Duke of Oil: 1976 Yamaha RD400C

Yamaha’s plan to update its popular 1975 RD350B model was simple: For model year 1976 they committed to building an even better version of America’s favorite 2-stroke motorcycle.

The solution was found in upping the little oil-burner’s capacity, which Yamaha engineers did by about 14 percent, stretching the stroke from 54mm to 62mm for a boost from 347cc to a full 399cc. The end result was the RD400C, perhaps the most advanced production 2-stroke motorcycle up to that time.

But the bigger-is-better philosophy was only the tip of the iceberg, as that bigger engine formed the foundation for a much improved street bike. To borrow from Cycle World magazine’s test in their March 1976 issue: “The Yamaha RD400C is the closest thing to a perfect motorcycle that we’ve ever run up against. As a matter of fact, there is only one item that keeps it from being the world’s first perfect motorcycle, but we won’t tell you what it is… at least not until we’ve told you about the rest of this beauty.”

If CW’s words come across as excessive hyperbole, consider Cycle magazine’s lead-in to its road test the following month: “The quickest, fastest, best-handling, and hardest-braking lightweight ever now joins the engine capacity creep and gets more comfortable without giving up as the world’s best, and only, midi-Superbike.”

So what prompted the two leading motorcycle publications of the era to treat this mid-displacement roadster like royalty? Let’s put Drew Immiti’s sparkling original jewel (his RD400C boasts original paint, chrome and seat upholstery) on its centerstand for a close walk-around of what Yamaha offered 42 years ago.

First things first

Deciding which to examine first, the engine or the chassis, is tough, because as Cycle World stated, this bike borders on perfection. But since most (if not all) of us are gear heads, let’s start with that blacked-out, air-cooled, 2-stroke engine, which was more than just a stroked-out 350. It was practically an all-new engine.

For starters, the ports (intake, transfer and exhaust), while similar in design to the 350’s, differed in terms of timing to compensate for the reconfigured stroke length. In addition, each of the RD400C’s cylinders were given a bypass hole that ran at about a 45-degree angle from a point 20mm above the exhaust port, leading directly to the exhaust passage. This helped facilitate kick-starting the longer-stroke engine, plus the holes helped reduce low-rpm surging, a common issue with 2-stroke engines of the era. Another benefit was reduced exhaust noise at low rpm, and with noise abatement increasingly at the forefront of motorcycle legislation, low noise was as important to Yamaha as high horsepower was to the customer.

Each piston had a small, roughly 4mm-high and 10mm-wide cutaway on the bottom edge of the front skirt, which allowed direct passage from the crankcase to the exhaust port for just a few rotational degrees before and after top dead center. This helped relieve crankcase pressure to further reduce low-rpm surging.

The new 400’s pistons also checked in with wedge-shaped keystone rings, replacing the L-shaped Dykes rings found in the 350B’s engine. Keystone rings were less prone to sticking in the ring groove, although Cycle editors felt the Dykes rings created less drag on the cylinder walls, translating to more top-end power. But top-end power wasn’t the only priority for many street riders; reliability was paramount, too, and Yamaha considered the keystone rings to be more trouble-free under most conditions than were the Dykes.

Topside, the 400C’s cylinder heads accepted 3/4-inch long-reach spark plugs that dissipated heat quicker and more efficiently than the RD350’s 1/2-inch-reach spark plugs. Small rubber blocks inserted within the 400’s cooling fins helped quell even more of the engine’s vibration noise.

Like the RD350B, the 400C took in its fuel and air mixture through a pair of 28mm Mikuni slide/needle carburetors. However,  the 400’s carbs were now equipped with external pilot air circuits to help eliminate an over-rich condition that often developed after riding the bike at low speeds — a condition that especially became apparent just before thwacking the throttle to show off the bigger engine’s snappy power that more often than not could put the bike’s front wheel high in the air. Wheelies were an RD rider’s signature.

The 400C had a more powerful ignition system, with beefier coils pirated from the 4-stroke TX500’s parts bin, and an ignition resistor that cut down voltage flow across the ignition points helped increase contact points life. The 400C’s new coils and brighter sealed beam headlight and twin-bulb taillight necessitated a higher output generator, now supplying 280 watts. And on the subject of lighting, the RD400C was one of the first motorcycles to check in with self-cancelling turn signals. Progress and perfection.

