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14 Reasons a Cub Cadet String Trimmer Won’t Start (SOLVED)

Before you give up on your string trimmer and think it’s reached the end of its life, try to repair it using the guide below.

A Cub Cadet string trimmer won’t start when the engine isn’t getting sufficient air, fuel, and spark.

This may be due to a plugged air filter, bad spark plug, plugged fuel filter, clogged fuel line, dirty carburetor, faulty recoil starter, plugged spark arrestor, plugged fuel tank vent, or old fuel.

You may simply have a flooded engine causing your Cub Cadet starting problem.

Before repairing your string trimmer, remove the spark plug wire and wait for all parts to stop moving. You can find safety precautions for your string trimmer in your Cub Cadet operator’s manual.

String trimmer

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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.

Reasons For Your Cub Cadet String Trimmer Starting Problem

If its been a while since you’ve replaced the fuel filter, air filter, and spark plug, I recommend replacing these maintenance items to rule out these items as being the cause of your starting problem.

Replacing these items should be part of the routine maintenance completed annually on your Cub Cadet.

1. Plugged Air Filter

The air filter offers protection to the engine by keeping dirt and debris from entering the air intake. It’s important to always have an air filter installed on your Cub Cadet string trimmer.

While the air filter offers protection to the engine, it can cause problems if it isn’t kept clean. When the air filter becomes so plugged with dirt, it can keep a good flow of air from passing through it.

The engine is starved of air and will not start. If you find this is your problem, never run your string trimmer without an air filter to get the job done.

This can be a costly mistake. Dirt can enter the air intake and cause engine wear and damage. It may cause so much damage, the string trimmer will have to be replaced.

SOLUTION: Remove the air filter and wipe out any remaining dirt from the air filter housing. Replace a dirty air filter with a new air filter.

Where string trimmer parts are located

2. Bad Spark Plug

The spark plug will become dirty over time with a buildup of carbon. This may cause the plug to misfire having intermittent starting problems.

Other items to look for is cracked porcelain or burnt electrode; a loose spark plug wire; and an incorrect spark plug gap. These items can also cause a starting issue with your Cub Cadet.

SOLUTION: You can attempt to clean a dirty spark plug with a wire brush and reuse it. I prefer to replace it. It is an inexpensive part and one of the primary items responsible keep your string trimmer running.

Make sure your spark plug is gapped to the manufacturer’s specification and the spark plug wire (boot) is securely attached.

3. Plugged Fuel Filter

The fuel filter is a small part inside the fuel tank. You will find it attached to the fuel line. Its job is to keep dirt and other contaminants from passing into the fuel system.

When the filter becomes plugged because it isn’t being changed regularly, the amount of fuel getting to the carburetor may be reduced causing starting issues.

SOLUTION: Locate the fuel filter inside the fuel tank and replace it.

  • Wipe around the fuel cap to remove any loose dirt so it doesn’t fall into the fuel tank.
  • Remove the fuel cap.
  • Gain access to the filter. A clean bent wire works well to hook the fuel line and pull the filter out of the tank.
  • Remove the old filter from the fuel line.
  • Install the new fuel filter securing the fuel line to the filter.
  • Place the filter back inside the fuel tank and install the fuel cap.

4. Incorrect Fuel Mix (2-Cycle Engine)

Cub Cadet string trimmers with 2-cycle engines require a gas and oil fuel mixture. Running straight gas through your trimmer will cause the engine to seize. This is a quick way to ruin your string trimmer.

Gas runs extremely dry. Without the added oil, the engine won’t get the lubrication needed for the internal parts to move freely.

A 2-cycle Cub Cadet string trimmer uses gas-to-oil 40:1 fuel mixture. A 40:1 mix equals 40 parts gasoline to 1 part oil.

When creating the oil and gas mixture for your Cub Cadet string trimmer, use unleaded gasoline with a minimum 89 octane rating (mid-grade) and maximum 10% ethanol content.

Add a 2-cycle premium oil that is ISO-L-EGD and JASO M345 FD certified that is designed to work air-cooled engines. Stay away from regular motor oil or outboard 2-cycle oil.

Mix it in an approved gas can before adding it to your string trimmer.

SOLUTION: Drain the fuel tank and fill it with the correct gas-to-oil mix. If you continue to have problems, have a small engine mechanic diagnose the problem and determine whether a cost-effective repair can be made.

You can find more information about the right gas to use in your Cub Cadet string trimmer here.

Cub Cadet 2-Cycle Gas to Oil Mix

Mixture1 Gallon Gas2 Gallon Gas2.5 Gallon Gas
40:13.2 oz Oil6.4 oz Oil8.0 oz Oil
Cub Cadet 2-Cycle String Trimmer Gas to Oil Mix

5. Wrong Gas (4-Cycle Engine)

Cub Cadet string trimmers with 4-cycle models require straight gas. Use an unleaded gas with a minimum 89-octane rating and maximum 10% ethanol.

Stay away from fuels with high ethanol contents like E15 and E85 fuels. Using an ethanol-free fuel is best.

SOLUTION: Drain the fuel tank and fill it with the correct gas.

6. Incorrect or Insufficient Engine Oil (4-Cycle Engine)

If you have a Cub Cadet string trimmer with a 4-cycle engine, you will have separate fill ports for the engine oil and fuel.

You will not mix oil and fuel together for this type of engine. It’s important to use the correct engine oil and the right amount of oil.

Never use 2-cycle engine oil in a 4-cycle Cub Cadet string trimmer. Cub Cadet recommends using SAE 30 engine oil in the string trimmer. This engine oil from Kawasaki works well.

Oil is required to keep the engine components lubricated. When the wrong type or not enough oil is used, friction can build in the engine and overheat causing your string trimmer not to start and possibly ruin the engine.

