• The Suzuki GT380 Sebring was the first sub-400cc street bike to break away from the 350cc class rating. It was also the first mid-displacement multi-cylinder roadster from Suzuki. In the wake of the performance bikes of the early Seventies, the Sebring navigated in a very different direction, and moved toward serene performance and exceptional comfort.
In designing the GT380, Suzuki's engineers mixed fresh concepts with proven parts. The bore and stroke of the Sebring are the same as Suzuki's 250cc street twin. By adding one cylinder the displacement was bumped up to 371 cc. Mild port timing, low compression and small carburetors level out the 380's power and separate it from pipey, performance-type two-stroke engines.
The six-speed gearbox is also of the same design as the GT250 twin's. Gear spans are progressively tightened up in the higher cogs and there's nothing unusual about that, but it does allow a rider to find a gear in which the GT380 is absolutely smooth on the highway.
For all intents and purposes the engine has remained unchanged since its release in 1972. The Ram Air System has proved efficient in increasing engine heat dissipation, and more importantly, reducing operating noise. The low compression motor runs trouble-free on regular grade gasolines. Suzuki's intricate oil injection system lubricates the pistons and crankshaft bearings individually and includes a recycling arrangement which removes fuel mixture accumulation from the crankcase areas and feeds it directly into the combustion chambers.
In 1974 a number of major changes were made to the chassis, carburetor intake, exhaust and instrumentation. The chassis was completely redesigned to improve handling and ground clearance. Better fork internals, shock dampers and springs delivered a better ride. New instruments were joined by the digital gear readout and larger warning lights.
Bell-crank operated carburetors replaced the cable-actuated mixers and the mufflers were moved up and in for additional ground clearance. Modifications to the intake system reduced objectionable induction drone and relocated footpegs and controls increased comfort.
Suzuki built the Sebring with a front drum brake only in its first year, moving to a disc in 1973. Rubber engine mounting is unchanged, having proven effective in eliminating vibration. The conventional triple-point ignition system is driven from an independent idler gear to prevent timing fluctuation associated with crankshaft flexing.
Initial saddle and gas tank designs have gone without alterations. Minor design improvement changes have been made to the GT380's through its five model series, but few are visible. Suzuki believes in improving the breed from the inside out, not the outside in. The GT380 has successfully survived four tough years, and Suzuki appears willing to retain the Sebring indefinitely.
• There is little new and nothing unconventional about the '76 Suzuki GT550
Indy. It is a four-year old motorcycle that was designed to ride the waves of progress, survive as a seasoned veteran and never get out of date. Paint scheme alone identifies the 1976 Indy as new.
Suzuki's innovative design of the Ram Air cylinder head shroud system has endured, unchanged, since the beginning. Ahead of its time in 1972, the RAS provides dual benefits. The scoop increases air-flow activity over the fins, and also functions as an excellent sound-deadener to minimize the amount of top-end piston noise.
There have been no performance changes made to the cylinders or pistons since the Indy's inception. The low-compression two-stroke triple was designed for durability and runs as happily on low-or no-lead fuels as it does on premium. Unchanged since the GT550's original design is its exceptionally effective rubber mounting system.
Only one chassis change has been made to the GT550 through the five-model series. In 1974 a number of major up-dates were built into the Indy—mostly to subdue noise and improve handling. The carburetion, intake and exhaust systems were modified to reduce operating noise levels. The exhaust pipes were tucked up closer to the frame, the side stand and center stand were moved in and the foot pegs relocated to give the Suzuki additional lean angle clearance. The frame changes amounted to nothing more than relocating foot controls and brackets to which they attached.
New instruments were fitted to the Indy in 1974 and included Suzuki's popular digital gear read-out and bigger idiot lights. The five-speed gearbox is identical to the transmission in the big 750cc Suzuki LeMans. The 550's clutch and primary drive are equally robust.
Suzuki went to the disc front binder in 1973. The rear drum brake and wheel have remained unchanged, as have the tire sizes.
Electric starting was in the first 550, and has remained without alteration. Most of the electrical components are the same as those used in the 750s. The plush saddle
and four gallon gas tank have been changed in very minor ways—a new piece of vinyl here and a fresh paint stripe there. Minute internal modifications appear in the parts books of each new Indy, but the motorcycle remains pleasantly the same. Unlike most re-vamped new models the GT550 has lost seven pounds since 1974 and the price has escalated only moderately. As Suzuki's most successful road bike, the GT550's reputation for dependability is a matter of record.
