Yamaha RD 400
Yamaha RD 400
Air cooled, two stroke, parallel twin cylinder,
reed-valve 'torque induction
Bore x Stroke
64 х 62 mm
2x 28mm Mikuni carbs
Battery, dual coils, breaker points
44 hp @ 8000 rpm
3.8 kg-m @ 7500 rpm
6 Speed / chain
265 mm disc
Braking 60 - 0 / 100 - 0
- / 37.1 m
14.1 sec / 90.1 mp/h 145 km/h
106.8 mph 172 km/h
Road Test 1979
They are gone now. The whooping, tire-smoking,
Kawasaki triples. The smooth, civil, dependable three-cylinder Suzuki GTs. Even
the venerable Suzuki T500 twin. Kawasaki and Suzuki once manufactured two-stroke
motorcycles exclusively, but now four-stroke bikes hold sway over their American
It wasn’t government regulation that doomed the
road-going two-stroke motorcycle. Nor a shattering improvement in four-stroke
technology. It wasn’t the industry’s lack of faith in the viability of the
two-stroke design. Americans just don’t want motorcycles that go poppity-ding.
As far as they are concerned, engines with cams and valves and a hearty exhaust
note belong on the highway. Engines that go poppity-ding belong in lawnmowers
There’s only one two-stroke street motorcycle in
the American market now, the Yamaha RD400E. Ironically it’s the same motorcycle
that did much to foster the enduring myths of the two-stroke performance. In its
previous R5 and RD350 incarnations, it fouled spark plugs, made indifferent fuel
milage, did wheelies at the slightest provocation and had a torque curve as
steep as the back of God’s head.
Of course, the RD400 is pretty civilized now.
When sales of the RD350 began to taper off in 1974 and 1975, Yamaha gave its
rowdiest high-performance motorcycle 50 more cubic centimeters and some
friendlier manners. The engine became more flexible, the ride turned soft and
the seat got thicker. As a result, the RD is as docile around town as a Honda
Hawk. You can slog it between the stop signs with a minimum of noise and
gear-changing like a commuter bike. On the freeway, you slip the gearbox into
sixth and loaf along at 4500 rpm.
In fact, -if you fail to venture into the upper
limits of the RD's tachometer you'll notice this two-stroke is no harder to ride
than most other motorcycles. It doesn't take more than a half-hearted stab at
the kickstarter to urge the RD to life. You could catch your pants leg on the
lever and still light it off. But instead of being rewarded with a sound like
sheetmetal screws rattling inside a glass jar, you'll hear the flat, hollow
crack of a two-stroke, as if a particularly expensive sort of popcorn were being
brewed up. It takes a little care to get the RD rolling because of a lack of
heavy flywheel effect and an abrupt clutch engagement, but only a little care.
And when you crank in more throttle, the RD always responds promptly and
smoothly. All sorts of carburetion strategies have been tried to make
four-stroke engines respond efficiently, but simple Mikuni slide/needle carbs do
the job on the two-stroke RD.
The only glitch in the RD400's performance is a
tendency to surge at lightly-loaded cruising speeds. It's particularly annoying
while riding in traffic. The only remedy is to accelerate slightly or shift into
a higher gear to increase engine load.
Other than this complaint about surging the RD400
never fails to provide a comfortable ride. The rubber-mounted engine, exhaust
pipes, foot pegs, handlebars and gas-tank filter out most of the vibrations. So
if you're intent on comparing this motorcycle to the gaggle of 400-cc
four-stroke twins on the market the Yamaha performs far better than the myths
about two-stroke idiosyncrasies would have led you to believe.
Underneath its raincoat of civilized manners
lurks the heart of a flasher. The RD is still a high-performance machine. Below
4000 rpm. the RD accelerates slowly but smoothly. It might even seem that the
throttle simply acts as a volume control for engine noise. But as the tach winds
past 4000 rpm, power builds until the engine bursts into life at 5500 rpm.
Between 5500 and 8000 rpm, the Yamaha is on the pipe, pulling like a roadracer
until it runs out of breath at the 8500-rpm redline.
