Two Cycle String Trimmer Will Not Start When Hot

Written by eric blankenburg
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Two Cycle String Trimmer Will Not Start When Hot
A two-cycle trimmer needs to keep air and gas flowing to run when hot. (line trimmers image by Horticulture from

Your two-cycle string trimmer operates through a constant stream of air and fuel mixing in a precise balance inside the carburettor. When the flow of air or fuel stops, even a little, the precise balance will be lost and the trimmer will start experiencing problems. Most often, problems starting your trimmer when hot are located in the carburettor, but you can make some basic fixes yourself before taking the trimmer to the shop.

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Keep Air Flowing

Your two-cycle string trimmer must suck in enough cool air to mix with the fuel inside the carburettor. Dirty air filters, clogged air vents and dirty mufflers will prevent the proper intake and outtake of fuel. If the airflow is reduced you will force other parts of your engine to work harder to maintain the proper air and fuel balance. Following the cleaning and maintenance schedule provided in your operator's manual can greatly reduce the wear and tear on the more vital engine parts like the carburettor.

Maintain Fuel Supply

Keeping the fuel system free and clear of obstructions will also help your engine run more smoothly. The fuel gets sucked up from the tank via the fuel pump inside the carburettor. The fuel pump's diaphragm pulses in time with the crankcase, drawing fuel into the carburettor when the pressure increases. If the fuel pump can't operate efficiently due to an improper fuel supply from the tank, your trimmer will stop working, especially when the diaphragm gets hot. Keep the fuel filter and fuel lines clean and unobstructed. The tank also contains its own vent valve to allow heated air to escape, which maintains the proper pressure inside the tank so fuel can get pumped into the carburettor. To check if your problem is the vent valve on your fuel tank, unscrew the gas cap about halfway and try to run your trimmer. If the vent is plugged, the loosened cap will allow these heated gases to escape. Replace the cap if your vent valve is blocked.

Check for Leaks

Sometimes an air leak somewhere in the fuel system can cause your engine to stop running when it gets hot. Air leaks can occur in the fuel tank, fuel lines, and around the valves where the fuel lines connect to the carburettor. If the carburettor isn't airtight that extra air will push the fuel out of balance and force your trimmer to die. More serious air leaks can also occur around the gaskets on your carburettor, which will expand when heated, causing the air leaks to worsen.

Servicing the Carburetor

Most often this problem results in a warped diaphragm in the fuel pump. When the engine gets hot, this warped diaphragm will no longer pulse in time with the crankcase and the fuel supply will shut off. Another common problem is the seating of the fuel inlet lever, which needs to be flush with the housing. When the lever gets knocked off its seat, too much fuel may be reaching the carburettor. Arrange for a professional to replace or service your carburettor.

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