The CB750K1 Four was sold from 1970-71 and was
available in one of four colors: Candy Ruby Red, Candy Gold, Valley Green
Metallic, or Candy Garnet Brown. The gas tank stripe was gold. The tank, side
covers, and upper forks were of the basic color (red, gold, green, or brown).
The side covers were smaller and there were no slots on the leading edge. There
was a two-throttle cable system (pull open and pull closed). The headlight shell
was also the basic color. The exhaust system was a 4-into-4. The engine was a
736cc SOHC 2-valve dry sump inline 4 cylinder linked to a 5-speed transmission
and chain drive. The serial number began CB750-1044650.
Honda CB 750 Four K1
Air cooled, transverse four cylinder, four stroke, SOHC, 2
valve per cylinder.
"The Finest..Must Be Somewhat Extravagant, To
Surpass Basic Need, Yet Be Incredibly Functional, To Enhance, Rather Than Merely
Satisfy The Riding Experience"
TIRED OF PEOPLE not noticing? In past years,
motorcycling marked you as a man apart. Not a freak. You didn't start riding to
be contrary. But when you started, you discovered a feeling, a free, physical
joy, privy to yourself. You glowed inside, glad that you had something that the
common man didn't share. But now everyone rides a motorcycle, and you've stopped
waving to the other guy, and things don't seem the same anymore. The only thing
that could relight your fire is the very best road bike in the world. It would
have to be extravagant, so that the envious bystander would be forced to say,
"But who really needs all that," proudly thumping the tank of his leaky Twin. It
would also have to be extremely functional. Roadable. Comfortable. Responsive.
You want a meaningful mechanical entity, not a bejeweled pig.
Owning a bike like this, you could thumb your
nose at the Honda Motor Company, which is most responsible for seeing that
hordes of nouveau riders crowd you on your private road. But if you had the
finest of all production machines, this two-wheeled answer to Ferrari-Maser-Vignale-Porsche-Lusso-Lambhorgini-Super-Pesante,
you would be riding a Honda 750cc four-cylinder. Soichiro-San would have the
It is so clear as to be beyond argument. Some
will say that it is too heavy, or the suspension is too stiff, or it is too
quiet, or that four cylinders is too many for a motorcycle. But the total is
greater than the sum of its parts. If the Four didn't run faster than 120 mph,
if it didn't turn a 100-mph standing quarter-mile, it would still be the finest.
This superlative covers many factors. Most people
take it for granted that you can't lean a 500lb behemoth around turns like you
can a good Single or Twin. Fact: it is nearly impossible to ground the 750,
which allows as much, or more, banking than the Superhawk. Further, the weight
seems to provide little handicap to proper handling. The springing is stiff, so
the chassis is traveling in the same direction as the rolling gear most of the
time. Cornering at speeds from 60 to 110 mph, the Four shows very little
tendency to "pogo" or shake its head.
Heart of Honda's new masterpiece is the engine,
of course. It's easy to fawn over as a modern-day marvel, but it would be more
appropriate to say that the motorcycle industry, in its present stage of
development, deserves nothing less. What is surprising about the 750 is that
development of the complete motorcycle took less than a year. Honda had not even
decided the basic engine configuration by summer of 1968.
The choice of the four-cylinder design and its
basic features was based, not upon pure drawing board theory, but upon Honda's
experience with other racing and production engines. Even Honda's Grand Prix
formula car plays a part in the heritage of the 750. That it would be a Multi
was obvious—for greater efficiency. That it would be an in-line Four was based
on the firm's seven-year experience with four-cylinder racing motorcycles, as
well as more recent experience with production car engines.
The 750 follows the classic four-in-line pattern,
with the outer crankpins opposed to the inner two by 180 degrees. There is a
spacer between the center cranks to allow room for twin single-row chain drive
sprockets, as well as the drive sprocket for the single overhead camshaft. The
five-mainbearing crank is forged in one piece, turned on a lathe, heat treated
and then finish ground. Firing order is 1-24-3 (cylinders numbered from left to
right). Power is taken by chain from the center of the crankshaft to the clutch
by way of the mainshaft. Center drive allows the clutch to be positioned
inboard, reducing engine width. There is relatively little difference between
crank speed and clutch speed, so the clutch size may be reduced, as it does not
have to bear the brunt of a severe gear multiplication. This does, however,
cause engagement to be rather sudden in relation to the amount of movement the
rider makes with the clutch lever. Practice will overcome this one idiosyncrasy
of the 750, so that smooth starts will be a matter of course.
Use of chain, rather than gears, for the primary
drive has clear advantages. It simplifies crankshaft construction, for one
thing. The chain transmits power more efficiently, and presents yet another way
to reduce engine weight. While power transfer by chain offers the possibility of
snatching, Honda seems to have eliminated it with the use of a spring-loaded
rubber tensioner for the twin primary drive chains.
