Honda Motosport Sl350
HONDA MOTOSPORT SL350
CYCLE WORLD ROAD TEST
It’s Getting To The Point Where We Will Have To Admit That Honda Has Built A Real Dirt Bike.
IGNORING SUCH technicalities as the coming of a new model year, Honda has embarked upon a campaign to drastically reduce weight on the "SL" series dirt machines. First one of the series to benefit from the new program is the big SL350.
To quote from our Dec. 1969 road test of that original SL350: “. . .In heavier going, it is heavy going. It weighs 336 lb. fully equipped—enough weight to bottom the forks at medium speed on the average trail.”
The 1970/4 version of the Motosport SL350 K1 is still fully equipped and street-legal, and its styling somewhat resembles that of the original machine. But the new one is about 50 lb. lighter. The extent of redesign is great, involving modifications to both the engine and the running gear, not only to pare off weight but to improve handling, and tractability. The result is a totally different machine, lighter feeling, more controllable in rough terrain, and less front heavy. While it is not perfect, it is much more likely to hold up under the serious scrutiny of a dirt rider who is accustomed to pure-bred, off-the-road motorcycles.
Several changes make the trusty 325-cc sohc vertical Twin engine much more appropriate for dirt use. A new, milder cam grind lowers peak horsepower (30 bhp at 9000 rpm, instead of 33 bhp at 9500 rpm) to benefit pulling power at low engine speed, the latter being much more important for slogging around in rough terrain. Carburetor throat size has been reduced from 32mm to 24mm, to keep intake velocities higher at low rpm, contributing to smooth running at slow speed. Actually, the standard SL350 engine was fairly tractable. But these two simple changes make the revised version much more of a stump puller. While you could lug a 350 with 32-mm carburetors down to 1500 rpm, you couldn’t always be sure that the flame wouldn’t go out. A pair of big throat carburetors yield good top-end horsepower, but become less effective at low rpm. As the velocity through the throat drops, mixing of fuel and air becomes less than perfect. The 24-mm carburetors are of the double-float piston valve variety, lighter than the constant velocity 32s.
Honda has also seen fit to dispense with the luxury of an electric starter on the new SL350, a measure which had already been taken on the SL100 and the SL175. This, of course, saves a good amount of weight. At the same time, the designers took the opportunity to redesign the crankcases, rather than leave all the fitments intended to accommodate the starter motor. In the process, the cases have been narrowed slightly, the oil filler plug moved forward to an easily accessible spot on the gearcase, and the kick starter relocated to a position where it doesn’t stick out and bark the shins as did the previous model. One interesting feature: the kick starter is linked with the primary drive gear, rather than through the transmission, which means that it may be used to start the engine in any gear with the clutch pulled in. As more than a few of these machines are likely to be raced in the desert or run in enduros, the primary drive start is particularly useful, in that it saves time usually wasted finding neutral if the rider has killed the engine by locking up the rear brake on a downhill or lugging down too far on a hillclimb. This feature will soon be found on all of Honda’s off-road machines.
The SL350 frame is a completely new design, a single-backbone, full double-cradle frame of small diameter tubing supplanting the former single down tube design. Heavily gussetted at primary stress points, the new frame is not only lighter but undoubtedly stronger than the old variety, and offers greater resistance to lateral distortion. This last attribute will be appreciated when you get your SL to going poing-poing at full power in the boonies, as it helps to eliminate a phenomenon that rodeo cowboys call “sunfishing.” The welding on the frame is sloppy, as it was on the other SLs. Honda officials say that these frames are hand-welded. But we have reason to think that it’s automatic: no hand welder could do that badly.
Suspension performance has been greatly improved. The Ceriani-style telescopic forks now have 6.7 in. of travel, an improvement of about an inch over the original legs. Experimentation by two American Honda service department staffers—Bob Young and Masahiro Semba—resulted in suggestions to increase both compression and rebound on the front forks, which the home company followed. This, in conjunction with the overall lighter weight of the machine, prevents the bottoming which set in at the slightest provocation with the old model. The rear shock absorbers have also been modified to yield increased rebound damping, thus eliminating rear-end bounce after heavy compression.
An overall weight change of the magnitude of 50 lb. usually requires extensive retuning of the suspension units, but modification was further made necessary when Honda chose to make lighter front and rear hubs for the Motosport 350, thereby reducing unsprung weight and changing the deflection characteristics of the wheel assemblies. Reducing unsprung weight is of obvious benefit, as it makes it easier for the wheel to follow deflections in the terrain, improving both traction and stability.