SOLUTION: Drain the engine oil and fill it with the correct oil grade. When running your trimmer in very cold or very hot temperatures, you may have to adjust the viscosity to your ambient temperature.

If you continue to have problems, have a small engine mechanic diagnose the problem and determine whether a cost-effective repair can be made.

Cub Cadet 4-Cycle String Trimmer Engine Oil

Cub CadetSAE 30
Cub Cadet String Trimmer 4-Cycle Engine Oil

7. Old Fuel

Old fuel left in a Cub Cadet string trimmer won’t only cause fuel restrictions, but it can also damage the carburetor and engine. Gasoline can begin to break down as quickly as 30 days after purchase.

The ethanol found in most gasoline attracts moisture from the air. This moisture and ethanol mixture gums up the fuel system and corrodes components.

Because gasoline can begin to go bad as soon as 30 days after purchase, the fuel must be consumed within this time frame. If you are unable to use it in this amount of time, add a fuel additive to stabilize the fuel so it extends its shelf life.

Always use unleaded gasoline with a minimum octane rating of 89 (mid-grade) and maximum ethanol content of 10% (E10). Never use E15 or E85 in the engine as this will damage the engine and most likely void manufacturer warranties.

SOLUTION: Drain any old fuel remaining in your string trimmer and fill it with fresh fuel. A 2-cycle engine requires a gas and fuel mix at a ratio of 40:1. A 4-cycle engine requires straight gas.

Add a fuel stabilizer like Sea Foam Motor Treatment or STA-BIL to stabilize the fuel and reduce moisture in the fuel.

8. Bad Primer Bulb

A cracked Cub Cadet primer bulb that won’t fill with fuel will not function correctly to get fuel to the carburetor for starting the string trimmer.

SOLUTION: Make sure the fuel line is securely attached to the primer bulb. Replace a cracked fuel bulb with a new primer bulb.

9. Fuel Line Blocked

Old fuel sitting in your string trimmer can leave gummy deposits behind and restrict fuel flow. This can clog the fuel line keeping the engine from starting.

SOLUTION: Replace a fuel line on your Cub Cadet string trimmer when it is cracked, kinked, or clogged.

10. Plugged Fuel Tank Vent

You may find the fuel tank vent attached to a line coming out of the fuel tank or built into the fuel cap. When this part becomes plugged and fails to work, the air is no longer able to pass through the tank vent and into the fuel tank.

When the fuel tank is no longer able to vent, a vacuum will form keeping fuel from flowing out of the tank to the carburetor. When the carburetor doesn’t get fuel, the trimmer won’t start.

A good indication you may have a fuel tank vent problem is when your string trimmer won’t start until you remove or loosen the fuel cap to allow air into the fuel tank.

If it does start, confirm the fuel tank vent is the issue by allowing the trimmer to run with the fuel cap tightened. Wait to see if shuts down and won’t restart until you loosen the fuel cap.

SOLUTION: Replace the fuel tank vent so the air can flow into the fuel tank.

11. Dirty Carburetor

The carburetor regulates the amount of fuel that is mixed with air to create combustion in the cylinder. It is common for old fuel sitting in your trimmer to gum up the carburetor clogging fuel passageways.

You will need to clean or replace the carburetor to get your Cub Cadet started again when this happens.

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 carburetor cleaner to clean it.

If the carburetor does not function after being cleaned, you may need to rebuild it or replace it with a new carburetor.

Depending on the model string trimmer you run and the price of a carburetor, it may be best to invest in a new Cub Cadet string trimmer rather than put money towards replacing a carburetor on an old trimmer.

Weigh the price, age, and model of your current Cub Cadet with a new string trimmer.

12. Bad Recoil Starter

Your Cub Cadet string trimmer uses a recoil to start the engine. A bad pulley, a loose or missing spring, or broken clips can keep your recoil from working.

SOLUTION: You can attempt to replace the spring and restring the recoil.

If it does not work because other components in your recoil are damaged, such as the clips or the pulley, you are better off just replacing the recoil assembly.

13. Plugged Spark Arrestor

You will find a spark arrestor in a Cub Cadet string trimmer with a catalytic converter. The spark arrestor is a small screen that is used to prevent hot exhaust material from shooting out of the Cub Cadet causing injury or a potential fire.

The screen can become plugged with soot and prevent hot exhaust air from leaving the trimmer. This can cause a Cub Cadet trimmer to not start.

SOLUTION: Disconnect the spark plug boot. Make sure your engine is not hot. Remove the engine cover and engine exhaust cover.

Remove the spark arrestor and clean it with a wire brush to remove the soot. If you are unable to sufficiently clean it or it has a hole in it or is damaged, you need to replace it with a new spark arrestor screen.

14. Flooded Engine

The engine may become flooded when attempting to start it and now it won’t start. This isn’t too serious of a problem and can easily be fixed.

The Cub Cadet engine can become flooded when the choke is in the closed position and the starter rope was pulled many times allowing too much gas into the carburetor.

It can also happen with the switch off and the starter rope being pulled multiple times or when the primer bulb is pushed too many times.

SOLUTION: Use the following procedure to “unflood” your string trimmer so the engine gets the correct fuel-to-air ratio required to start and run.

How to Fix a Flooded Engine on a 2-Cycle Cub Cadet String Trimmer

  • Make sure the on/off switch is in the on position. This switch is automatically in the on position for many Cub Cadet string trimmers.
  • Place the choke lever in the open (run) position. This is position “3” on many Cub Cadet trimmers.
  • Press the throttle trigger while pulling the starter rope over and over. This can take anywhere between 5 and 15 pulls before it starts. Your string trimmer engine will sputter first. Continue to pull a few more times and it should start.