Project SiR The Bicycle Pages Project Yamaha RD400F






The Last Two Stroke Sport Bike Holds One Advantage Over Its Four-Stroke Rivals: It Wheelies At Will.

[Note: This article, from the July 1979 issue of Cycle World, covers the U.S. model RD400F; the Canadian model, which I have, does not have the dashpot mechanism found on the U.S. model, and the Canadian model does have electronic ignition, instead of the U.S. model's points ignition.]

The Yamaha RD400 is still with us, carrying the flag as the largest two-stroke street bike now sold in the United States. 1979 is the RD's fourth year of production and the machine that made its name as the slightly bigger replacement for the RD350 finds itself in a changed marketplace. If you wanted a fast lightweight motorcycle in 1974, you bought an RD350. If you wanted enough cheap horsepower to embarrass 750cc riders with less-than, lightning reflexes in stoplight GPs in 1976, you bought an RD400. Anybody who wanted to be competitive in the 400cc class--either Box Stock or Production--at 1977 club road races had to have a Yamaha 350 or 400 of one vintage or another.

Times change. With more and more emphasis on clean air, riders became self conscious about leaving clouds of blue smoke behind at intersections. Two-strokes became a Social embarrassment. Writers prone to prediction declared that increasingly strict exhaust emissions standards would kilt off the two-stroke street bikes. Motorcycles would be four-strokes. Heavier. Slower. Less fun.

It didn't happen that way. The two-strokes have given way to four-strokes because most people bought four-strokes. That the RD400 is alive today is testimony to its performance image. Yamaha engineers have met the stiffer emissions standards with typical cleverness and skill, making the RD400F cleaner, quieter and just as fast as the earlier models.

In the meantime the four-strokes have gotten faster. The last comparably equipped Honda Hawk we rode ran the quarter-mile in 14.08 sec. at 90.90 mph. Our test RD400F did the quarter in 14.18 sec. at 91.37 mph. Honda Hawks are taking over the 410cc Box Stock class in California club races, with several Hawk pilots winning and placing in the class. And in American Federation of Motorcyclists (AFM) 410cc Production (the equivalent of Modified Production m many parts of the country), Cycle World's project Hawk has been finishing in steadily improving positions, from fourth in its first two races to second in its third event, with more modifications to come. Clearly, the four-strokes demanded by the buying public are not the heavy slugs once predicted to follow the demise of two-strokes. The RD400F instead faces a marketplace dominated by highly competent, competitive 400cc four-strokes.

Yamaha tells us that the RD400F appeals to the younger male, average age 23, who would like a larger bike but can't afford one. He's attracted by the RD's sporty image and wants quick acceleration and handling. The RD400F's styling and marketing are heavily keyed to the overwhelming success of the TZ250 and TZ350 road racers, which explains the 1979 RD's full name, the RD400F Daytona Special.

The reaction to this cafe-oriented machine is, as might be expected, mixed. Specials are the hot sellers, but the laid back, semi-chopper Specials without the "Daytona" prefix. The RD400F sells best in areas on the East coast and around the San Francisco bay area of northern California. It isn't as popular in middle America, nor in Southern California. The machine is, in many ways, a cult bike, which contributes to its concentrated areas of popularity in a sea of customized semi choppers selling well across the country.

It is no longer less expensive that its four-stroke competitors, selling for a suggested retail price of $1694. A Honda Hawk II with comparable equipment sells for $1698, while an economy Hawk I (with spoke wheels, kick starter, drum front brake) is only $1498. Yamaha's economy model four-stroke XS400 lists for $1559.

Pic: RD400F Side Shot But the RD400 remains relatively simple. The engine is a 398cc air-cooled two stroke Twin with reed-valve induction and 28mm Mikuni carburetors. It's oversquare, with bore and stroke of 64 x 62mm. Lubrication is provided by an injection pump which feeds oil into the intake manifolds at a rate determined by engine rpm (the pump is driven off the crankshaft) and throttle opening (the pump is connected to the throttle linkage by a cable). Helical gears drive the wet clutch, which powers the six-speed transmission.

Several significant changes have been made for 1979, some dictated by clean-air requirements, others not. The RD400F is fitted with a butterfly valve at each exhaust manifold to decrease emissions during the deceleration mode of EPA testing. By reducing emissions in that one mode, the overall score is lowered enough for the bike to pass the overall test, yet performance is not significantly compromised.

The valves are controlled by engine vacuum and throttle opening. When the carburetor slides are raised less than 3.3mm, a valve located at the carburetor linkage remains closed. With that control valve closed, vacuum from the intake manifolds opens a dashpot located underneath the carburetors, behind the cylinders. That dashpot pulls butterfly valves closed via an adjustable rod linkage which runs underneath and between the cylinders to the exhaust valve assembly.

