Weed eater motors are compact, durable and versatile engines that can be used for many other projects--as long as you are willing to think outside the box. Many small engine shops, as well as go kart and mini-bike suppliers, will have driveshaft, clutch and chains that will retrofit onto weed eater motors. From there, and with a little engineering, you can use that motor to power a bevy of projects.
Motor Powered Bike
One of the more practical ways to use a weed eater motor is in a bicycle conversion. There are many kits and plans to do this that are available on the Internet, which makes this one of the easier projects to tackle. Depending on how much money you want to spend and the time it will take you to complete, this might make a good family, school or science project.
Inexpensive Boat Motor
You may not even have to remove the shaft of the weed eater for this project. Any marine store will carry a wide range of boat propellers to choose from, and they can be mounted directly onto the original threaded mounting screw on the end of the shaft. Tighten it down with a lock nut, and after that, just figure out a way to attach it, shaft and all, to the back of your boat. Obviously, the smaller the boat the more practical this idea is, but for those of you that like the water, this might be a great idea.
Power Your Skateboard
There are two ways to power up your skateboard. The first is to engineer a clutch and chain system to a drive wheel on a skateboard. You may have to change wheels to accomplish this, but the small size of a weed eater engine makes this a very practical application.
The other way is to fit a small propeller on the engine shaft, and your skateboard would be propelled much the same way an air boat is. You'll have to build a safety guard around the prop, but this would be a much simpler way than a clutch and drive wheel configuration.
Much like a skateboard, you can harness the power of the engine to propel a scooter. Easily steerable, this can be a lot of fun.
RC applications for aircraft or boats is a very reasonable idea. Most larger RC motors are about the same size, so adapting a weed eater motor may be a relatively simple process.
Adapting a weed eater to a mini-bike frame would also be possible. A drive train would need to be engineered to power the rear wheel, but a high-revving 2-cycle weed eater engine can make any mini-bike a lot of fun to ride.