Test riders loved this new big-bore 400cc enduro. Go just about anywhere, do just about anything and do it with zip! The DT1 250 had grown up, first to 360 size, and now 400, and had become an extremely seductive machine.
Weighing less than 300 pounds, with equal amounts of horsepower and torque, the oversquare single-cylinder two-stroke could tackle a dune in the Mojave Desert, climb gnarly trails high in the Rocky Mountains or hare along old logging roads in Maine.
These magazine riders were pretty darned experienced, and enjoyed pinning the throttle, fore and aft, making maximum use of the power at all times. However, when it came down to puttering along rural roads, the 400’s engine was not quite as smooth as many relaxed riders would have liked—good for the hurly-burly of competition, or the adrenalin-seeking individual off on his own, not so comfy for the commuter crowd.
We should go back to the beginning, which was that excellent DT1 250 of 1968. The Japanese had finally realized that there was a specific “American” market, which did not exist in Japan or Europe—and it was those riders who wanted to make use of our vast publicly owned wilderness, and do it at a pace that could give a minor rise to the adrenalin level. The misnamed “street scramblers” were just not suitable, being way too street oriented and not nearly scramblerish enough. One of the real blessings of this great land of ours are those huge expanses of unoccupied land, and all that was needed was a properly set up motorcycle to enjoy the tens of thousands of miles of dirt roads and trails…and a big gas tank was very useful, too.
The DT1 was a superb motorcycle right off the drawing board, as its two-stroke, piston-port single with five ports provided the good lowend grunt that play-bikers wanted. Yamaha realized that there was a big difference between a machine that could win races and a machine that a noncompetitive rider would enjoy.
This unitized powerplant was bolted into a cradle frame using a double loop, plenty strong enough to cope with the claimed 22 horsepower. It was great fun without being too finicky. And just in case somebody wanted to race it, a Genuine Yamaha Tuning kit with a new cylinder, piston, expansion chamber, etc., was available as well.
Dealers could not keep them in stock. And the competitive MX and YZ models that followed were eagerly snapped up by racers. As was the smaller 125cc AT1 Enduro that soon followed, and, in 1970, the bigger 351cc RT1 360 Enduro with a bore of 80mm, stroke, 70mm. Then in 1974 the RT1 was modified and reintroduced as the DT360A Enduro, the Yamaha marketing types realizing that the success of the original DT1 250 gave value to those DT letters. The DT360A used a lot of MX parts, including the chassis, to give it both sportier handling and more power. Also new was the reed-valve induction system and CDI ignition, which did away with the oft-times bothersome points.
But the DT360A was a one-year only machine, as Yamaha soon realized that there had not been enough differentiation from the previous RT—so lo and behold, in 1975 the DT400B appeared, the 360A having been bored out to 85mm for a total of 397cc. It was crowned with a very sexy new head with radial finning. The Mikuni carburetor was the 32mm size, and with the reed valves this made for a rather thirsty engine, getting only 30 mpg or less when honking down a sand wash—and the smallish gas tank only held 2.4 gallons. That certainly restricted the range, even though the oil tank for the automatic lubrication held 1.6 quarts, and was good for two full gas tanks.
Primary power went through helical gears to the multiplate clutch, and then into the five-speed transmission. At the rear wheel the dyno measured almost 24 horsepower at 5,500 rpm, and 24 lb-ft of torque at 5,000 rpm. Happiness for the experienced rider was best found at 5,000 rpm, and in fifth gear on a straight stretch of road the bike could happily exceed 80 mph.
However, with a steepish rake of 30 degrees, and 5.1 inches of trail, such speeds were not pursued for long. The fork used progressive springing with three stages; the Thermo-Flow shocks at the rear had double springs and remote reservoirs. A 3.00 x 21 Dunlop Trials tire was on the front, a 4.00 x 18 on the back…though a 4.50 knobby could be squeezed in. The single-leading-shoe brakes were intended for dirt riding, and could cause a mild panic when attempting a fast, unplanned stop on the pavement. The wheelbase was 56 inches.
The biggest problem was in starting the bike. A 400cc single two-stroke is a hefty engine to get fired, and while the automatic compression release worked like a charm, the CDI could be less cooperative when starting off first thing in the morning. Also, when the engine was hot, a rider could spend a lot of energy kicking. As one magazine put it, there was no sure-fire drill that could be followed, and starting lacked any predictability.
Once running, the DT400 was a trifle lurchy on the road, as the engine did not really take kindly to a constant throttle—unless it was constantly wide open. On first look, dealers thought they would sell these bikes by the thousands, even at $1,370. However, a year later the noncurrent 1975 models were on sale for $948. Even with the less-than-expected sales, Yamaha went on improving the DT400, making the rear suspension a mono-shocker for 1977.
Yamaha was also trying to enlarge the market for these big-bore twostrokes, offering a race version YZ400, and for 1976 they put lights and a muffler on the YZ and presented it as a serious enduro model, the IT400, trying to compete with the European ISDT models. But the two-stroke era was coming to an end, and Yamaha knew it.
Which is why in 1976 Yamaha introduced a 500cc four-stroke single in the road-legal XT and off-road TT versions. The DT400 was kept on the payroll until 1979, and then retired.
(This Retrospective article was published in the February 2010 issue of Rider magazine.)