If you want to keep the RD in its pocket-rocket
mode, you have to enjoy using the gearbox. Unlike Suzuki's old GT two-strokes,
the RD wasn't designed for high-gear roll-on acceleration. When you want to
climb a hill or pass another vehicle, it's best to shift down a gear.
Unfortunately, some riders during our test complained that too much pressure on
the shifter was required to change gears, possibly because of a shift-return
spring that's too stiff. Missed shifts into sixth gear were another complaint.
We understand that this behavior is caused by the shift mechanism failing to
rotate the shift drum far enough. An adjustment of the shift-stop eccentric
should cure this problem however.
On the whole, the RD400 is the sort of motorcycle
that rewards a rider who wants to participate. You don't just straddle the
RD400—you operate it. When the engine is warm, twisting the throttle wide open
as you come off the line will cause the front wheel to clear the ground by a few
inches. But when the engine is relatively cool, the same application of throttle
can produce an attention-getting wheelie. But if you keep the skyshots under
control, the Yamaha will hustle down the quarter-mile in 14.35 seconds at 90.4
mph, several heartbeats ahead of its nearest rival, the Honda Hawk (14.73
To match that performance, Yamaha added mag
wheels and disc brakes to the RD formula at the same time it bolted on self-cancelling
turn signals and a thick seat. The Yamaha's light weight, good tires and strong
brakes also encourage you to get the most out of braking. Under controlled
conditions, the RD screeched to a 124-foot stop from 60 mph and halted in 28
feet from 30 mph. Unfortunately, it's also easy to lock the brakes unexpectedly,
especially the rear disc. This problem with the rear brake led to some large
bobbles when diving into the turns of a few mountain roads.
During serious swoop sessions, you really begin
to appreciate how much you can demand from the RD. But at the same time the bike
also demands a lot from you. Though it's hardly a flyweight anymore, the RD is
still lighter than anything else that's as fast. And the weight feels as if it's
in the right place. So you can flick the Yamaha from side to side almost as fast
as you can think. The steering is ultra-quick and the bike leans over easily,
but it's still possible to make sudden changes in your line, even at the limit.
You do, however, have to be aware of changes in the road surface while riding
this Yamaha. A cam-her change, humps or even a change in your line at the wrong
time can cause the RD to get wiggly. This flaw seems partially a function of the
soft suspension and, on our test bike, rear shocks that lost some damping after
only a few thousand miles.
When you're trying real hard in the swoops and
darts, the RD will run out of cornering clearance long before it runs out of
grip. Although the suspension has been stiffened slightly since we last tested
the RD400C in 1976, sporting riders will still have an easy time of grinding the
footpegs because their mounting brackets are routed beneath the pipes. But at
least the rubber-mounted brackets allow the footpegs to flex a little when they
touch down so they don't help you off the road.
There are other side effects to keeping a
two-stroke motorcycle like the RD on a leash in your garage besides its
wonderfully schizoid personality. Compared to a four-stroke motorcycle,
maintenance requirements are limited. There are no valves or cam chain to
adjust. No oil filter. Only ignition timing and carburetor synchronization are
required to help the RD400 feel its Wheaties. Our bike smoked heavily just after
starting and when run hard after a casual slog, but we averaged just over 400
miles to a quart of injection oil. Not all of that oil went through the engine,
however, because a construction error in the oil tank breather allowed it to
puke Yamalube for nearly 100 miles after each top-up. Yamaha has never heard of
this problem before. Certainly our two previous RD400 test bikes never leaked,
so we must assume this oil-loss was peculiar to our test bike. And in over 4000
miles of all kinds of testing, our bike never threatened to foul a spark plug.
The only problem with the RD400 is the fact that
you're forever forced to explain so much to the inquiring public. You have to
explain that this motorcycle doesn't foul spark plugs, that it doesn't smoke
badly, and especially that it won't try to spit you out of the saddle every time
you twist the throttle a little aggressively.
