4-Stroke Fun: 1977-1982 Yamaha XS400

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1976-1982 Yamaha XS400.
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1974-1979 Kawasaki KZ400.
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1977-1978 Suzuki GS400.

Yamaha XS400
Years produced: 1977-1982
Power: 32-36hp @ 8,500rpm
Top speed: 96mph (period test)
Engine: 391cc air cooled, SOHC parallel twin
Transmission: 6-speed, chain final drive
Weight/ MPG: 391lb (wet)/53-58mpg
Price then/now: $1,348 (1977)/$600-$2,000

Until the early 1970s, with the exception of Honda, mid-size Japanese motorcycles were always 2-strokes. But with the pending demise of 2-stroke street bikes in the U.S. market looming, the other three big Japanese bike builders had little choice but to look to the Otto Cycle for their future — even though they continued to make 2-stroke street bikes until the end of the decade.

Kawasaki was the first to offer a sub-500cc 4-stroke with the KZ400 twin in 1974. Yamaha followed in 1976 with the XS360, and Suzuki introduced the GS400 in 1977. Launched in the U.S. in 1977, the XS400D was essentially a bored-out XS360 (66mm bore for the 360, 69mm for the 400). As such, it was an air-cooled 4-stroke parallel twin with two valves per cylinder and a 180-degree crankshaft of 52.4mm stroke. Drive to the single overhead camshaft was by chain with an automatic tensioner. The crankshaft ran on three split-shell plain main bearings, with plain bearings also used for the big ends. A pair of 34mm Mikuni CV carburetors fed the iron-sleeved cylinders. Starting was electric, with a kickstarter fitted as backup. Drive to the 6-speed transmission was via a straight-cut gear primary and wet multiplate clutch. Unlike its competition, the XS had no balancer shaft. 

The unit construction power unit fitted into a mild steel tubular frame with a single downtube and dual cradle below and behind the engine. A non-adjustable telescopic fork controlled the front wheel, while a swingarm and 5-way adjustable shocks were fitted at the rear. Both front and rear wheels were cast alloy with a single disc brake. The XS400 was especially anticipated, because its 2-stroke sibling had been the fastest of the 400s to that point. Much of the XS400D’s styling was influenced by the RD400, with which it appeared to share some components. Though listed as a “commuter” model, the XS400D came equipped with full instrumentation and self-cancelling turn signals.

The squared-off gas tank and seat on the XS400D gave it classic lines, but for 1978 a restyled teardrop gas tank and slightly stepped seat were fitted on the XS400F, “Capturing the essence of the ‘American Triumph’ look,” Cycle Guide said. Also available in 1978 was the XS400-2, a spoke-wheel, drum-brake, kickstart-only model missing self-cancelling signals and with a lower MSRP. A further redesign for 1980 gave the cruiser-like XS400SG a deeper step to the seat, pull-back bars, shorty mufflers and a drum brake replacing the rear disc. The XS400 was replaced in 1982 by an all-new double overhead cam, counter-balanced twin, the Seca 400. 

So was the XS a worthy successor, or a different animal altogether? A Cycle World tester called the XS400 “the roughest running 400 4-stroke twin on the market,” with vibration making the mirrors useless at speed. Cycle was a little kinder, noting the little twin “pulls evenly and willingly to and past redline, and during short bursts, the vibration is not annoying. But if you jump on the highway … you may find it disappointingly buzzy.”

The engine started easily hot or cold, and it had smoother throttle response than its competition. However, Cycle World also found the gearshift occasionally balked, while the clutch sometimes grabbed and dragged when hot. The front suspension was compliant, though the rear “beat the rider’s kidneys,” CW said; and while Cycle found the shocks “acceptable,” they noted that on their softest setting “the bike pogos over bumps.”

Performance-wise, the Yamaha was generally quicker than the KZ400 and the GS400, but slower than Honda’s 400 Hawk. Braking was excellent, though testers noted the rear disc could be made to lock too easily. Possibly being too kind by half, Cycle magazine politely summed up the XS as “neither the fastest in its class, nor the least expensive, nor the most comfortable. It is nevertheless a handsome, functional and economical motorcycle — easy to maintain, miserly with gas, and still competitive in a hotly contested class.” MC

400cc Rivals to Yamaha’s XS400

1977-1978 Suzuki GS400
Years produced: 1977-1978
Power: 26.4hp @ 8,500rpm (measured)/96mph (period test)
Engine: 398cc air-cooled, DOHC parallel twin
Transmission: 6-speed, chain final drive
Weight/MPG: 400lb (wet)/51mpg
Price then/now:  $1,295 (1977)/$600-$1,900

Suzuki’s stroker heritage showed in the GS400 with its 180-degree built-up crank, ball and roller main bearings and roller bearing big ends. Drive to the two overhead camshafts was by adjustment-free chain, while gears drove the balance shaft. The cam followers acted directly on the valves with shim-over-bucket adjustment. Helical gears drove the wet clutch and 6-speed transmission, and 34mm Mikuni CV carbs provided the fuel mixture. A full dual-cradle frame housed the powertrain, with a non-adjustable front fork and dual 5-way adjustable rear shocks. It ran on spoked wheels, with a front disc and rear drum brake combo. The dash featured a gear-position indicator — but the electrics were protected by just one fuse. If that blew, the GS became a boat anchor.

Cycle Guide’s tester liked the lack of drivetrain lash (compared with the KZ400’s “slop and lurch”) and light, smooth controls. But, although handling was “quick and light,” a general lack of suspension damping made the GS “wallow a little — or a lot if you’re carrying a passenger — in rough corners.” They summed up: “On the whole we rate the Suzuki as the best urbobike we’ve ridden so far, and the best part of the package is the engine.

1974-1979 Kawasaki KZ400

Years produced: 1974-1979
Power: 36hp @ 7,000rpm
Engine: 399cc air-cooled SOHC parallel twin
Transmission: 5-speed, chain final drive
Price then/now: $1,180(1974)/$600-$1,900

First out of the gate with a 400cc 4-stroke twin was Team Green. Its 360-degree crank ran on four plain bearings with an adjustable central chain driving a single overhead camshaft operating four valves. A maintenance-free chain-driven balance shaft quelled vibration. A pair of 36mm Keihin CV carbs fed the two cylinders, and drive to the wet multiplate clutch and 5-speed (later 6-speed) transmission was by Hy-Vo chain. 

The powerplant fitted into a conventional mild steel tube frame with a telescopic fork and swingarm rear end with 5-way adjustable shocks. Brakes were a single floating 2-pot caliper disc at the front and a single-leading-shoe drum at the rear. Equipment included electric start (with kickstart backup), gear position indicator, crankcase oil level sight glass, and a blown stoplight bulb warning light.

Cycle liked the KZ400’s easy starting and the smoothness and even power delivery of the engine. Criticism centered around drivetrain lash, the “inadequately damped” front fork, and poor rear shocks. Reviewing the final model in 1979, Cycle World concluded: “It’s an easy to use motorcycle which would seem to be just what commuters have been asking for.

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