Blocked Air Flow

• The carburetor needs clean air to mix with the fuel. If dirty air were to get inside the cylinder, it would blow out your piston and cause it to seize up. This is why above the carburetor, on every string trimmer, there's an air filter to purify the air. If this filter is dirty or blocked up by large matter, then no air will enter the carburetor. When the air flow drops off, too much gas will be in the carburetor and the engine will die unless some of that gas is moving to the engine. When the air filter is dirty you may also experience starting, acceleration, and other performance problems. Wash the foam filter in water and dish soap, allowing it to thoroughly dry before reusing it.
  
  

Bad or Old Gas

• When you mix gas for a two-cycle engine, this gas will start separating after about three weeks and the oil will settle at the bottom. Bad gas that wasn't mixed properly may also have too much or too little oil. In either case this bad gas will enter the fuel system and start degrading parts, like the fuel hoses and carburetor. When this dirty gas mucks up the fuel system, it will choke off the proper amount of fuel needed in the carburetor. The engine will then start running only when you depress the throttle and let in the maximum amount of fuel. If your fuel tank, fuel filter, or fuel lines are blocked or clogged in any way, you'll notice these idling problems.
  
  

Adjusting the Carburetor

• After you've cleaned the air filter and gotten rid of any bad gas, check the settings on the carburetor. If the problem still persists, your carburetor may no longer be taking in the proper supply of fuel. An easy way to check is to increase the idle speed screw on the carburetor. If the idling problem goes away after increasing the idle speed, your carburetor is either dirty or needs servicing. Don't consider idle speed adjustment as a permanent solution to the problem, as the carburetor will continue to get dirty and you'll need to make continuing adjustments. Too many adjustments to the carburetor can lead to serious damage.
  
  

Serious Carburetor Issues

• Sometimes, especially if you used old or bad gas, the carburetor will get so dirty you'll need to disassemble, clean, and service it. The pump diaphragms will often cause idling problems, as they'll start sticking together and won't be pulling in enough fuel during idling but may come unstuck when the crankcase starts moving at full speed when the pressure increases. It's best to leave carburetor issues to the professionals as you can seriously damage the equipment.