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String Trimmer Won’t Prime

Most string trimmers are not complicated tools, per se. However, these tools have several parts that must function in perfect synergy, including the tiniest components. Otherwise, you are likely to have several issues in due course, including instances when your string trimmer won’t prime. 

A string trimmer won’t prime if there is any kind of blockage in the fuel lines, filters, valves, carburetor, or engine. A common problem is a broken, leaking, or clogged primer bulb and a stuck valve inside the unit. Therefore, you must inspect all these components. 

A typical hurdle in troubleshooting a string trimmer that won’t prime is the various necessary inspections. You need to rule out one potential issue at a time, eventually leading you to the problem. Read on to know why your string trimmer won’t prime. 

5 Reasons Why a String Trimmer Won’t Prime

A string trimmer isn’t used frequently. Thus, you may find more than one unpleasant surprise after the tool has been idle for a while. 

In some cases, a string trimmer won’t prime if the gas and oil mixture is stale, which is unavoidable if you store a string trimmer with a loaded gas tank (source).

The String Trimmer Was Stored With a Loaded Fuel Tank

Every string trimmer needs gas and oil mixed in a specific proportion, as recommended by the manufacturer. Also, the gas and oil should blend thoroughly before the mixture is drawn through the fuel filter. Storing a loaded gas tank may clog the fuel lines, carburetor, and thus primer bulb. 

Therefore, if your string trimmer won’t prime after you’ve stored it with a loaded fuel tank, you should: 

  1. Empty the reservoir. 
  2. Try to drain the residual fuel from the lines or hoses, engine, and carburetor. 
  3. Get fresh gas and oil, and mix it in the recommended ratio. 
  4. Load the fuel tank. 
  5. Test the string trimmer’s primer bulb. 

You may have to prime the purge valve several times in sets of ten or fifteen to create the suction necessary to pump air through the return line. This process draws the gas and oil through the fuel filter in the tank to the inlet hose (source).

The String Trimmer’s Primer Bulb Is Damaged or Has Leaks

The primer bulb on a string trimmer is connected to the carburetor and return fuel line, hence pressurizing and purging the system. Generally, the absence of fuel inside the primer bulb is one of the first symptoms you may notice. However, this symptom doesn’t have one cause (source).

  1. Inspect the string trimmer’s primer bulb thoroughly. Even if the plastic bulb appears to be intact, you may have some leakage, and your string trimmer won’t prime. 
  2. Check for leaks through the primer bulb’s gasket, seal, or ring, whichever type is on the model. 
  3. Check the primer bulb valve. A stuck primer bulb valve will disable the purging and pressuring mechanism. 

A damaged primer bulb needs replacement. Otherwise, compression problems require further investigation. If you have already checked the gas tank, you can rule out the clogging of the fuel intake and return lines at those ends. However, you may have blockage and leakage elsewhere. 

The Fuel Lines Are Clogged or Leaking

The intake and return fuel lines are plastic hoses in most string trimmers. Like the primer bulb, these fuel lines or hoses may crack and thus leak. 

Also, debris buildup is likely if you haven’t used the string trimmer for a while, as gas and oil can get dry and become sticky in due course. 

Press the string trimmer primer bulb and feel the fuel lines simultaneously. You may be able to feel air or fuel leaks. 

Translucent fuel lines can give you visual confirmation if some gasoline is flowing out of the tank. If there is no fuel in the intake and return lines, you have to look for clogging, whether at the fuel filter or in the carburetor. 

The Fuel Filter Is Dirty and Clogged

A string trimmer won’t prime if the fuel lines and filter are dirty and clogged. The fuel filter is in the gas tank, so you may need pliers to access it. The intake fuel line is connected to the carburetor, whereas the return line is hooked to the purge or primer bulb. 

Unplug and clean these hoses if they are clogged, or replace the lines if you find them damaged or worn out. 

Technically, a dirty or clogged air filter shouldn’t prevent your string trimmer from priming. You may have issues when running the engine, though. You can check the air filter, as you may need to access and remove it anyway to get to the carburetor in the subsequent step. 

The Carburetor Is Dry, Clogged, or Broken

Your string trimmer won’t prime if the carburetor is clogged or broken. Like the filters and valves, the carburetor can have debris and residue build up over time. Storing a trimmer with a loaded fuel tank leads to gummy deposits due to the gas and oil drying, especially during the winters. 

Access the carburetor and inspect it. A dry carburetor is a sign of clogging. Also, you may have a dirty carburetor, which you can quickly remedy with a carb cleaner. 

However, you may need to seek technical assistance if the metering unit is broken, such as a malfunctioning needle valve. 

Clean the carburetor, remove any grime and greasy debris, ensure all the ports are unclogged so that the fuel can pass through without any obstruction, and let the unit dry before you lubricate it for reassembly. Test the purge bulb and see if your string trimmer primes.


When a string trimmer won’t prime, start with the easiest inspections: the bulb, fuel lines and filter, and gas tank. Proceed to the gaskets, valves, carburetor, and other internal components after ruling out the first set of potential issues. 

Depending on the problem, you may need a new primer bulb, fuel line, filter, or carburetor. If you’re unlucky, you may need to replace more than one broken component. In such scenarios, weigh the pros and cons of spending on the required repairs or replacements. 

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