The Yamaha RD400, built between 1975 and 1980, is a 400 cubic-centimetre, two-cylinder, two-stroke motorcycle known for its quick acceleration and racing capabilities. Closely related to the popular RD350, the RD400 was developed to be bigger, smoother, quieter and faster than its 350cc counterpart. The increase in engine size required a complete redesign to add 51cc to the engine, including new crankcases, cylinders and cylinder heads. According to Real Classic's Yamaha RD400 vital statistics listing, the factory issued carburettors were two VM 28mm Mikunis, one for each cylinder, identified by Yamaha Part numbers, Mikuni markings and measurements.
Inspect the carburettors. The carburettors are located behind the cylinders toward the back of the motorcycle and will have "Mikuni" cast on the sides of each, followed underneath with "Japan." A serial number unrelated to identification is between these two markings. This identifies the carburettors as genuine Mikunis.
Look for the Yamaha part number. According to RD400.com, the part numbers for the RD400 left carburettor is 1A1-14101-00-00 and the right carburettor is 1A1-14101-00-00. The first two sections of the part number may be stamped somewhere on the carburettor assembly. The part number is not always stamped on the carburettor.
Measure the front spigot of the carburettor. Loosen the clamps and remove the rubber hose on the front spigot. The spigot hose runs from the carburettor to the cylinders Measure the diameter of the carburettor spigot with a machinists ruler. According to Motorcycle Carbs.com, the diameter of the spigot on the Mikuni VM28 is 35mm.