Other changes which reduced the weight of the 350 are the high-mounted fenders, which are now made from pressed aluminum instead of steel, a lighter seat, smaller lighting units, and a reshaped fuel tank. At this point, the styling department came into direct conflict with the engineering department when they chose to retain the second spark arrester/muffler unit. As the exhaust pipes already have been channeled close together under the cradle, it would have been no problem to join the pipes into one muffler and save about 10 lb. This becomes even sillier when you see that unstressed points on the front fork tubes near the triple clamps have been tapered to reduce excess metal, and holes have been stamped into nearby metal shrouds with the same idea in mind. Okay, we admit the double pipes look neat, but really, fellows!
The new frame design provided opportunity to reproportion the seat/peg relationship. The footpegs have been moved slightly rearward, which makes it easier to jump the front wheel over bumps. The seat itself is as deeply padded as before but has been lowered from 33.1 in. to 31.5 in.—a change which will be welcomed by the short ones among us.
The simple act of sitting on the machine and pushing the handlebars from side to side will render pleasant surprises. It’s less top-heavy, and you are not cowed by visual evidence of excessive bulk, imagined or real. The 2-in. increase in wheelbase is visually evident, and makes the Motosport seem more attractive and trustworthy than its predecessor.
It starts easily, albeit in the usual cold-blooded fashion one associates with Honda engines—along with the usual two minutes playing with the choke lever until the engine reaches proper operating temperature. Warm starts on the new SL350 are a bit “iffy,” without the aid of the electric starter, as the kick lever does not engage at the top of the stroke and turnover speed is therefore slow. So a machine-gun style of kick—the same sort you’d use for a recalcitrant two-stroke— works well.
Point the bike down your favorite stretch of fireroad and you’ll discover the machine is really at home there. The 54.5-in. wheelbase, combined with 62-degree rake and 4.5-in. trail, yields fast handling of the sort which is quite useful on TT courses for snapping the back wheel out on power, to get the machine squared off quickly out of a tight turn. In combination with a broad, smooth, four-stroke type of power band that begins to work at 2000 rpm, a five-speed gearbox, and moderate weight, the Motosport geometry makes for a delightful slider’s machine. We had great fun putting it on a few “serious” 250 and 360-cc dirt machines, which happened to wander onto the Saddleback TT course while we were doing some hot laps. How disgruntling it must be to watch this stylish creature from Japan with lights and mufflers on—a traditionally cumbersome trailbike with a rather undistinguished dirttrack history—go sailing by you and disappear with a muted hum!
While we were engaged in this impromptu bash, a remarkable thing happened: the exhaust and mechanical noise from the 350 was so low that you hear the tires howl every time you cocked the bike over, got on the gas and got it sliding on the graded hardpan dirt. The tires are block pattern Bridgestones with rounded profile, which we find preferable to the usual flat-profile trials tire when the bike is used on roads or graded tracks.
On rough ground, the front forks provided excellent control, much improved over the Motosport’s predecessor. They could not be bottomed easily, they damped well on rebound and they had good lateral rigidity. The rear shock absorbers were less than ideal, however, and allowed the back end to bounce around excessively, even on smoother stretches of dirt. The serious rider would do well to substitute a set of competition-grade rear dampers and experiment with rear spring tension.
Woods riders will find the 350 quite adaptable to slogging around, as it turns quickly and is not so top-heavy as one would expect with that big sohc engine. This is because much of the weight removal has been done on the upper parts of the machine. The lower seat height makes it much easier to paddle the bike through sticky terrain, and the seat is extremely soft and comfortable. Stock gearing, ideal for fire-road riding, is a trifle high for forested or hilly areas. A bash plate will be necessary to protect the exhaust pipes and, while there is none, Honda has provided extra bolts and tabs for quick mounting of that optional accessory.
If you are willing to do without street-legal gear, the weight can be brought below 300 lb.—by removing lights, horn, tachometer and speedometer. This may not be impressive to two-stroke fans, but it is excellent for a motorcycle powered by a four-stroke Twin that began life as a roadster engine, particularly one that pulls with the steady, solid feel of a 500 Single.
Honda’s new SL350 is not yet a motocross machine, but it is not intended for that purpose. Rather, it is a beautifully conceived, tractable, reliable playbike with new-found rough terrain capability. It’s amiable and has great potential.
HONDA MOTOSPORT SL350