Unusual (or unexpected, we should say) is Honda's
use of plain bearings in all major engine bearing surfaces including crankshaft
mains, connecting rods and camshaft. The public has always associated Honda
motorcycles with constant high rpm running, to which the use of rolling bearings
is also associated. However, there is much argument in favor of the plain
bearing, and Honda's departure from its "usual" practice does not seem so
radical if it is viewed in the light of the company's use of plain bearings in
its production car engines— and even in its latest 12-cylinder air-cooled
Formula One car. Compared to a rolling bearing of equivalent load capacity, a
plain bearing is lighter in weight, costs less to assemble, and is smaller in
size—all important factors in a multi-cylinder engine that must be crammed into
a roadable bike chassis. The plain bearing is also quieter. In a Multi, where a
greater number of bearings are required, they contribute greatly to Honda's
desired goal of silent engine operation.
As for load capacity, the arguments are also in
favor of plain bearings. The maximum load capacity of a rolling bearing is at
zero rpm, and decreases as rpm increases due to flexing of its components and
fatigue. But a plain bearing has relatively little flexing problem. It is
hydrodynamic (it rides on an oil film and metal surfaces do not touch at all)
and as the "hydro" action increases with bearing speed, so does its load
The rolling bearing is commonly thought to be a
friction-free bearing. This is a myth. Rolling bearing components roll, which
creates friction, and the balls or rollers also have a tendency to throw oil
from their path. When properly lubricated, a plain bearing tends to retain its
cushion of oil and the cushion becomes more effective with engine speed, a
factor which produces less friction.
Naturally, lubrication must be optimal with plain
bearings, so oil in the new Four is pressure-fed at 60 psi (it never drops below
30 psi even at low engine speed). It is worth noting that Honda didn't "mix"
rolling bearings in with plain bearings; Honda engineers reason that, as long as
a high pressure system has been created to lubricate plain bearings somewhere in
the engine, it makes sense to unify the system as much as possible. This
explains the presence of plain bearings throughout camshaft, connecting rods and
Yet another reason for a unified pressure system
is the heat factor. Oil carries away heat, and flow must be optimal in these
critical engine areas. For the converse reason, the transmission, which shares
engine oil, is on the scavenge side of the system; heat is less critical there.
Naturally, rolling bearings are used for the main transmission components, as
they work best under lower pressure conditions. Both the engine and transmission
are dry-sump, for minimal oil drag. For the first time, Honda has employed a
separate oil reservoir, the main reason being to make the engine package tidier
Another departure for Honda is the CB750's "undersquare"
bore/stroke ratio (63 by 67 mm, to give a displacement of 736 cc). No other bike
in the Honda line has a long-stroke configuration, and, at first glance, there
seems to be no advantage to engine efficiency in going undersquare. But, the
CB750's 8500-rpm redline and power peak isn't all that high (compare this to the
10,500-rpm redline of the CB350). So, evidently, Honda has eschewed being at the
top of fashion in favor of narrowing the bore, to ultimately reduce engine
width. At 8500 rpm in a production engine, the narrower bore makes little
difference in efficiency or piston speed; in fact, the engine seems capable of
being turned at much higher speeds, with appropriate valve train modifications.
The 750 is in. an extremely mild state of tune,
with intake/exhaust valve overlap being not much more than that of a good
touring car. One of the reasons for this mild tuning is to achieve tractability.
Honda was successful in this tack, as the 750 may be pottered around town
between 2000 to 3000 rpm. The other reason for mild overlap is to achieve good
gasoline mileage, which was one of the three most important criteria set up for
the 750 before the engineers went to their drawing boards. In one of our testing
sessions, we subjected the 750 to a six-hour "70 percent run," which would
correspond to a combination of fast touring and frequent sporting bursts from 65
to 110 mph through the swervery. We achieved a figure of 29.9 mpg, which was
extremely impressive under such agitated throttle conditions. In steady freeway
touring, the CB750 rider could expect 25 to 30 percent better mileage, which
would net about 150 to 180 miles on a tank of gasoline.
There is nothing unusual about the CB750's valve
gear layout. The valves are operated by rockers, and paired inner/outer valve
springs. The tappets are easily accessible for adjustment through rocker box
caps, although the fuel tank must be removed to reach them. There is one
innovation in that the intake and exhaust valves are offset slightly to minimize
the possibility of collision. While it may seem curious that Honda didn't go
all-out with a double overhead cam layout, it must be remembered that the
benefit from the extra cam would be doubtful for a large-bore touring engine.
