Don't look for older model Yamaha XT and TT engine model parts on Yamaha's latest 4-stroke offering. Yamaha built the YZ 400 engine based on computer modeling studies. It shares little, if anything, with its ancestors.

Power is generated in the ceramic composite plated cylinder via 5 valves: three intake valves and two exhaust valves. Chain driven, dual overhead camshafts perform the valve lifting duties. A single port exhaust exiting into a single header pipe performs the exhaust chores

Advanced technology abounds on the YZ 400. The new YZ 4-stroke engine is designed to run on 92 octane gasoline. This unusual stoichiometric accomplishment is achieved by employing a very unique carburetor and digital (computer microchip) ignition system.

This ignition system works like this:
Throttle position, along with the engine RPM, determines the ignition timing. The throttle positioning sensor, located on the 39mm Keihin carburetor, and magneto constantly send electronic signals to a microprocessor. This computer chip in turn advances or retards the ignition timing. This adjustable timing, along with the accelerator pumped carburetor, provides unusually strong engine performance. A nice touch to the Keihin carburetor includes bearings in the throttle slide guide. This makes for easier throttle twisting work.

The 400's transmission ratio's are the same as the '98 YZ 250's; however, the transmission gears and shafts aren't interchangeable between the models.

Chassis wise, the '98 YZ 400 and the '98 YZ 250 share the same chassis! The differences are the YZ 400 carries an oil tank near the steering head, some of the engine/lubricating oil is carried in the front part of the frame, and the YZ 400 has stiffer fork and shock springs. Rake, trail, forks, shock, and swingarm linkages are the same as the YZ 250.