When the carburetor slides are raised more than 3.3min, the control valve opens a line leading from the airbox, dashpot vacuum drops, and the butterfly valves spring open. The valves are either open (under most conditions) or closed (under deceleration and very small throttle openings) and do not hold at any position m between. When the control valve is open, the line from the airbox constitutes a coitrolled air leak, which is compensated for in carburetor jetting. (A one-way valve in the intake manifold vacuum lines prevents any reversal in the emissions system air flow.)

The valves are effective because a twostroke is less efficient with the throttle closed, especially when decelerating. The rich air/fuel mixture tends to run right through the cylinders and out the exhaust without complete combustion, and the lack of compression braking effect allows a decelerating two-stroke to coast with the throttle shut for relatively long distances. That adds up to lots of emissions during deceleration. The butterfly valves work on a sort of potato-up-the-exhaust-pipe principle - with the exhaust plugged, emissions aren't a problem. In actual fact, it isn't that dramatic. When open, the exhaust manifolds are 1-7/16 in. diameter, for a combined exhaust area of 3.25 sq. in., not counting the obstruction caused by the open butterfly valves. With the valves closed, exhaust exits through two quarter-inch holes in each valve, for a combined exhaust area of 0.2 sq. in. It isn't a case of total blockage, but it gets the job done.

Pic: RD400F Front & Front End The carburetors, while the same size as the carbs used on earlier RD400s, are new for 1979. Overall jetting is leaner, internal air passageways are changed and more air bleeds with pressed-in jets have been added. Most important, the slides are controlled by a mechanical linkage similar to the ones found on modern multi-cylinder bikes. In 1978, a single cable ran from the RD's twist grip to a junction box underneath the gas tank, where it connected to three other cables-one each for the carburetor slides and one for the oil injection pump. In 1979 a single cable leads from the twist grip to the linkage, making the connection at the carburetors. The linkage operates the slides and should eliminate the need for routine slide sychronization. A short cable runs from the end of the linkage to the oil pump, which means that throttle cable slack can be adjusted without requiring that the oil pump be readjusted.

The carburetors also feature vacuum take-off fittings to allow use of gauges in balancing idle settings. Several changes were made inside the engine to meet emission standards. The exhaust port was lowered 2mm from 35.5mm below the top of the cylinder in 1978 to 37.5mm below the top of the cylinder in 1979. Combustion chamber volume is reduced 3cc, from 26cc in 1978 to 23cc in 1979. Those changes raised the compression, and to keep c.r. within reasonable bounds Yamaha engineers lowered the piston dome height slightly. The end result is that the RD400F has a c.r. of 6.7:1, compared to 6.2:1 for the 1978 RD400.

Raising the compression results in a hotter flame and better combustion, while lowering the exhaust port gives the charge more time to burn before being swept out the exhaust system. A side benefit of the new port timing and compression is an increase in mid-range torque when the RD400 is compared to earlier models. On the other hand, those changes-in concert with the leaner carburetion-make the RD400F run hotter than its earlier counterparts.

To help make sure that the extra heat generated by the emissions-controlled engine doesn't cause a lubrication failure or seizure, the RD400F's pistons have oil retaining grooves machined into the skirts-the earlier RD's had plain skirts. The latest RD also has a one-piece airscoop-style cylinder head reminiscent of the "Ram Air" heads seen on the now discontinued Suzuki two-strokes. In addition to better cooling, the one-piece head also aids in noise control-the solid top scoop ties together cooling fins and thus reduces the rattle common to non-linked fin ends. Similarly, extensive rubber dampers inserted into the fins on the side of the cylinders reduce both engine operating noise and fin rattle.

Pic: RD400F Engine Detail Shots An incidental change to the 1979 piston was the elimination of a notch cut in the bottom of the exhaust skirt. That notch helped reduce surge and bucking under deceleration for earlier model RDs. Yamaha engineers dropped the use of the notch with the adaptation of the lower exhaust port which produces much the same effect as the notch and the butterfly exhaust valves.

