Your mower starts, but won’t stay running unless you leave the choke on. Finding and fixing the cause will improve your engine performance.
A Kubota lawn mower may only run with the choke on when the engine is getting 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 Kubota operator’s manual for all safety precautions. This includes removing the ignition key and the spark plug boot prior to making repairs.
Reasons your Kubota 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
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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 Kubota Mower Only Runs With the Choke On
1. Old Gas
Fuel restrictions can cause you to have to use the choke to keep your Kubota mower running. This is to restrict a little air so the air-to-fuel ratio required for combustion is met.
The root cause of most fuel restrictions is old gas. Gas can begin to break down as soon as 30 days after purchase.
The ethanol found in most gas today attracts moisture from the air. This ethanol and water mixture not only leaves behind varnish that coats the fuel system causing fuel restrictions, but it also is corrosive to the fuel system and engine.
Kubota 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.
It will also keep the gas stable so it lasts a little longer before it breaks down. Check out why I choose to use Sea Foam in every small engine I own.
Start the engine and allow your Kubota to run and allow the treated fuel to work its way through the fuel system.
Your engine may begin to run better and may even continue running without the choke engaged. If it doesn’t, continue to check the items below.
2. Dirty Carburetor
A Kubota carburetor is used to regulate the amount of gas that is mixed with air for combustion. When it becomes dirty, most likely due to old gas, fuel passageways, and internal parts can become clogged and fail to function.
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 Kubota 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 the 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
An inline fuel filter is placed between the fuel lines to strain fuel as it comes out of the fuel tank. This is to keep dirt and other contaminants from entering the fuel tank causing wear.
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 Kubota mower to move fuel uphill to the carburetor. Most Kubota 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
The gasket that sits behind the Kubota carburetor can wear or deteriorate so it no longer seals. A bad gasket can allow additional air to be drawn 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 vents out of the gas cap. You can use a pressure gauge to check if there is a vacuum buildup or you can follow these steps if you don’t have a gauge:
- 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 Experiencing Problems with Your Kubota Lawn Mower?
Own a lawn mower long enough, you’ll start running into problems with it starting, not continuing to run, smoking, leaking gas, giving a bad cut, vibrating, or another issue.
To help you save time and money, I have put together a guide to help you troubleshoot the next problem that develops on your Kubota mower.
You can find this guide at Common Kubota Lawn Mower Problems & Solutions.
If you are unsure how to perform diagnostics and repairs on your Kubota lawn mower safely, it’s best to have a professional complete the repairs.
This will help you avoid personal injury or additional damage to the mower. Your local Kubota lawn mower dealership or lawn mower repair shop will be able to help you solve your problem.