In 1975 the DT 400 appeared. It had been bored out to 85mm for a total of 397cc. The carburettor was a 32mm Mikuni and with the reed valves this made for a rather thirsty engine.
The fuel tank, which held just nine litres, certainly restricted riding range, even though the oil tank for the automatic lubrication held 1.5 litres and was good for two full tanks of petrol.
At the rear wheel the dyno measured a claimed output of almost 24hp (18kW) at 5500rpm and 24lb-ft (33Nm) of torque at 5000rpm. In fifth gear on a straight stretch of road the bike could happily exceed 130km/h.
The fork used progressive springs with three stages. The Thermo-Flow shocks at the rear had double springs and remote reservoirs.
The biggest problem with the DT 400 was starting the thing. A 400cc single two-stroke is a hefty engine to kick over and while the automatic compression release worked like a charm the CDI could be less cooperative first thing in the morning. Once running, the bike was a little rough on the road, as the engine did not like sitting on a constant throttle unless it was held wide open.
Yamaha was trying to grow the market for big-bore two strokes, offering a race version YZ400 and, in 1976, adding lights and a muffler to the YZ and presenting it as a serious enduro model called the IT400. In 1976 Yamaha introduced a 500cc four-stroke single in the road legal XT and off-road TT versions. Nevertheless, the DT 400 stayed in production until 1981 when it was retired.
$200 to $500 (The Red Book, May 2014)
The single leading-shoe brakes worked okay in the dirt but struggled to pull the bike up quickly on the road.
The head and cylinder were covered in large cooling fins designed to dissipate engine heat into the passing air. The fins were painted black but had a cool polished edge to them.
The 32mm Mikuni carburetor, together wit reed valves, large engine ports and a 85mm piston, meant these engines chewed through the 9-litres of fuel
really quickly, but packed a punch while doing so.
The twin rear shocks had progressive springs but the DT 400 was never designed to handle massive jumps.
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