• More than a trace of R5C/RD350 heritage is visible in the Yamaha RD400C, but it's not just an old hat cleaned and blocked in the hope it will last another season. The 14-percent displacement increase was obtained by stretching the engine's stroke from 54mm to 62mm, which required larger, stronger crank-cases and taller cylinders. They decided to use the extra volume to broaden the power range instead of lifting the power peak, and that led to new porting specifications, trickery with little exhaust/ crankcase bleed holes and reed petals. Put all the changes together and they amount to a new engine that is slightly more muscular and much more agreeable than the one that powered the RD350. The bike also has bigger, better spark coils, which have slightly diminished its appetite for plugs.
Yamaha's redesigning and updating didn't stop with the engine. The RD400C got a new closer-ratio six-speed transmission, and the whole power package is now held in vibration-isolating rubber-bushed mounts. The frame was reshaped to take a bigger air cleaner box, and to move the engine forward 20mm for better-balanced handling. Ride quality was improved by several orders of magnitude with a suspension reworking that included a new teflon-sleeved, stiction-free front fork. And the Yamaha's already phenomenal braking abilities were further strengthened with a switch to a disc rear brake and slightly fatter front tire.
Cast aluminum wheels are fitted on the RD400C, in part because they're more durable than laced wire wheels and also as a part of an overall restyling job. There's a new paint scheme, a new fuel tank with a quarter-gallon more capacity, and a new and more comfortable seat. Other new features include an oil-level warning light, Yamaha's clever solid-state self-canceling turn indicators, and a more powerful alternator to cope with the electrical load created by the bike's new ignition coils and brighter headlight.
Very nearly the only part of the RD400C that hasn't had a big upward boost, compared with last year's RD350, is its price. You get the new package, disc brakes, cast wheels and all, for just eight dollars more than Yamaha asked for the RD350.
The RD400C is faster and better handling than its fore bearers, faster than its displacement gives it any right to be, and a $1219 bargain.
• All of us here at Cycle loved Yamaha's original eight-valve 500cc twin; we weren't much taken with the versions that followed. Last year's XS500 was stone-slow and cursed with (among other things) the most diabolical combination of drive-train backlash and abrupt off-idle throttle response we've ever happened upon. The bike was hard to ride smoothly, harder to enjoy, and our test report was a litany of sour appraisals. Maybe somebody at Yamaha read our report; maybe they already were aware of the 1975 model's shortcomings. What's important is that the '76 XS500C is much-improved, and that the list of improvements covers our complaints like a blanket.
The off-idle lurching got a lot of attention. Switching to a set of more carefully calibrated Mikuni carburetors helped, as did a general tightening of the transmission. More correction was provided with a change from 5- to 10-degrees of ignition advance at idle, and yet more in a 10-percent increase in the crank's flywheel effect. There may even be some assistance in the new throttle actuation, which has a single cable pulling against a rather soft return spring instead of the old Kei-hins' dual-cable, wrist-wrestler system. There's still too much lurch and lash, and mountain road exigencies turn smoothness into an impossible dream, but this side of the 500's personality is more agreeable than before.
Performance has been improved, despite a half-point drop in compression ratio, by a reworking of its ports to give the engine better midrange pull—and by changing from 17/43 to 16/42 final drive sprockets. A Yamaha spokesman said the change in overall gearing was in recognition of the national 55 mph limit, but the increased leverage can also be said to help a smallish engine move a heavyish motorcycle.
Last year's Kayaba fork, which was very good, has been replaced with a Showa fork, which is superb. The new fork legs mount the front brake caliper behind the axle, and there's now a disc rear brake and cast aluminum wheels. And there's new, hard-edged styling that's distinctive if not to everyone's tastes, with a tank capacity upped to four gallons. The XS500C's looks also are altered by the new one-piece cylinderhead/cam-box casting, which has eliminated the oil leak that smudged older models. Your look will be altered by the Yamaha 500's new seat, which can be sat upon for many miles before that familiar pain and pained expression develop.
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