Considering the crisis of confidence in the
two-stroke motorcycle as a whole, even the persistent appearance of the RD400 in
the Yamaha line-up must be explained. Because the RD400E is virtually identical
to the last year's D-model, Yamaha manufactured these motorcycles last fall to
avoid the 1978 emissions regulations and market this bike as a new model.
We can only speculate that Yamaha itself truly
appreciates the RD400's stature as both the last remaining two-stroke motorcycle
and the only small-displacement, high-performance road bike on the market.
Still, tough new emissions laws are waiting in the wings. Yamaha seems prepared
to deal with the problem because there will be a 1979 RD400. But Yamaha's faith
in this motorcycle must be matched by renewed public enthusiasm for this
company's interpretation of two-stroke performance. Because if RD400 sales
continue to decline. Yamaha will find it economically unfeasible to continue
selling the bike in this country.
If any bike can turn the tide in the war against
the two-stroke. it will be the RD400. It has helped to explode the two-stroke
myths. It is the most modern, most completely finished two-stroke motorcycle
ever cut loose for the American market. Even more compelling is the fact that
the RD is just a good motorcycle. The measure of its design concept is the fact
that the RD can alternately perform as a dead-reliable commuter bike and as a
keen-edged street-racer. And that is why it should survive.
The RD400E is not significantly different from
the RD400C introduced two years ago. The machine did not have to change to keep
up with its competition because it has no competition. Most of the machines that
provide equal or superior performance weigh almost 100 pounds more than the
RD400. And most of them have ten times as many parts to wear out. And few
motorcycles of any size or type are as comfortable to ride as Yamaha's RD.
The RD400 is an exceptional motorcycle partly
because of—not in spite of—its scornful two-stroke engine and partly because of
the machine's careful engineering and many years of development. The RD's
crankcase seals are one example of Yamaha's thoughtful engineering. Crankcase
sealing is commonly done with neoprene lip-type seals. Though their life is
long, these seals do wear out. When the seals are located at the ends of the
crankshaft they can be easily replaced, but on multi-cylinder engines the seals
between crankchambers can normally be replaced only by pressing the crankshaft
apart. And that's a big job.
Yamaha solved this problem (though they were not
the first to do so) by using a labyrinth seal between the crankchambers. The
labyrinth seal does not actually contact the crankshaft and therefore will not
wear out. Labyrinth seals work by creating a maze (a labyrinth) that discourages
leakage. They are not positive seals because they do leak, but changes in
pressure between crankchambers are so rapid, even at idle, that the leakage is
thoughtfulness of Yamaha's engineers is also
evident in their efforts to prevent light-load surging. To lessen surging, the
exhaust skirt of each RD400 piston has a notch at its bottom that connects the
crankcase directly with the exhaust port at top-dead-center. There is also a
4-mm hole located 13 mm above the exhaust port and connected with the exhaust
port. This small passage not only reduces surging but makes the engine easier to kickstart as well by lowering cylinder pressures at cranking speeds. At higher
engine speeds the passages are too small to significantly affect performance.
Yamaha has used reed valves for many years, but
it is worth recalling that they were one of the first companies to use them
successfully in high-speed engines. At high speeds the reeds flutter at an
incredible rate and as a result their life span was short. The technique that
Yamaha adopted, which has since become the norm, was to apply a thin rubber
coating to the reed block. As the stainless steel reeds repeatedly slam closed
against the reed block, the rubber provides just enough cushion to prevent
In-line twin cylinder engines have a reputation
for being shakers. Many twins, the Suzuki G5400, the Honda Hawks and the
Kawasaki KZ400 for example, use supplementary counterbalance shafts to offset
inherent engine imbalances. But these shafts are heavy and costly, and their
drives are sometimes both noisy and troublesome. But when of the same
configuration, a two-stroke engine will be smoother than a four-stroke engine
because the two-stroke has less reciprocating mass and its power impulses are
softer and more frequent. Still, the RD400 engine does vibrate.
