A STIHL string trimmer only runs with the choke on when the engine is getting too much air or too little fuel.
This could be caused by old fuel, a puncture in the fuel line, a plugged fuel filter, a bad carburetor gasket, a dirty carburetor, or a plugged fuel tank vent.
Follow the safety guidelines provided in the STIHL operator’s manual. Wait for the engine to cool and remove the spark plug wire prior to performing repairs.
6 Reasons a STIHL String Trimmer Only Runs With the Choke On
Old Fuel in a STIHL String Trimmer
The type of gas and its quality have a major impact on the performance of your STIHL trimmer. Old gas can leave behind varnish that coats the fuel system components and causes failures and fuel restrictions.
When the flow of fuel is restricted, the choke may have to be used to maintain the ratio of gas-to-air required to form combustion.
To reduce fuel-related problems due to old gas in the future, always use fresh gas and mix it properly with 2-cycle oil. Consume the fuel within 30 days and no longer than 60 days when using a fuel stabilizer.
STIHL string trimmers require gas and oil mixed at a rate of 50:1. Use unleaded gas with a minimum octane rating of 89 and a maximum ethanol content of 10%.
Mix with high-quality two-cycle engine oil. You can learn more about choosing the right gas and oil for a STIHL string trimmer here.
SOLUTION: Remove old fuel from your string trimmer. Mix a fresh fuel mixture using gas and 2-cycle engine oil. Add a fuel additive like Sea Foam or STA-BIL to help clean the fuel system and reduce moisture.
Add this mixture to the fuel tank. Start the trimmer and allow the fuel mixture to work its way through the fuel system. Continue to check other items in this list if you continue to have to use the choke to keep the trimmer running.
Puncture in the Fuel Line on a STIHL String Trimmer
When the engine is getting too much air, the choke must be used to reduce the amount of air entering through the carburetor throat.
Check the fuel line for a puncture where additional air can be sucked into the line.
SOLUTION: Remove and replace any damaged or cracked fuel line. If you find a fuel restriction in the line, clean the line or replace it.
Bad Carburetor Gasket on a STIHL String Trimmer
Another area that can draw in too much air is around the gasket that sits behind the carburetor. This gasket can deteriorate over time and no longer seals properly.
When this happens, additional air is allowed into the system causing the trimmer to run lean with more air and less fuel. The choke may need to be used to keep it running.
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 STIHL trimmer.
Plugged Fuel Filter in a STIHL String Trimmer
The fuel filter is used to strain fuel in the fuel tank to keep dirt and debris out of the fuel system and engine. When it isn’t replaced regularly, the filter can become plugged and won’t allow sufficient fuel to pass through it.
I recommend replacing the fuel filter annually and using fresh clean fuel to reduce the likelihood the filter causes a fuel restriction.
SOLUTION: Replace a plugged STIHL fuel filter. Wipe around the fuel tank cap before removing it to keep dirt from falling into the tank. Pull the filter out of the tank.
A clean bent wire works well to hook the fuel line 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.
Make sure the filter sits at the very bottom of the tank. Reinstall the fuel cap.
Dirty Carburetor on a STIHL 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 STIHL string trimmer.
A buildup of varnish and gummy deposits can make the carburetor not function right and it won’t be able to get fuel to the engine. Its passageways can become clogged and internal components can stick and fail to function properly.
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.
Before spending money on a replacement carburetor, evaluate the condition of your string trimmer and the cost to replace it with a comparable STIHL string trimmer.
If you have an entry-level string trimmer, you may be better off replacing the STIHL string trimmer and not replacing the carburetor. It’s a financial decision you will have to make based on the string trimmer model, its age, and the cost of repairs.
Plugged Fuel Tank Vent on a STIHL String Trimmer
The fuel tank must vent to allow air to pass through 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 in order to keep it running. A good indication you may have a fuel tank vent problem is when your trimmer runs sluggishly with the choke off and begins to run better after loosening the fuel cap.
SOLUTION: Place the STIHL 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.