Yamaha engineers also upgraded the clutch hub with redesigned rubber cushion dampers to soften the power delivery to the transmission, which in turn sported new, closer-ratio gearing with a slightly higher first gear. As Cycle stated in its report, “there is so much torque available and the clutch operation is so smooth that getting launched is much less hassle than you might anticipate.” Cycle editors also waxed eloquent about the transmission’s closer-spaced ratios during upshifts: “The tack needle will not swing back so far after a shift that the engine will bog. Production-class road racers will absolutely love the new ratios.” Considering that RDs all but ruled the 410cc production class, that was great news, indeed.

Framed

Those and other small yet significant upgrades (including an improved and more throttle-sensitive oil pump) to the engine created the new RD400C’s heartbeat. The chassis and bodywork — its skeleton and flesh, if you will — boasted even more refinements and improvements.

Start with the engine’s relocation within the RD’s stout frame. Much of its basic configuration and geometry were carryovers from Yamaha’s first successful Grand Prix 250-class racer, the heralded RD56 of the mid-1960s.
To simultaneously reduce intake noise and increase incoming air volume for better performance, Yamaha reconfigured the 400’s frame to accept a larger air box, in the process moving the engine forward 20mm. Shifting the engine forward improved steering input and overall handling, too, as the added weight up front helped the RD400C’s front tire stick better than the 350’s while cornering. RD riders and road racers from that era are especially familiar with how quickly the perky little oil-burner could steer. And once in the corner at speed it took a steady throttle and full concentration to maintain the line so the bike wouldn’t twitch left and right. Seasoned RD racers knew this was one of the 350’s quirks, the price paid for a quick-steering motorcycle, and generally the solution was for the rider to shift his weight forward to compensate for more traction while tracking through a turn. The RD400C retained that mongoose-like agility exhibited by the RD350B into and out of corners, but with less drama when its 3.25 x 18-inch front tire fought for traction.

It helped, too, that the RD400C’s Yokohama rubber was mounted on solid cast aluminum wheels as opposed to the RD350B’s laced-spoke steel rims that most certainly generated a degree of spoke flex during full lean. These were among the first cast wheels offered on a production motorcycle, and even though the 400’s seven-spoke hoops looked massive compared to the 350B’s more conventional spoked wheels, they were a mere 100 grams heavier. Again, progress mixing itself with perfection, or as Cycle touted, “thus passing another race-track innovation to one of the most sporting street machines of its time.

New, fancier suspenders

In terms of suspension, the RD400C checked in with some innovative features for the time. The fork legs were one inch longer than the 350’s, and Teflon bushings between the tubes and sliders reduced stiction to help keep action smooth and steady. The crew at Cycle magazine also performed a simple modification to their test bike that many amateur road racers of the era already knew about, changing fork oil to a slightly heavier mixture, a trick that especially helped slow rebound damping. Cycle editors reported that adding Torco 20-weight fork oil “stabilized the front end.” Seasoned racers also experimented with fork oil volume in their RDs, some claiming that an extra dollop or two in the mix helped stabilize fork movement even more.

The rear shocks were improved, too, and they have an interesting back story to their development. According to Cycle World, Yamaha equipped one of their prototype RD400s with S&W aftermarket shock absorbers, and after experiencing favorable testing results they shipped the lot back to Japan. Coincidently, those same shock absorbers were favored by many production-class RD350-mounted road racers in America, and since Yamaha designers had dedicated themselves to making the best 2-stroke motorcycle ever, engineers in Japan emulated the S&W’s spring and damping rates as best they could into the 400C’s rear suspenders.

Engineering improvements didn’t stop there. Like the RD350B, the new 400 came with a superb front disc brake, but a similar hydraulic disc found its way onto the 400’s rear swingarm. The added stopping performance was instantaneous. Said Cycle‘s editors in their report: “The little bike just squares down on its springs and stops. There is no wallowing or squirming or rear wheel hop. It just stops.” The added disc brake, coupled with various other refinements, added about 10 pounds to the RD400C’s overall weight. At least they were “perfect” pounds.

But stable high-speed handling coupled with rapier-quick steering wasn’t the RD400C’s only calling card. The bike was also comfortable, its new seat offering the rider a pillowy place to perch himself during a ride. Moreover, the engine, handlebars and footpegs were rubber-mounted to help isolate the rider from vibration, and to further quell the nasty vibes the exhaust system sported rubber connectors between the mufflers and header pipes. A rather smooth ride ensued, and coupled with 45mpg-or-so fuel economy, the RD400C wasn’t a bad touring bike, either.

