Basics of Choke
Your grass trimmer allows you to regulate the fuel and air mixture in the carburetor with manual choke settings. The carburetor — or carb for short — runs on a mix of fuel and air. The mix burns in the engine cylinder and produces power. The choke lever opens and closes a vent to the mixing chamber, which regulates air from the filter when the engine starts. When the choke is on, the vent is closed, blocking air from reaching the chamber. When the choke is off, air flows in from the air filter.
Check the Gas Cap
How is the fuel flowing in your machine? When your trimmer starts and runs on choke, it could be because the check valve in the gas cap is clogged. Start the trimmer and gently unscrew the gas cap halfway off. Turn the choke off and assess. If the engine is still running, that means the gas cap may be the culprit. Turn off the engine and remove the gas cap. Clean it thoroughly with warm, soapy water and gently rinse. If it is too clogged or dirty, a gas cap replacement is needed.
Install New Fuel Lines and Fuel Filter
The fuel lines are connected to the carburetor and may need replacing to ensure the machine is receiving the right amount of fuel. The filter can become clogged, same as the gas cap check valve. Replacing both may be the solution. Before you remove the fuel lines, carefully empty the machine of any fuel in the tank. A pair of needle-nose pliers can help pull the fuel filter and detach fuel lines out through the empty gas tank. You can also remove the cover plate for the carburetor and remove the fuel lines on that end. New fuel lines can be found online or at any home improvement store. Attach the new fuel line to the carburetor elbow connectors and insert the new fuel filter to the fuel line.
Check the Carburetor
The carburetor may simply need a good cleaning as well. Check the seals to ensure they are tight and are not clogged or damaged. The carburetor can be removed from the machine for inspection and cleaned using a carburetor cleaning solution. If the carburetor is damaged in any way, skip the cleaning and replace it entirely. Before taking the carburetor apart for cleaning, purchase a carburetor rebuild kit from the manufacturer or at a home improvement store. Not only will it have instructions on disassembling and reassembling your carburetor, it will contain replacement parts for pieces that are often the root of the problem. Follow instructions on cleaning solution and on disassembling your carburetor for best results.
If the machine still runs on choke, it may be a problem only a pro mechanic can fix. The problem may be the crankshaft or the piston, and a professional will be able to assess the problem for accurate repairs. If the machine is older, and the mechanic estimate paired with previous spending for replacement parts is worth more than your trimmer, consider investing in a new string trimmer.