After the engine warms, you find you are not able to place the choke in the off/open position without your string trimmer shutting down. However, you are able to keep it running by keeping the choke on or halfway on.
Finding and fixing the problem is necessary to get your trimmer to perform at its best.
A Cub Cadet string trimmer only runs with the choke on when the engine is getting too much air or too little fuel.
This may be the result of a fuel restriction due to a dirty carburetor, plugged fuel filter, clogged fuel line, plugged fuel tank vent, or old fuel; or the result of excessive air due to a punctured fuel line or a bad carburetor gasket.
Remove the spark plug wire prior to making any repairs. Be cautious and follow all safety procedures in the Cub Cadet operator’s manual.
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Follow all safety instructions provided in your equipment operator’s manual prior to 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.
6 Reasons a Cub Cadet String Trimmer Only Runs With the Choke On
1. Old Gas in a Cub Cadet String Trimmer
Allow gas to sit in your Cub Cadet string trimmer can be the root cause of your running problem. This is because old gas can leave behind gummy deposits that restrict fuel flow.
You may find the only way you’re able to keep the mower running is by using the choke to correct the fuel-to-air ratio needed for combustion.
Gas can begin to break down and become less effective as soon as 30 days after purchase. Purchase fresh gasoline and consume it within 30 days.
Most gas sold today includes ethanol, a fuel that attracts moisture to the fuel system. The water and ethanol mixture will separate from the gas and oil mixture. It leaves behind varnish that can restrict fuel flow or cause component failure.
Ethanol is not good for a small engine. Never use fuels with high ethanol content. Avoid fuels sold as E15 and E85 as they contain up to 15% and 30% respectively.
A Cub Cadet string trimmer requires an unleaded gas that has a minimum 89-octane rating and contains no more than 10% ethanol.
- Cub Cadet 2-cycle string trimmers: Require a gas and 2-cycle oil mixture at a ratio of 40:1.
- Cub Cadet 4-cycle string trimmers: Require gas only (No oil is mixed with fuel. A separate fill port is used for engine oil.).
Read this article for more about choosing and caring for the fuel used in your Cub Cadet string trimmer.
SOLUTION: Remove old fuel from your string trimmer. Mix fresh fuel in a fuel container with an additive like Sea Foam or STA-BIL to stabilize the gas, clean the fuel system, and reduce moisture.
Add the fuel mixture to the fuel tank. Start your Cub Cadet trimmer and allow it to run for about 5 minutes to run the treated fuel through the system.
If the trimmer doesn’t begin to run well without the choke on, continue checking the items below.
2. Clog or Puncture in the Fuel Line on a Cub Cadet String Trimmer
The gummy deposits left behind by old fuel can stick to the fuel line narrowing the passageway. This restricts fuel flow.
You may also find the fuel line has developed a crack or has been punctured allowing air to be pulled into the fuel line.
A clog or puncture in the fuel line can result in needing to use the choke to correct the fuel-to-air mixture needed to keep the engine running.
SOLUTION: Remove and replace a clogged or punctured fuel line.
3. Plugged Fuel Filter in a Cub Cadet String Trimmer
Another item that can restrict fuel flow is a plugged Cub Cadet fuel filter. A fuel filter is used to prevent dirt and other debris from running through the fuel system and engine.
When the filter isn’t changed regularly or you’re running very dirty fuel, it can become plugged not allowing sufficient fuel to pass through the filter. It’s best to stay on top of your fuel filter maintenance by changing it out annually.
SOLUTION: Replace a plugged fuel filter. The filter is located inside the fuel tank. Wipe around the fuel tank cap before removing it to keep dirt from falling into the tank.
Take note of the location of the fuel filter inside the fuel tank to ensure the new filter is installed in the correct position. Pull the filter out of the tank.
A clean bent wire works well to hook the fuel linen to pull the filter out of the tank. Remove the old filter and attach a new fuel filter to the end of the fuel line and place it back inside the fuel tank. Reinstall the fuel cap.
4. Bad Carburetor Gasket on a Cub Cadet String Trimmer
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.
This is when there is a higher concentration of air and less fuel than required by the engine.
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. You may need to clean the carburetor while it is removed from the Cub Cadet trimmer.
5. Dirty Carburetor on a Cub Cadet String Trimmer
The carburetor’s function is to regulate the amount of fuel that is mixed with air to form combustion to start and run the string trimmer.
A buildup of varnish and deposits can make the carburetor not function right so it isn’t able to get fuel to the engine.
SOLUTION: If you are a little mechanical you should be able to handle cleaning your carburetor. Clean the carburetor by taking it apart and using a carburetor cleaner to remove deposits left behind from old fuel.
If the carburetor does not function after being cleaned, you may need be able to find a rebuild kit to rebuild the carburetor or you may have to replace it.
6. Plugged Fuel Tank Vent on a Cub Cadet String Trimmer
The fuel tank must have a vent so air can pass into the tank to equalize the tank pressure and air pressure. Without a vent, the fuel tank will create a vacuum that will restrict the amount of fuel getting to the engine.
This may cause you to have to run the trimmer with the choke on. A good indication you may have a fuel tank vent problem is when your Cub Cadet trimmer runs for a while before it starts to run sluggish or shuts down.
The engine only starts to run better once the fuel cap is loosened to allow air into the fuel tank.
SOLUTION: Place the string trimmer on a level surface. Loosen the cap and don’t allow gas to spill. Start the trimmer. Place the choke in the off position.
If the trimmer runs well with the choke in the off position, tighten the cap and allow it to continue to run with the choke off. If it dies, you may have to replace the plugged fuel tank vent.
Most Cub Cadet string trimmers vent from the fuel cap. Replace a bad cap.