Lawn Mower Automatic Choke Problems (Diagnose + Fix)

It seems that as lawn mowers become more advanced, they become more confusing. My old lawn mowers used to have a choke on the throttle lever and a primer, and now they have only automatic chokes. Working the choke lever or giving the primer an extra squirt seem to be things of the past. So, what do you do when you’re having automatic choke problems? Well, let’s take a closer look at the mechanism and figure out how to fix your automatic choke.

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An Overview of Lawn Mower Automatic Choke Problems (The Short Explanation)

When a lawn mower is cold, the choke is held open/on by usually a spring-loaded mechanism. Then, as the lawn mower starts/heats up, the auto-choke is closed by either the airflow from the flywheel or heat from the exhaust pipe. A defective part, poor adjustment, or dirt in the mechanism can cause automatic choke problems.

4 Lawn Mower Automatic Choke Problems + How to Diagnose Them

Automatic chokes come in many different designs, but they all seem to work pretty much the same. There are a couple of jobs that automatic chokes need to take care of. First, the choke needs to choke the lawn mower when it is cold and then stop choking when the lawn mower warms up. Second, the lawn mower needs to be able to keep the choke off when the lawn mower is started hot.

So, you could either have a problem starting the lawn mower or keeping the lawn mower running if the automatic choke is sticking in either the ON or OFF positions.

Here are the parts you will need to inspect to diagnose automatic choke problems.

Air Vane
Air Vane Spring
Thermostatic Cut Off
Connection Bars

To inspect the automatic choke, it’s usually easier to remove the engine cover so you can get a good look at the choke mechanism. Normally you’ll find delicate parts like the carburetor, choke, and exhaust hidden under the cover. So, grab a socket wrench or screwdriver and pop off the cover.

Inspecting the Air Vane

The air vane is a long plastic part that sits up close to the flywheel. Once the flywheel spins, the air vane catches the airflow and closes the choke. The air vane should be free of any damage and be securely attached to the engine.

If you move the air vane back and forth, it should move very easily. If, for any reason, there is resistance on the air vane, you’ll need to carry out a fix. Also, this is probably why the automatic choke is sticking.

Testing the Air Vane Spring

To return the air vane back to the ON position, the lawn mower uses a spring. This spring is connected to both the vane and the body of the lawn mower.

So, the test you want to do is to check that the spring is able to pull the air vane back into the ON position. If the spring is damaged or stretched out, it is probably not going to work correctly.

Also, it’s common for the spring to be completely missing. So, if the spring isn’t working as it should be, then you’ll need to fix it.

Diagnosing the Thermostatic Cut-Off

As I mentioned before, there is a thermostatic cut-off for the automatic choke that is connected to the exhaust pipe. Once the exhaust pipe gets hot, the thermostat extends a bar to switch off the choke.

This is a test that is best performed when the lawn mower is warming up. So, if you can, start the engine and observe the thermostat bar. You want to watch how the bar moves and make sure that it engages the air vane. The bar should move towards the air vane, then push it back to the OFF position. If it’s not moving the air vane as it should, then you will need to repair it.

Connection Bars

More often than not, the carburetor is on one side of the lawn mower, and the exhaust is on the other. To connect the carburetor to the thermostat cut-off, some lawn mowers use a connecting bar. You want to make sure that the bar is in place and that it’s not damaged.

I have had cases where the bar gets bent and prevents the choke from turning off. So, take a look at the mechanism and the bar and see how it operates.

Solutions to Problems with a Lawn Mower’s Automatic Choke

By now, you should have a better understanding of how your automatic choke works and how it can go wrong. Now it’s time for the repairs. I’m pretty sure that your lawn mower’s automatic choke problems will be gone in no time. Let’s take a look.

Air Vane Repair

The first part of this fix is to remove the choke spring. I find that you can pretty easily remove the spring from its mounting brackets by using a pair of pliers. Once you have the spring off, take a socket wrench and remove the shoulder bolt from the air vane. Usually, this is the only bolt that holds the air vane to the lawn mower.

Next, take your new air vane and place it onto the lawn mower. You will want to make sure that you position the new air vane correctly on the choke arm. I suggest that you take a minute or two to see how the air vane connects to the carburetor before you remove it. This will make it easier when you place the new air vane on the mower.

Next, grab the shoulder bolt and reinstall it back onto the lawn mower, making sure it’s nice and snug. With the new air vane installed, check to make sure that it moves easily. You want to make sure you didn’t over-tighten the shoulder bolt. Finally, take your pliers and reinstall the air vane spring back onto the mower.

Tools & Parts Required to Replace the Air Vane

Replacement Air Vane
Pliers
Socket Wrench Set

Replacing the Automatic Choke Spring

Well, there’s not much to say here as you already removed the spring. So, a new spring and a pair of pliers are all that’s needed. The only extra thing is to double-check the mounting points to make sure they are in good shape.

Tools & Parts Required to Replace Choke Spring

Pliers
Choke Spring

Adjusting the Thermostatic Cut-Off

Nine times out of ten, the thermostatic cut-off bar is bent, so you’ll need to make an adjustment. Now, this really needs to be done when the lawn mower is both cold and hot. So, when the mower is cold, make sure the bar isn’t making any adjustment on either the connection bar or the air vane. You want to make sure no adjustment to the lawn mower’s automatic choke is being made when it is cold.

Then, warm the mower up and make sure the bar completely switches off the choke. To make your adjustment, I normally use a pair of pliers to bend the bar. This is a bit of a back-and-forth fix that can take a few attempts, but if you are patient, you’ll get it fixed.

Tools & Parts Required to Adjust the Thermostatic Cut off

Pliers

Repairing the Connection Bar

If your connection bar looks a bit bent out of shape, then you are going to want to grab a pair of pliers, or maybe just using your hands will work. Now, you want to check how the connection is working. It would usually be a case that the mechanism isn’t moving far enough as the bent bar doesn’t travel as far. So, try your best to reshape the bar to restore the required travel length.

Again, this fix is a bit of a hit-and-miss. It’s going to take a few tries to get it just right. If it’s proving to be too difficult, you might want to think about replacing it. But I’ve always found I can repair this issue with a bit of patience.