How to Adjust a Carburetor on a Stihl FS 80 Weed Trimmer

Stihl no longer offers the FS 80 string trimmer on its web portal, but it was a popular model in its day, and there are still plenty of them around. The FS 80 has an interchangeable trimmer head that can be replaced with a cutting tool, a power tool attachment, or a combitool. When you replace the head with one that does a more difficult task, you sometimes have to tweak the performance of the engine by adjusting the carburetor. You might also have to adjust the carburetor if you live at a high elevation because the unit is factory-adjusted for sea-level conditions.

The adjustment screws for the FS 80 trimmer are located on the side of the engine housing just behind the handle for the starting cord. There are three of them, and unlike later models, they all have Phillips drive heads, so you can make adjustments with a #2 Phillips screwdriver. There's no need to purchase a special tool.

The screws are arranged in a vertical triangle. The two at the top are marked "H" (high speed) and "L" (low speed) and are for making high-speed and low-speed adjustments, respectively. The third screw at the apex of the triangle is the idle adjustment. It's marked "LA."

After many hours of use, the carburetor screws can slip and cause the engine to sputter or run unevenly. To restore performance, the manual provides instructions for restoring the factory settings. Before making these adjustments, it's important to clean or replace the air filter and adjust the throttle cable to make it tight and responsive.

1. Start by turning the H screw counterclockwise by 3/4 of a rotation or until it stops.
2. Next, carefully turn the L screw clockwise until it stops and then back it off by one full turn.
3. Start the engine, let it warm up, and then turn the LA screw clockwise or counterclockwise as necessary to make the trimmer head stop turning. If the engine idles unevenly or sputters when you make this adjustment, turn the L screw clockwise about 1/4 turn or until the engine idles smoothly.

Since the factory settings are designed for sea level, Stihl recommends moving the H screw 1/4 turn away from the stop position for every 1,000-meter (3,300-foot) change in altitude. If you make this adjustment with the engine running and the throttle fully engaged, you'll notice the engine steadily speeding up until you reach a point at which it stops increasing. This is the correct position for the H screw.

When you mount a cutting tool, you may also have to readjust the idle speed using the L and LA screws. If the engine stops while idling, turn the LA screw slowly clockwise until it runs steadily. If you go too far, the cutting tool will rotate when the engine is idling, so turn the LA screw counterclockwise until the tool stops rotating. If the engine accelerates too slowly at this setting, turn the L screw counterclockwise (richer mixture). On the other hand, if the idling is erratic, turn the L screw clockwise (leaner mixture) until the engine runs smoothly.