The Yamaha YZ125 is a motorcycle

Motorcycle

A motorcycle is a single-track, two-wheeled motor vehicle. Motorcycles vary considerably depending on the task for which they are designed, such as long distance travel, navigating congested urban traffic, cruising, sport and racing, or off-road conditions.Motorcycles are one of the most...

 with a 124 cc two-stroke engine  designed for motocross

Motocross

Motocross is a form of motorcycle sport or all-terrain vehicle racing held on enclosed off road circuits. It evolved from trials, and was called scrambles, and later motocross, combining the French moto with cross-country...

 riding and produced by Yamaha Motor Company

Yamaha Motor Company

, is a Japanese motorized vehicle-producing company. Yamaha Motor is part of Yamaha Corporation and its headquarter is located in Iwata, Shizuoka. Along with expanding Yamaha Corporation into the world's biggest piano maker, then Yamaha CEO Genichi Kawakami took Yamaha into the field of motorized...

. It was first produced in 1974 (originally titled "YZ125 Monocross") and has continued production with a new model being produced every year to date. The YZ 125's produced from 1996 to 2002 are considered by some to be the best dirt bikes ever, due to its wide powerband and excellent handling. The YZ125 has been ridden to five AMA National Motocross Championships, and multiple AMA Regional Supercross Championships.

In 2001, Yamaha released a bike designed to complement the YZ125, the four-stroke YZ250F. The two bikes share a rolling chassis and are eligible to compete in the same racing class.

Engine

The YZ 125 has had a liquid-cooled 124 cc, reed valve inducted, two-stroke engine since 1981. The models from 1974 to 1980 were air-cooled and also two-stroke. It breathes through a 38 mm TMX carburetor

Carburetor

A carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....

 made by Mikuni

Mikuni

is a leading Japanese carburettor manufacturing company that was founded in 1923 and incorporated in 1948. Their business activities are centred on carburettors, fuel injectors and other automobile related equipment...

. Stock, the engine produces 33 hp at the rear wheel.

Transmission

The YZ125 has been built with five- or six-speed sequential gearboxes depending on model year. The 2005 model has a Constant mesh wet, multiple-disc coil spring clutch

Clutch

A clutch is a mechanical device which provides for the transmission of power from one component to another...

.
The revised gear ratios as of the 2005 model are:

• Gear Ratio - 1st Gear 31/13 (2.385)
• Gear Ratio - 2nd Gear 29/15 (1.933)
• Gear Ratio - 3rd Gear 27/17 (1.588)
• Gear Ratio - 4th Gear 23/17 (1.353)
• Gear Ratio - 5th Gear 24/20 (1.200)
• Gear Ratio - 6th Gear 23/21 (1.095)

Frame

From 1974 through 2004, the YZ 125 had a single backbone frame made from steel. It generally averaged from 176 pound. For the 2005 year, however, Yamaha switched to a single backbone frame constructed from an aluminum alloy. This frame material change dropped the weight to 190 lb.(dry weight) For 2008 models, additional weight savings in the wheel assemblies and front fork suspension have yielded additional weight savings making wet weight, no gas sub-200 lb. Aluminum framed YZ125's are notably 'flickable' and sometimes this trait is seen as a drawback since they tend to become more difficult to control on rough surfaces.

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