How to Troubleshoot Gas Weed Eaters

Gas weed eaters can make quick work of tall vegetation around the yard. The two-cycle engines are easy to operate as long as you remember to mix the gas properly with the correct engine oil. Failure to use a gas/oil mix will ruin an engine. Gas weed trimmers can also be finicky when starting. Manufacturers list a basic starting procedure on the top of the plastic-covered engine. Weed eater engines that will not start after following the instructions can have only a few things wrong with them. By following a logical process you can troubleshoot the problems and get the engine running.

Things You'll Need

  • Fresh gas/oil mixture
  • New spark plug
  • Spark plug wrench
  • Fuel line
  • Fuel filter
  • Small slotted screwdriver
  • Carburetor rebuild kit

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Be sure you add only a fresh gasoline-and-oil mixture to the engine. Dispose of gasoline more than nine months old, following local regulations, or add gasoline stabilizers to the fuel to extend its life. Smell the gas mixture. If a hint of “varnish” hits your nose, discard the fuel.

Using a spark plug wrench, pull the plug from the engine and check its condition and connection. Replace the plug if the end that fits into the engine is covered in black soot. Generally, a spark plug should last a few years in a regularly operated weed eater. New plugs sell for only $2 to $3 so, if in doubt, replace the old plug.

Observe fuel flowing from the gas tank into the carburetor. Pull the plastic fuel line that attaches to the inlet stem on the carburetor. Fuel should be flowing from the small-diameter plastic line. Replace cracked, broken or leaking fuel lines. If fuel is not flowing from the end of the line, replace the fuel filter.

Empty the fuel into an approved container. The filter resides inside the tank. Turn the fuel tank upside down and shake the unit until the small filter comes out of the fuel-filling hole. You may have to pry the filter from the tank opening. Replace the filter and fuel line at the same time.

Adjust the “Hi” and “Lo” carburetor set screws. The “Hi” screw is for the high engine running. The “Lo” screw is for idle operation. If the engine pops and fails to run continuously, use the small slotted screwdriver and perform the following adjustments. Turn both screws in a clockwise direction until they will not rotate. Do not force the fuel-mixture screws closed. Open both screws 1 ½ turns counterclockwise. Start the engine. Turn the “Lo” idle screw slightly, left or right, to smooth out the engine at idle speed. Depress the throttle and let the engine operate at full speed. Turn the “Hi” screw, left or right, until the engine again smooths out. This will take practice. Various mixtures of the fuel/oil ratio will change the way these screws are adjusted to the operation of the two-cycle engine. You may have to “tweak” the adjustment between different fuel mixes.

If all of the above steps have been performed and the engine still fails to run or even start, rebuild the carburetor. Remove the carburetor from the rear of the engine, where it is typically held in place with four screws. Note the type of carburetor or brand name. The most typical brand is a “Walbro,” made for two-cycle engines. The rebuild kit is a generic type with all the parts needed for many styles of carburetors.

Set the carburetor on old newspapers on a flat workbench. Remove both sides of the small metal fuel mixer with the screwdriver and lay the parts on the newspaper. Match the old gaskets and diaphragm fuel pump with the new parts. Replace all parts. Reassemble the carburetor in the exact, opposite steps of disassembly. Attach the carburetor to the engine. Follow the Step 5 directions to adjust the fuel-mixture screws.

Tips & Warnings

  • The most common problem with small two-cycle engines is the carburetor. Typically, these units must be rebuilt every three to five years, depending on use. Generally, the rebuild takes less than an hour. Rebuild kits can be found online for less than $10, including shipping.
  • The engine runs but the head of the weed eater fails to turn. The clutch mechanism is either stuck or the long torque rod is broken. Pull the front cowling from the engine and inspect the dry clutch and the torque rod. Replace broken torque rods.
  • Oil in the clutch mechanism will cause the clutch to slip. Clean all oil and debris from the mechanism.
  • Clutch and torque rod is in good shape, but head still fails to turn. The head bearing is frozen or bad. Replace either the head unit or bearing. At times, over long use, the whole assembly may become severely corroded. This can make removal of the bearing almost impossible. Replace the entire drive head unit.


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