Other alterations to the RD have no relation to emissions output. New forks are used with stanchion tube diameter increased for production reasons, not because the earlier RDs suffered from a lack of front-end rigidity-that wasn't a problem. The 1978 RD400's fork tubes were 34mm, but the 1979 RD400F's fork tubes have an outside diameter of 35mm, an o.d. shared by several other current Yamaha models. Front wheel travel has been increased to 5.5 in., up 0.8 in. from the 4.7 in found on the earlier RD400s, and at 28 in. from axle center to tube top, the forks are a little longer as well. The longer forks increase the wheelbase 0.7 in. from 1978's 51.8 in. to 1979's 52.5 in., but rake and trail stay constant at 27.5 and 4.3 in. thanks to careful juggling with the frame. The same rear shocks are used this year, but there's an extra 0.060 in. rear wheel travel (3.6 in. this year versus last year's 3.54 in.) due to the upper shock mount being moved down .060 in.-that tiny bit of extra shock laydown angle produces the small rear wheel travel increase.

Both the front and rear brakes have one piece stainless steel discs which weigh 4 lb. 4 oz., considerably less than the 1978 RD400E disc assemblies (which consisted of stainless steel rotors riveted to aluminum carriers. and which weighed 5 lb. 8 oz.). The new discs are the same outside diameter (10.5 in.) but are thinner, 5mm thick compared to the older RD's 7mm, and are shared by the RD400F with several other Yamaha models. The RD's calipers and caliper mounts are changed this year also, again for production reasons.

The gas tank is larger for 1979 as well, holding 4135 gal. compared to the earlier RD's 3.4 gal.

Probably best of all, the latest RD has the footpeg mounts routed above the mufflers, instead of the previous mount system of 1ooping underneath and around. With the old setup the RD's biggest problem was available cornering clearance-it was easy to ground the peg mounts, pry one or both wheels off the ground, and crash in a shower of sparks. Unfortunately for racing enthusiasts, the mufflers themselves are not raised, and as anyone who has installed rearsets and racing tires on his Production class RD racebike knows, it's not uncommon to drag the entire length of an RD muffler through a turn. Ground clearance is better than it used to be, and is entirely adequate for sporting street use - only the footpeg tips and the centerstand on the left dragged during our street testing. But for racetrack use, cornering clearance still needs improvement, especially considering that it's impossible to drag any part of the Honda Hawk exhaust system even when racing slicks are used.

Along with the relocated footpegs, the latest RD has a lower seat, 31 in. versus the earlier bike's 32.5 in seat height. The net effect of the changes is that the R D400 has lost some of the top-heavy, sitting-on-the-top-of-the-world feel of the earlier machines. It feels better and more natural. especially to taller riders.

The important part of any-and all-changes is how they translate into overall function.

The RD400 may have no advantage over its four-stroke adversaries in terms of the hard numbers. dragsrap time. terminal speed and so on. But it feels sportier, less refined. nervous. almost twitchy. in spite of the engine changes to please the EPA--the net result of which were improved midrange with a definite falling offof power at 7500 rpm-and in spite of slightly different weight distribution-44.1 percent front. 55.9 percent rear for 1979 versus 43 percent front and 57 percent rear-in 1978 the RD400 still wheelies with abandon. It's the only 400 that lofts the front wheel effortlessly, without even a tug on the bars needed.

In fact, it's difficult to leave a stop light at any rate faster than a Car-like crawl without bringing up the front wheel. For show-offs who like deliberate wheelstands, simply running up to 4500 rpm in first and stabbing the throttle open instantly lifts the front end as high as the rider dares, while doing the same thing in second gear and tugging on the bars yields the same result with more effort.

Pic: RD400F Wheelie & Rear End It's pretty difficult to not feel sporty when the front wheel is hopping two feet off the ground and pedestrians are staring and pointing. Perhaps the best part of it all is the fact that a rider so inclined can wheelstand the RD400F across an intersection and not be travelling 65 mph before the far crosswalk. It's kind of like riding a 1176cc R.C. Engineering Suzuki GSI000, except with less imminent danger to one's driver's license. The police may not appreciate the wheelies, but at least they're less likely to bust you for speeding, too.

The powerband, then, is more abrupt than the powerbands of its four-stroke competitors. The RD makes good power from 4500 or 5000 rpm up to 7500, but the transition from below 4500 to above 5000 rpm isn't much of a transition at all. First, you have nothing, then you've got it all.

That's in the lower gears. On the highway, the RD turns about 5000 rpm in sixth gear at an indicated 65 mph. But to pass a slower vehicle or accelerate with any kind of authority the rider must downshift twice-bringing engine speed up to at least 6000 rpm. Rolling on the throttle at 5000 rpm in sixth doesn't have much effect.

Then there is the surging. At very small throttle openings, the RD bucks and can't maintain a constant speed. It's noticeable at speeds of about 55 to 65mph in sixth gear on the highway, and also around town in fourth, fifth and sixth gears at speeds of 35 mph and above. Any time the RD is traveling at what the rider wishes was a steady state, constant rate with little load on the engine, the bike can't seem to make up its mind whether it wants to accelerate or coast or decelerate. It's surge, and it's a problem when-as near as we can tell-the butterfly valves are closed and the potatoes are in the pipes. Open the throttle to accelerate, drag the front brake to add load and compensate with more throttle, and the bucking halts. Maybe the 1979 RD400 needs that 1978 piston notch after all.

