Weed Whacker-Powered Bicycle
A version of the same thing was on Instructables, and that's what partially inspired me to make this. ;)
The engine mounts are made out of wood, because I still have not learned to weld, but they hold up just fine.
The engine is an ten-year-old Echo weed whacker. It has 10,000 max RPMs. The bike peg attached is 2" in diameter and gives about 25mph top speed.
There is no clutch! The one that came with the engine was way too flimsy for a vehicle like this so I took it out. Saved a lot of space, too.
For the throttle, I disconnected the back brake line and connected it to the engine. The front brakes are still intact, and you can turn the engine off to slow the vehicle as well.
I added the extra heatsink to the engine to help keep it cooler, as it was never meant for this kind of exertion. It gets as hot as the engine, so I guess it conducts the heat away pretty effectively.
There is a 10-LED bargraph to show the engine temperature. It uses an LM3914, an op-amp, and a thermistor. There is also a 555 to drive a buzzer when it gets too hot.
The bike has head and taillights that are both driven from a joule thief. They are not very bright; they are more for being seen than to see.
Here's a youtube video, taken almost a year ago:
Some more photos and a little more information: