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Toro Mower Only Runs With the Choke On (FIX IT NOW)

You were able to start your mower, but this time when you adjust the choke to the open position to keep the engine running once it warms, it sputters and dies. You can only keep the engine running with the choke on.

When looking for the reason a Toro mower only runs with the choke on, identify items or areas that allow the engine to get too much air or not enough fuel.

This may be due to a dirty carburetor, old gas; plugged fuel filter, faulty fuel pump, bad gas cap, clogged or punctured fuel line, or bad carburetor gasket.

Refer to your Toro operator’s manual for all safety precautions. This includes removing the ignition key and the spark plug boot prior to making repairs.

Reasons your Toro lawn mower only runs with the choke on:

  • Lack of fuel
    • Old gas
    • Dirty carburetor
    • Clogged fuel line
    • Bad fuel pump
    • Plugged fuel filter
    • Bad fuel cap
  • Too much air
    • Bad carburetor gasket
    • Puncture in the fuel line
Toro engine only runs with the choke on

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Follow all safety instructions provided in your equipment operator’s manual before diagnosing, repairing, or operating. Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.

7 Reasons a Toro Mower Only Runs With the Choke On

1. Old Gas

Gas doesn’t stay fresh for long. It can begin to break down as soon as about a month after you buy it. The effects of old gas will gum the fuel system and corrode components.

Most gas sold today includes an alternative fuel called ethanol. This product is better for the environment but it is not good for small engines like the one used on a Toro.

Ethanol attracts moisture from the air to the fuel. Ethanol and water will separate from the gasoline and sink to the bottom of the tank.

It leaves behind sticky deposits when it evaporates and the water is corrosive and can damage fuel components.

Fuel components that don’t work right or clogs that develop in the fuel system can keep the engine from getting a good amount of fuel. In order to keep the engine running, you may have to use the choke to correct the gas-to-air ratio required for combustion.

Toro mowers require unleaded gasoline with a minimum 87-octane rating and a maximum 10% ethanol content. Never use gas that contains more than 10% ethanol.

Read more about selecting the right gas for your Toro mower here.

SOLUTION: Drain old gas from the fuel tank. Add fresh gas with a fuel additive like Sea Foam Motor Treatment or STA-BIL mixed in to help clean and reduce moisture in the fuel system.

It will also keep the gas stable so it lasts a little longer before it breaks down. Check out this article for why I choose to use Sea Foam in every small engine I own.

2. Dirty Carburetor

The carburetor is the part that regulates the amount of gas that is mixed with air to form combustion. This is the place a little gas is stored after it leaves the fuel tank.

Old gas is often the reason a Toro carburetor fails. It coats the inside of the carburetor clogging fuel passageways and causing internal carburetor parts to stick.

The fuel restriction can require you to need to use the choke to correct the ratio of fuel to air to keep the engine running.

The carburetor will need to be cleaned or replaced so the engine can get the right amount of fuel to run well.

SOLUTION: If you are a little mechanical and don’t mind working with small parts, you should be able to remove and clean the Toro mower carburetor yourself following these instructions.

If the carburetor doesn’t work right after you clean it using these steps, you may have to rebuild it or replace it. Have a small engine mechanic clean or rebuild the carburetor if you don’t want to tackle the job.

3. Puncture or Clog in the Fuel Line

A puncture in the fuel line can cause air to be sucked into the line so the engine receives too much air. Or, a clog can develop in the line that restricts the amount of fuel the engine receives.

Either one of these issues may cause your mower to only run when the choke is on. The choke needs to be used to adjust the air getting to the engine to control the right fuel-to-air ratio needed for combustion.

SOLUTION: Inspect the fuel lines. Follow the line coming out of the fuel line and follow it up to the carburetor. Look for punctures or cracks that can introduce air to the fuel system.

Next, check for a clog in the fuel line that can restrict fuel. First, shut off the fuel flow using the fuel shut-off valve. Then remove the end of a section of the fuel line and place it in a container to collect fuel.

Turn the fuel flow back on and watch for good flow coming out of the fuel line and into the container. If you are not getting good flow, shut off the fuel supply and remove the fuel line from your mower.

Spray carburetor cleaner into the line to loosen the clog. Follow this with compressed air to remove the clog. Repeat as necessary.

If you just can’t get the fuel line clean and the clog removed or you find a puncture, replace it with a new section of the fuel line with the same diameter.

4. Plugged Fuel Filter

Another area you can experience a fuel restriction is with the fuel filter that is installed between the fuel lines. This filter is required to keep dirt from entering the fuel system and wearing on the engine.

When the filter isn’t regularly replaced to ensure you keep the filter on your Toro in good condition, it can become plugged. This will keep a good flow of fuel from getting to the carburetor.

This is a part that should be replaced annually to keep it in good condition. When it becomes plugged, the amount of fuel that is able to pass through the filter may be restricted.

SOLUTION: Remove a plugged filter and insert a new inline fuel filter between the fuel lines.

You should find an arrow on the side of the filter housing. The filter must be inserted so the arrow is placed in the direction of the fuel flow.

5. Faulty Fuel Pump

A fuel pump is used on your Toro mower to move fuel uphill to the carburetor. Most mowers use a vacuum pump. This type of pump uses the vacuum off the engine to move fuel from the fuel tank to the carburetor.

To check the fuel pump, you need to confirm the pump is getting fuel to the inlet port. Then remove the fuel line from the carburetor and place it in a container to collect fuel as you test the flow coming out of the line.

SOLUTION: If you aren’t getting sufficient fuel flow or find any cracks or fuel leaks, replace the fuel pump with a new one.

6. Bad Carburetor Gasket

You will find a gasket that sits behind the carburetor. This gasket can wear or deteriorate and no longer seal efficiently. This can cause air to be sucked into the system causing the engine to run lean.

Running lean is when there is a higher concentration of air and less fuel than required by the engine. The choke will need to be used to compensate for the extra air being pulled into the engine through a bad gasket.

Inspect the carburetor and make sure all bolts are securely attached and the gasket is in good condition.

SOLUTION: If you find the gasket no longer seals, gain access to the carburetor and carefully remove the linkages and bolts attaching the carburetor. Remove the carburetor and gasket.

Reinstall a new gasket and reattach the carburetor, bolt, and linkages. This is also a good time to determine if you need to clean the carburetor when you have it off the mower.

7. Bad Gas Cap

The fuel tank must be able to vent allowing air to enter the tank. A vacuum forms in the fuel tank when the vent is plugged and air can’t pass into the tank. This vacuum won’t allow fuel to get to the carburetor.

The fuel tank vent is located in the gas cap on a Toro mower. You can use a pressure gauge to check if there is a vacuum buildup or you can follow these steps:

  • Loosen the gas cap to allow air into the tank.
  • Place the choke in the off position.
  • If the engine continues with the choke in the off position, the cap may be the problem.
  • Try to replicate the problem to confirm the cap is bad.
  • Tighten the cap and allow the engine to continue to run with the choke off.
  • If the engine begins to sputter after a while and won’t run well until you loosen the cap, you most likely have a bad cap.

SOLUTION: Replace a bad gas cap.

Still Having Problems with Your Toro Lawn Mower?

It would be great to own a problem-free lawn mower, but it’s never the case. No matter what brand mower you own, you’re going to run into problems the longer you own it.

To help you troubleshoot your mower problems, I have put together a list of common problems along with causes and solutions to fix them. Check out Common Toro Lawn Mower Problems and Solutions to learn more.