The RD400F feels flighty. This is a motorcycle that will-and does-change direction at the slightest input, whether from the road surface, the wind, the rider's hands upon the bars. It moves around in its lane at freeway speeds, just a little to the left, a little to the right, requiring constant correction to maintain a precise course unless the air is dead still and the road perfectly smooth.

Pic: RD400F Engine Details But what seems to be nervousness in a straight line at cruising speeds changes to an incredible manuevering capability both in traffic and at top speed. You want to turn, here and now? Done. No indecision. No waiting. If anything, you've turned before you wanted to turn. That means you can change lines m a corner to pass a slower rider on the track, or to dodge a rock on the road, instantly. The RD's weight is carried relatively low, since the engine doesn't have cams or valves riding at the top of the engine mass. The relatively low center of gravity and steering geometry combine to make the bike steer very quickly. The forks do an admirable job of absorbing road expansion joints and other small jolts, and handle the big bumps as well. But the shocks, even set on the lowest spring preload, allow the road to jar the rider. They need more compliance for cruising.

On twisty roads, the suspension works. The forks dive instantly with a touch of the almost-too-powerful front brake, but we've come to expect that of the current crop of compliant fork motorcycles. The shocks do their job, and the bike doesn't wallow or wobble when run hard. As proven on the racetrack time after time, what weaknesses the RD may have at speed don't include suspension and handling flaws, at least not with a stock engine.

The brakes. They are, in Yamaha tradition, almost too responsive, too touchy. It takes far less lever pressure to lock the front wheel even at 60 or 65 mph-than most other bikes. The rear is difficult to control in quick stops, especially as road speed approaches zero. But at 90 or 95 mph, the brakes are perfect, although the rear is again hard to control at turn entrances as the speed drops to 60 or so mph. The brakes fit the character of the machine, as if to say, "This is a sport bike, and if the binders are a bit touchy for the less-skilled, then let 'em buy an economy model XS400 with drum brakes!"

The instruments are fine: well-lit at night and easy to read. The mirror image is often blurred, not surprising since this twostroke Twin has a typical high-frequency vibration. (It seems smoothest around 5500 rpm. an indicated 70 mph, but don't tell the police we said so). New for 1979 are Yamaha's automatic self-canceling turn signals, and judging by the number of riders we see going straight with blinkers on, that's a good thing.

Also changed is the headlight control system. It used to be that the headlight came on with the engine, just like this year. But in 1978 simply unplugging a relay gave the rider control of when the headlight was on and when it was off, via a normal on/off switch on the right control module. That's gone this year. Now, the relay trips the headlight when the engine starts, as always. But now the headlight stays on even if the engine is turned off with the kill switch or stalls, and stays on until the ignition key is switched off. If a rider unplugs the relay this year, he gets no headlight at all, engine on or off.

The ignition system is the same, battery and points while the competitors are moving to electronic systems. Timing has been advanced, again for smog reasons, from 2.3mm BTDC in 1978 to 2.4mm BTDC in 1979. Changed are the ratings of the voltage regulator (up to 15 amp. from 4 amp in 1978), the rectifier (up to 15 amp, from 12 amp) and the headlight high/low beams, up to 50/35 from 40/30.

At its core, the RD400F is a sports bike, more than any other Japanese motorcycle given that name by accident or intent. It isn't as comfortable as its four-stroke competitors, and to some zealots, sounds wrong, (like all two-strokes). In terms of performance numbers, it has no advantage over the latest four-strokes. It is different in feel and fact. It's a two-stroke. It runs best with the throttle open. It wheelies.

It's a scratcher's bike. For getting really great performance for not much money, RD400s have always been good buys, and we know that in at least one manufacturer's case, that's still true. Bolt on some of these chambers, he says, and the thing is a rocket, exhaust valve and all. For the man ready to pay for porting, pipes, and carbs, it's possible to get a streetable RD400 into the mid- 11 -second bracket at the dragstrip (with wheelie bar).

Perhaps the biggest appeal of the latest RD isn't what it is, but what it can become. Stripped of any pretense of stockerism and built for serious road burning may be where the RD400 is meant to be.

It is two-stroke island in an ocean of four-strokes, a cafe racer in a world of choppers, a cult function bike in a universe of styling fads.

If that's what you want, then you want an RD400F.

Pic: RD400F CW Spec Sheet

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