My trusty motorized bicycle
There are many hills in my area and it can be a pain to bike up each one on my daily bike commute. To speed things up, I decided to attach an motor to my bicycle. It's a great project that's useful, fun, and can teach you a lot about basic small-engine mechanics. WARNING: If you decide to do this project, check your local laws on the legality of motorized bikes and where you can ride them. Also, be safe.
A little research shows that kits are readily available to motorize your bike in both electric and gas powered options. There's tons of information on the web detailing the relative merits and drawbacks of gas versus electric, but it basically boils down to this: Electric is clean, but weaker and costlier; Gas power pollutes, but gas motors are stronger and cheaper. Mostly for the sake of cost, I decided to go for a gas powered bike.
Kits for converting your bicycle to a gas-powered motorized bike run around $120 if you go for a 2-stroke engine (which I did). Typically what you're getting is a chinese-made, nominally 80cc (actually a little less), single-cylinder engine, a gas tank, a carburetor, and all the components needed to install them onto your bike. They are comphrehensive kits, but often low quality, so expect to possibly have to replace some of the hardware if it breaks. Also look out for the bicycle specs required by the kit. There needs to be enough space in the V made between the down tube and the seat tube to fit the motor as well as the carburetor, which attatches to the side of the cylinder. I was able to avoid these two issues by buying a broken, used motorized bike and then putting a working motor, carb, and tank onto it from a new kit. That way I got a bike that already fit the motor and was able to replace some small things, like the chain tensioner, when they broke.
Despite the cheap quality on some parts like the soft steel bolts, the reliability of the actual motor and carb can be pretty good. The engine I got only lost compression once, and that was remedied quickly by tightening the headbolts. As of this point, the bike's been running for almost a year with almost no service. It's been dropped, abused, run in the rain, and taken almost 20 miles on one ride, but it's still ever-dependable to take me where I need when I need it.
There's wealth of information online about how to set up kits like this, so I won't bother going over that. Instead, here's just a couple tips from personal experience:
Bend on exhaust pipe
1. Chances are, even if you pick a bike with the right dimensions to fit the motor, the exhaust pipe will still stick out in the way of the pedals. The solution to this is to bend the pipe. Don't bother trying to bend it cold. Some people report that it's possible, but you'd have to be superman (or at least have some powerful tools) to bend steel pipe cold. Instead, pick up a small torch, like a Bernzomatic propane torch. Heat up the pipe part of the exhaust (not the chamber) until it glows red and then lean on it until it bends out of the way of the pedals. I recommend not heating it too liberally because it can cause the pipe to get too soft and collapse on itself. Heat it just enought to bend.
As you can see in the picture, I heated the pipe for too long and it collapsed slightly. The result was a slower top speed (22mph instead of 30+mph), but great low-end torque. Seriously, this thing takes steep hills like a champ. Anyhow, the pipe collapse wasn't detrimental, but it should be avoided.
Heating will leave a blue-purplish color on the chrome (see the picture), but nothing too unsightly. Also, if you have a powder-coated pipe (some are painted black), you should rub off the paint before heating it because the paint will burn and release some nasty chemicals.
A final note about the exhaust: don't touch it. That should be obvious, but there's been times when the engine was still cool but I unthinkingly touched the exhaust and it was much hotter. Also, watch out for burning your calves on the exhaust when wearing shorts. Speaking from experience, it's not too bad. But it can definitely be avoided if you're careful.
Lever configuration on the motorized bike
2. Your shifters and brake lever might get in the way of the clutch lever. Some people say to just take them off and hook up the bike to only use the front brake, but trust me, you'll want all the braking power you can get. Instead, keep the shifters and left brake lever and just turn the clutch lever 180 degrees so it faces towards you. That way, you can push in the lever with your palm while riding and still have access to the brakes with your fingers. If you adjust the angle right, you should even be able to leave the clutch out and comfortably hold the grip while you're motoring without the lever getting in the way of your wrist.