only a small vacuum will form in the fuel tank, limiting but not starving the engine’s fuel intake.
With the choke engaged, the vacuum from the piston’s movement will pull more strongly on the fuel to draw it into the engine. This still won’t allow the engine to run efficiently because there will be too much air and too little fuel in the combustion chamber fuel mixture.
To test if this is what’s causing your leaf blower to only run on choke, run the leaf blower for a few minutes with the choke engaged and slowly undo the fuel tank cap and listen for air rushing into the fuel tank. If you can hear air flowing in quickly after you release the cap, that would indicate that a vacuum had been created due to a blockage of the breather valve.
Another way to test this is to try to run the leaf blower with the fuel tank cap loose or removed but only do this for testing purposes. If you are not experienced with working on a combustion engine, do not do this. Take it to a professional for servicing.
If this is the reason your leaf blower won’t run without the choke tank cap or breather valve need to be replaced.
Reason 3: A Fuel Line Leak
Another cause could be that there is a leak in the fuel line that is limiting the amount of fuel that reaches the combustion chamber. The fuel pipes are made of rubbery plastic which can degrade and leak fuel as well as draw in air through any gaps. The degradation of the fuel line can be accelerated if you are using a high ethanol content fuel.
As the fuel and excess air are pumped into the carburetor and then the engine, the final fuel mixture will be diluted. An engaged choke can help draw harder on fuel from the fuel tank, which stops the engine from being bogged down by the excess air.
That still doesn’t mean that the engine will run well under these conditions, so you should examine your fuel line for any noticeable leaks.
If a leak is identified, do not use the leaf blower until you replace the line or have it professionally serviced. Always use common sense when working with a gas-powered engine.
Other Potential Causes Of A Leaf Blower Only Running On Choke
Remember that issues with the air-to-fuel ratio is usually what causes a 2-stroke engine to only run with the choke engaged. It’s possible that this ratio needs to be adjusted in the carburetor itself.
Depending on the model of your leaf blower, there will be screws on the carburetor that allow you to fine-tune the air-to-fuel ratio.
Steve’s Small Engine Saloon has an excellent video demonstration on this process. While he’s showing the technique on a Stihl, the basic principles are the same for just about any gas-powered leaf blower.
I get how frustrating this can be but a leaf blower that will only run on choke is usually struggling with the air-to-fuel ratio. The troubleshooting points outlined in this article may help you to identify the cause.
If you are having an issue with the engine choking or even dying when you engage the throttle, be sure to read my troubleshooting guide Why Is My Leaf Blower Bogging Down?