Weed trimmers are gasoline powered lawn care devices that allow a standing man to trim the weeds along a fence, sidewalk or tree. When this device wears out or breaks down, the gasoline engine is usually still in working condition. Most long-time home owners have one or two broken weed trimmers in the garage -- with fully functional gasoline engines. These small gasoline engines are excellent for DIY projects. Putting a motor on a scooter is one fun way to recycle an old weed trimmers engine.
Take the engine off the weed trimmers. There will be a set screw that holds a pulley or gear onto the shaft of the gasoline engine and there may be several bolts holding the engine onto the weed trimmers frame. Save these bolts for possible later use. You will need to get two gears -- one to fit onto the shaft of the gasoline engine and another to fit onto the rear axle of the scooter near one of the wheels. The gear that fits onto the engine shaft should be 3 or 4 times larger in diameter than the other gear. You should also get a length of bicycle chain long enough to fit over both gears.
Attach the large gear to the engine and the small gear to the rear axle of the scooter near one of the wheels. Put just enough bicycle chain to engage both gears and have a little play. The chain should not be loose enough that you can easily pull it off the chain without disconnecting any links. You should test this mechanism while holding the scooter off the ground. You want to know that the engine runs smoothly, there is no scraping or catching of the gears and that the chain does not come off. It is better to find out all these things before you get on the scooter.
Run a control cable from the engine control to the handlebar of the scooter so you can operate the clutch and shut the engine off while riding the scooter. Test the controls while the scooter is on its side or up on a support with the wheels off the ground. You want to make sure the controls work smoothly before you get on the scooter.
Test the scooter throughly in a safe place -- like a parking lot -- before you take it out on a crowded sidewalk or near traffic.
In an emergency it is convenient to have an engine shutoff other than the one that is on the handlebars. Many weed trimmers engines have a large easy-to-grasp and easy-to-pull-off wire to the spark plug. Pulling this wire off disconnects the spark plug and turns off the engine. If your engine does not have this safety feature, you can buy one at any large department or hobby store.