Rather than complicate the engine with auxiliary
balancers (which they have used on several engines), Yamaha has built a smooth
motorcycle by isolating the engine's vibrations from the rider. The engine,
footpegs, handlebars and fuel tank are all mounted in rubber. The engine
vibrates but most of the vibration does not reach the rider.
The flexibility of the motor mounts may in some
small way affect handling, however. When thrown into a hard sweeper, the RD400
does not feel as solid as an RD350. Certainly the 400's soft suspension accounts
for much of this feeling but the rubber engine mounts might also have an effect.
The RD350 and RD400 use very similar frames but the solidly-mounted RD350 engine
stiffened its frame. Because the RD400 engine is rubber mounted, it contributes
very little to the rigidity of its chassis.
As a consequence, the RD400 frame would have to
be more robust to achieve the rigidity of the RD350 chassis.
The RD's clutch is basically a conventional wet,
multi-plate design but for one interesting characteristic: The drive plates are
not symmetrical discs. Some material has been cut from one side of each disc to
intentionally make them out-of-balance. As the clutch spins, centrifugal force
holds the light side of the plates against one side of the hub and so prevents
the plates from rattling. Also when the clutch is assembled, the light sides of
the plates are staggered so that the clutch assembly is not thrown
out-of-balance. Several other Yamaha 'models share this feature.
Another interesting, if less unique, clutch
detail is the use of rubber rings between the plates to prevent clutch drag.
When the clutch is engaged, the clutch springs force the plates together and
slightly compress the rubber rings. When the clutch is disengaged, the rubber
rings expand and help separate the plates completely.
The RD400 engine/gearbox unit is a light package
despite the motorcycle's 353 pounds. The RD400 is about 25 pounds heavier than
an RD350 but the 400 engine is only about five pounds heavier than the 350
engine. The RD400 motorcycle is actually heavier than the four-stroke XS360D and
is as heavy as the Honda Hawk Type I (the kickstart-only model), but the RD
engine is more than 20 pounds lighter than the Hawk engine. Any way we look at
it, the RD400 is about 20 pounds heavier than it should be.
We suspect that some of this weight is in the
wheels and brakes. Yamaha claims that each RD aluminum wheel is only a quarter
of a pound heavier than a wire-spoked wheel, but we think they may have been
using a very heavy wire wheel in the comparison. The RD400 also has
double-action brake calipers at both wheels, though most machines use lighter
single-acting calipers on the front wheels.
Even though the RD400E could be a few pounds
lighter, it is a well thought-out motorcycle. It is also, unfortunately, an
We recently attended a Motorcycle Industry
Council conference on motorcycle exhaust emissions regulations. In addition to
the depressing information that was the subject of the meeting, we were told
that 1978 marked the virtual end of the road-going two-stroke motorcycle. We
have heard this rumor too many times.
Those people, at the MIC conference and
elsewhere, who have doomed the two-stroke road bike to extinction insist that
the facts could hear no other logical conclusion. Fact: Shortly after the
Federal government made evident its intention to regulate motorcycle emissions,
Suzuki began developing four-stroke engines and subsequently stopped importing
two-stroke road machines here. Fact: Yamaha evaded the 1978 emissions standards
by producing all the RD400Es destined for the United States before January 1,
1978. Deduction: Two-stroke engines are not able to meet Federal exhaust
If Sherlock Holmes had been so adept at the
science of logic, Professor Moriarty might have been the youngest King of
England. For if any of the soothsayers at the conference had bothered to ask,
Yamaha would have informed them that there will, in fact, be a 1979
When we tested the RD400C in January 1976 we said
it was the best bike in its class. It was fast, comfortable, simple, practical
and relatively light. In the following months we received many letters from
people who bought RDs on the basis of our test and who said they were in
complete agreement with our conclusion.
The two-stroke engine could be, and perhaps
should be, the powerplant of the future. But if the public continues to believe
the old wives' tales; if they ignore this final example of the type; we can look
forward to riding motorcycles that are heavier, more complicated and more
expensive to maintain than they have to be.