A "moped" by legal definition is a vehicle that can be powered either by pedals, or by a combustion engine not capable of surpassing 30 mph in most states. Home-built mopeds can be a fun and efficiently way to get around, and most state laws say they can legally be operated on public roads without license, helmet or registration. This is one way to do it, but there are endless configurations.
Essentially, what you'll be building is a gasoline-powered bicycle, which is a moped (a portmanteau of "Motorized" and "Pedal" powered) by any definition. One of the simplest ways to do so is to use a Cruiser Style frame (like Wal-Mart's Del Mar), and install a two-stroke motor kit. These kits are available from a number of sources online, and the better ones run about $150. Cheaper kits are available for around $100, but these are almost always made by Chinese or Korean manufacturers, and many are notorious for unreliability.
Cruiser-style bikes have classic good looks, and the curves of a pre-war Indian motor-bike. The bad news is that most are single-speed bikes designed for tooling around town, which will make it almost impossible to pedal-assist the motor at anything over 20 mph. Get a bike with front and rear hand brakes, as opposed to the pedal brakes found on cheaper models. These pedal brakes lack the one-way clutch that allows the rear wheel to move without engaging the pedals.
Most people utilize chainsaw-style two-stroke engines, as they are generally light, cheap, powerful and reliable when properly maintained. However, they do lack the low-end torque of a more expensive four-stroke engine. Four-stroke engines are better for heavier riders using steel-framed bikes, and don't require the addition of oil to the gasoline for lubrication.
Either way, most states require that the engine be 49 cc or less in displacement and be rated at less than two horsepower. The word "rated" is operative here; though the engine must be factory rated at less than two horsepower, there are rarely any state laws that specifically prohibit modification. Almost all states forbid any sort of transmission that requires clutching or shifting while in use, but most are silent when it comes to automatics.
The assembly and tuning process varies from kit to kit, but the basic premise is to mount the engine to the frame and connect it via chain or belt to a sprocket mounted on the rear wheel. Whatever kit you purchase will come with instructions, but the instructions that come with American-sourced kits are usually the most detailed. When used with the intended frame, a motor kit should take less than three hours to install for a first-timer. An average 49cc kit will sustain 30-35 mph with a180 lb rider.
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