2-stroke tour bike

Back in the late 1970s, it was customers like a young man named Drew Immiti that Yamaha enticed into dealer showrooms where they could check out the new RD400C. When Drew visited Yamaha of San Luis Obispo (California) looking for a new bike, he was a 20-year-old college student attending nearby California Polytechnic State University, San Luis Obispo. “I actually bought it in 1977,” explains Drew today. “It was a leftover from ’76 and I paid $1,285 for it out the door.”

Impatient and bursting with energy fueled by bubbling young male hormones, Drew immediately set out to log break-in miles on the fresh engine. “The dealer told me to bring it back for its first service after I put 500 miles on it,” Drew recalls. “That was on Wednesday. I told him I’d be back on Friday for the service check. He looked at me like I was nuts. Anyway, I finished my finals on Thursday, then rode the bike until it showed 500 miles on the odometer, and like I promised, I showed up Friday morning for the service. When that was finished, I rode the bike down to my mom’s house in Pasadena.”

That three-day caper was only the beginning of Drew’s affection for his RD400C. The next day, he visited some college friends at San Diego State University (aka The Party School), and when the fun and excitement began to recede there he indulged himself and his bike in another adventure, riding the little RD to the nearby Mexico border, where he promptly turned around and headed north, his sights set on Canada.

His unofficial and somewhat circuitous Tri-Flag route took him to visit other college friends at Chico State and Humboldt State colleges (aka Party School 2 and Party School 3), who let him sofa surf at their apartments in between parties (notice a particular pattern forming here?). Yet despite the distractions, young Drew eventually wound up at the Canadian border, where he and the ring-ding Yammie took a ferry boat ride to Vancouver Island to check things out before heading back to California. Quite an adventure, eh?

For the record, Drew had only a tank bag and rear-seat bag for luggage, complimented by a small wad of cash in his pocket. Who says youth is wasted on the young? The RD was Drew’s first motorcycle, and the cross-country baptism forever etched a love of motorcycling into his somewhat twisted, yet maturing, psyche. As proof, today we present Drew Immiti, the adult, who with James Henderson is co-owner of Superbike Corse in Laguna Hills, California. As for Drew’s first RD400C, he eventually sold it to upgrade to a BMW R100RS, but that’s another story for another time.

Which brings us up to Drew’s current RD400C, a bike he bought from a close friend who modestly prefers to remain anonymous. It turns out that the mystery man owned the bike for about 15 or so years after acquiring it in its original condition. Although the engine shows less than 2,500 miles on the odometer, the previous owner refurbished the engine with all new seals and gaskets, a wise move for an aging 2-stroke engine that idly sat for many years, because air leaks can prove quite troublesome in terms of performance and engine longevity.

The original paint and chrome were buffed out, and the seven-spoke cast aluminum wheels and other raw aluminum components were detailed to regain their original beauty. He mounted a set of IRC tires, similar to what were found on Japanese performance bikes of that era, onto the wheels, and for show purposes a pair of new-old-stock mirrors were regally perched onto the handlebar. And beneath the flip-up seat rest the bike’s hidden electrics and frame top rails, all in as pristine condition as the rest of the bike. A few years ago, the whole ensemble took best in its class at the annual Motorcycle Classics Vintage Bike Show at the Barber Vintage Festival.

But that surviving showroom-fresh condition hasn’t stopped Drew’s inner child from having fun on this little ringy-dingy. Shortly after these photos were snapped and archived he rolled the red-hot RD into his shop to replace its period-correct tires with a set of modern, stickier Pirelli Demon skins, and the bike’s sedately high handlebar made way for a slightly lower-profile BMW-style handlebar. Ditto for the mirrors, which will be preserved in their original condition for another day, because Drew has plans to take the little smoker on its own extended road trip sometime soon.

Perfect ending?

So, what about that near-perfection on two wheels that Cycle World alluded to? According to the Cycle World test, the RD400C did have one minor flaw: its horn. Anemic in its aural performance, the horn, CW’s editors said, “wouldn’t make a hungover wino flinch, let alone inform some quadraphonically deafened cigar-puffing lardo that he is blindly stuffing his gas-sucking smogmobile into the lane you are occupying.”

Harsh words? Perhaps, but as a former RD250/350/400 owner myself — and I speak with confidence on behalf of the thousands of other raging RD riders, too — we never used our bikes’ horns anyway. We were too busy popping wheelies and strafing apexes to worry about beeping the horn. Because, as guys like Drew Immiti found out so many years ago, there can be as much perfection in the ride as there is in the bike itself. MC