Your mower starts but then dies when the choke is adjusted to the off position. The only way it keeps running is when the choke is placed in the on position or halfway between the on and off position.
A zero-turn 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.
Before performing repairs, shut the mower off, remove the ignition key, and remove the spark plug wires. Wait for the engine to cool. You can find a list of safety precautions in your operator’s manual.
Reasons your 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 Zero Turn Only Runs With the Choke On
You may not know gas can begin to break down as soon as 30 days after purchase. This is why it’s important to purchase the quantity of gas you are able to consume within this time.
Not only is it important to consume gas quickly, but it is also necessary to use the right type of gas. Gas-powered mowers require unleaded gasoline with a minimum 87-octane rating and maximum ethanol content of 10%.
Never use gas that contains more than 10% ethanol. Ethanol is not good for small engines so, if used, it must be used in limited amounts.
Ethanol is an alternative fuel made from plants. It naturally attracts moisture to the fuel system which has negative effects on the fuel system and engine.
This water and ethanol mixture can leave behind varnish that restricts fuel flow and corrodes fuel components. It also separates from gas over time sinking to the bottom of the tank.
SOLUTION: Drain the fuel tank. A fuel siphon pump works well for this.
Add fresh gas with a fuel additive mixed in to help clean the fuel system and reduce moisture. I like to use Sea Foam Motor Treatment. You can read more about the advantages of Sea Foam on your fuel system and engine here.
Puncture or Clog in the Fuel Line
A puncture in the fuel line can cause air to be sucked into the fuel line so the engine receives too much air. A clog can also develop in the line that restricts the amount of fuel the engine receives.
Either one of these issues can cause the mower to only run when the choke is on.
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 the zero-turn.
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 fuel line with the same diameter.
Bad Carburetor Gasket
The gasket that sits behind the carburetor can deteriorate and become worse over time. When this happens, it no longer seals properly allowing additional air into the system causing it 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 on to compensate for the extra air being pulled into the engine through a bad carburetor gasket.
SOLUTION: 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.
Plugged Fuel Filter
The fuel filter is a small part that is placed between the fuel lines to keep dirt and other contaminants out of the fuel system causing wear on the engine.
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.
A reduced amount of fuel may require the choke to be used to correct the ratio of fuel to air required for combustion.
SOLUTION: Remove the fuel 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.
The carburetor’s function is to regulate the amount of fuel that is mixed with air to form combustion to start and keep the engine running.
A carburetor failure is often due to old gas clogging fuel passageways and causing internal components to stick. You need to get the carburetor to function correctly by cleaning it or replacing it.
SOLUTION: If you are a little mechanical and don’t mind working with small parts, you should be able to remove and clean the carburetor yourself following these instructions.
If the carburetor doesn’t work right after you clean it, you may have to rebuild it or replace it.
Bring your mower to a small engine dealer to have cleaned if you don’t want to tackle the job.
Faulty Fuel Pump
A fuel pump is used on your zero-turn mower if the carburetor is placed higher than the fuel tank. Most riding mowers have a vacuum pump that 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.
Bad Gas Cap
The gas cap is designed to allow air to pass through the cap to vent the fuel tank. When the cap breaks or becomes plugged, the fuel tank will form a vacuum when it isn’t able to vent properly.
The faulty cap will restrict the amount of fuel getting to the carburetor.
SOLUTION: You can try to identify whether the gas cap is bad by loosening the cap to allow air to get into the tank. If you are able to move the choke and the engine continues to run, the cap may be bad.
Try to replicate the problem to further confirm the cap is bad. Tighten the cap while allowing the engine to run.
If it begins to run sluggish and you have to engage the choke to keep it running, but you can remove the choke when the cap is loosened, the cap is most likely the problem.
Still Experience Problems with Your Zero Turn Mower?
Many different types of problems can develop in a mower. It doesn’t matter what brand you own.
While some mowers are built with stronger materials, bigger filters, better engines, and tougher spindle housings, they are all going to break down and cause issues at some time. Some may just not develop problems as quickly as others.
I put together a guide with common things that go wrong on zero-turns. In this guide, you will find a list of causes and solutions for a mower dying, smoking, vibrating, not starting, having cutting issues, and more.
Check out my guide at Common Zero Turn Mower Problems: How to Fix Them
If you still can’t find the solution to your problem or you don’t feel comfortable troubleshooting or repairing your mower, it is best to have an experienced mechanic check out your mower.
You can visit your local dealership that provides repair support for your brand mower. You may also find a lawn mower repair shop with experienced small engine mechanics.