The general procedure for starting a gas-powered string trimmer is to switch it on, close the choke and pull the starting cord, but trimmer designs vary from manufacturer to manufacturer, so it's important to read the instructions supplied with your model. If your trimmer has a priming bulb, which is a small, clear plastic button attached to the trimmer housing over the carburetor, you have to press this several times before pulling the starting cord to get gas into the carburetor. The exact number of times you need to press it varies from four to 10, depending on the model, and you shouldn't overdo it, or you can flood the engine.
Starting Sequence for a String Trimmer
Step 1: Stabilize the Trimmer
Set the trimmer flat on the ground or on a workbench. This allows you to hold it still with one hand while you pull the cord with the other.
Step 2: Check the Gas and Oil
Open the gas tank and make sure the trimmer has gas.
Step 3: Turn On the Power Switch
The on/off switch may be located on the motor housing or on the trimmer shaft. Turn it on.
Step 4: Close the Choke or Push the Priming Bulb
On models with a choke, you'll see a small lever near the bottom of the gas tank. Move this lever all the way to the other side to the "on" or "choke" position. If your model has a priming bulb instead of a choke, press it in the number of times specified in the manual.
Step 5: Pull the Starting Cord
Hold the trimmer securely with one hand while you pull the starting cord sharply with the other. When the motor turns over and the trimmer starts, move the choke back to the "off" or "closed" position.
Don't Flood the Engine
The trimmer may not start on the first pull, but it probably will after repeated tries. There's a limit, however, to how often you can prime the engine without flooding it. If you smell gas after several attempts, the engine is probably flooded, so wait 10 or 15 minutes before trying again to give the gas time to drain. One of a number of things could be wrong if the trimmer won't start at all. Some of them are easy to fix, while others aren't.
What If the Trimmer Won't Start?
The trimmer needs gas, air and spark to start, so the first and easiest thing to check is the air filter. If it's dirty, it's probably preventing air from getting into the carburetor, so clean or replace it. The next thing to check is the spark plug. Remove it with a spark plug wrench and clean the contacts with sandpaper or replace the plug. If these simple fixes don't work, the carburetor or fuel line may be clogged, and a simple hack may save you the job of cleaning them.
Remove the air filter and spray a one-second squirt of engine starting fluid — which you can purchase at any car parts or hardware store — into the carburetor through the air intake passageway. Without setting the choke or pressing the priming button, immediately pull the starting cord. The engine should turn over and in doing so may burn off some of the gunk clogging the carburetor. If this doesn't work, your best bet is to have the trimmer serviced, or if you're handy with machinery, clean the carburetor yourself.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.