If your FS 76 string trimmer starts fine but bogs down when you give it throttle, and you’ve already ruled out the carburetor as the problem, don’t worry — there are still several other common issues that could be causing it. Here’s a breakdown of what to check next and how to fix it.

1. Clogged Spark Arrestor Screen

Over time, carbon builds up on the spark arrestor screen located in the muffler. When this screen becomes clogged, exhaust gases can’t escape properly, causing poor performance and bogging under load.

Fix:

Remove the muffler cover.

Find the small screen (usually behind the muffler outlet).

If it's clogged, clean it with a wire brush or burn off the carbon with a torch (only if you’re experienced).

If it’s too clogged or damaged, replace it.

2. Dirty or Restricted Air Filter

A dirty air filter limits the amount of air getting to the engine, creating a rich fuel mixture and causing the engine to bog.

Fix:

Take out the air filter and inspect it.

If it’s dusty or oily, clean it with warm soapy water (for foam types) or replace it if it’s paper or beyond cleaning.

Tip: Run the trimmer briefly without the air filter (only for testing in a clean area). If it revs fine, the air filter is the issue.

3. Exhaust Port Carbon Build-up

Just like the spark arrestor screen, the exhaust port itself can get clogged with carbon. This is especially common in older 2-stroke engines that have been run with old or improper fuel mix.

Fix:

Remove the muffler and inspect the exhaust port on the engine.

If you see heavy carbon build-up, gently scrape it off with a plastic pick or small screwdriver. Do not let debris fall into the cylinder.

4. Clogged Fuel Filter or Fuel Line Issues

Even if the carb is working fine, if the fuel filter is partially clogged or the fuel line is cracked, the engine might get enough fuel to idle but not enough to run at full throttle.

Fix:

Replace the in-tank fuel filter (cheap and easy).

Inspect the fuel lines for cracks, stiffness, or leaks. Replace them if needed.

5. Faulty Ignition Coil or Weak Spark

When ignition coils begin to fail, they often still spark enough to idle the engine but not enough to fire under full load. That can cause bogging or cutting out.

Fix:

Test the spark with a spark tester or by checking how strong and blue the spark is.

If the spark looks weak or inconsistent, consider replacing the ignition coil (also called the module).

6. Improper Valve Timing (on Rare 4-MIX conversions)

While the FS 76 is a 2-stroke, if someone swapped or modified the engine with a 4-MIX or if the wrong parts were used in a repair, poor timing or valve issues could cause bogging. This is rare, but worth mentioning in unusual cases.

7. Old or Improper Fuel Mix

Even if your carburetor is clean, old or incorrect fuel mix can cause bogging. Ethanol-blended gas left sitting for weeks can gum up the entire fuel system and affect engine performance.

Fix:

Use a fresh 50:1 gas/oil mix with ethanol-free fuel if possible.

Add a bit of fuel system cleaner or stabilizer if needed.

Final Tip: Don’t Overlook the Basics

Sometimes the cause is as simple as a loose spark plug, a chipped trimmer head causing drag, or grass build-up blocking cooling fins, leading to overheating and power loss.

Wrap-Up

The FS 76 is a solid workhorse of a trimmer, and bogging doesn’t always mean a bad carburetor. Checking the spark arrestor, air filter, exhaust, and fuel delivery system can usually get your trimmer running strong again. If you're doing your own repairs and need reliable replacement parts — air filters, fuel lines, spark plugs — you can find affordable, dependable options from Hipa Parts.