A fuel filter is a device placed in the path of fuel flow which prevents contaminants from reaching the carburetor.
Many fuel petcock valves already have basic filters over their inlets. However, these are typically not very fine filters, and they can also fall off and corrode with time.
Carburetors also typically have a coarse fuel filter near where the fuel supply line connects, usually in a banjo fitting. It is important to remove and clean this filter to ensure a sufficient fuel flow.
The other common type of fuel filter is an in-line fuel filter. This type of filter is spliced into the fuel supply line between the petcock valve and the carburetor. In-line fuel filters are very popular because they are able to catch very fine contaminants, like rust, which the coarser fuel filters miss. Rust is very common in moped gas tanks, and when bits of it make it to the carburetor bowl, they can often clog up the main jet or cause other problems. There are many styles of in-line fuel filters, but most of them allow you to see the filter element so that you can tell when the filter is dirty or clogged. Some in-line filters are disposable and should be thrown away when they have clogged or become dirty. Others have permanent filtering elements which can be removed and cleaned.
You can buy an in-line fuel filter at most motorcycle, powersports, hardware, or small engine repair stores. Make sure to buy one that fits the size of fuel line you're using. (Most mopeds use 3/16" fuel line.)
Troubleshooting Fuel Filter Problems
If you have gas flowing from your petcock valve and your moped will start when you spray some starter fluid or premix in the spark plug hole or carb throat, but it won't start normally, a dirty or clogged fuel screen in the banjo fitting is often to blame. Another common culprit is a dirty carburetor.
In summary, fuel filters are good, and can save you a lot of time and trouble. Put one on your moped today!