Honda considers the reduced head size of an sohc engine to be of greater
advantage, as it may fit more compactly into the frame. There is also the
consideration of a dohc engine's greater weight; the CB750 head, being sohc,
weighs hardly more than the dohc head of the smaller CB450.
The cylinder block is, in effect, split into two
halves—two left-side cylinders and two right, with a gap in the middle for the
cam drive. The finning is generous and each cylinder is separated from the
others by large air passages for optimum cooling. The great flexibility of
shaping is made possible by shell mold casting. Honda engineers found that air
cooling the Four presented no problems at all. In testing of early production
models in high speed runs on the open-limit Nevada roadways, cylinder head
temperature was the same on the inside cylinders as it was on the outside.
The CB750 pistons appear run-of-the-mill, with
two compression rings and an oil scraper ring. But they possess minute features
that are the product of many hours of testing. On examination, the first thing
you notice is the greenish Teflon coating on the face of the scraper ring
blades. Its purpose is to eliminate excess wear during the first few, and
critical, operating hours of the engine. The coat soon wears away, having
accomplished its purpose. (Honda also removes the burr at the bottom of the
cylinders, a by-product of cylinder honing, to prevent piston scuffing.) The top
of the piston looks flat, but actually has a slight dome, for the sole purpose
of increasing the compression ratio. A true flat top piston would be preferable
to avoid the dome piston's tendency to collect oil mist blow-by at its peak,
which makes itself evident with a light puff of smoke from the exhaust as the
throttle is rolled off and turned back on.
In laboratory testing, Honda engineers tested the
pistons by running them for 200 hours at 70 percent power (at 6000 rpm), then 20
hours at full throttle (8500 rpm). Piston scuffing from metal-to-metal contact,
and excess heat, proved a problem in the first prototype designs. This was
eliminated by reducing thickness of the piston top so that heat would be
transferred away to the walls at a more rapid rate. It was also necessary to
taper the piston in progressive stages. There are four stages of taper (five,
counting the ring area) to provide the right combination of looseness (for
maximum power output) and tightness (for low engine noise).
The Four uses a new type of electrical system for
Honda - an excited field alternator with a contact point type voltage regulator.
This is an automotive approach, and is necessary because of the heavy demands
made on the system, which must support sealed beam headlight, taillight, turn
signals, instrument lights, horn, electric starter, and ignition for four
cylinders. The effect of the system, in which extra output is gained by exciting
the main source of power with another small current, is to provide more than
enough power to meet the Honda's needs (apparently word got back to Japan that
some of us are so mad as to install extra lights and radios on our machines).
Carburetion is handled by four separate Keihins,
one to a cylinder. These are piston valve, double float items with no difference
from the standard fare, other than a provision for a vacuum gauge. They share a
common gigantic air box with replaceable paper filter element. The four
separately adjustable throttle cables meet in a junction over the engine which
is operated by single cable from the throttle grip. Choke levers on each are
connected by a rod; the length of rod between each carburetor is changeable to
allow individual adjustment of each unit.
The five-speed transmission is hefty and the size
of its gearsets appear well up to handling the CB750's maximum rated torque
(44.12 lb.-ft. at 7000 rpm). In basic design, it differs only slightly from
previous Honda practice. Where the shifting drum, in other models, acts as fork
carrier, it is used only for locating the forks in the 750. The forks actually
pivot on two large pins. The seven clutch plates are of a semi-neoprene
material; Honda has used metal plates but found them troublesome. Provision for
automatic final drive chain oiling is made through the drive sprocket; oil
passes through a drillway in the shaft and is thrown out onto the sprocket by
centrifugal force. There is an extra shaft in the transmission—a transfer shaft,
which picks up power from the countershaft to deliver it to the final drive
Of most interest in the effective, but mostly
conventional chassis/rolling gear assembly is the hydraulically operated
single-caliper disc front brake. One of its most welcome features is that the
disc is cast stainless steel, which does not rust, as do the non-stainless discs
on other disc brakes preponderantly available. As the disc is exposed to view,
it is nice to know that it will look as nice after a few years use as when it
came off the showroom floor. There is a chance of more brake noise due to the
use of stainless, which has mediocre sound dampening quality. To counter this,
Honda has made the puck, or brake pad, slightly spherical over its gripping
surface. The puck is self centering.
Unlike several other double and single disc
brakes we have sampled, the Honda unit is not at all touchy. It is reasonably
fade-free for a 120-plus-mph machine and possesses enough stopping power to
break the front wheel loose at any speed. However, the lever action must be firm
to do so, and there is a responsive, broad zone of gradation between full-off
and full-on—a decisive safety factor. This excellent feel and broad gradation
may be a by-product of rounding the brake puck friction surface. When the brake
is applied mildly, only part of the puck may come into full contact with the
disc. Full contact and full stopping force is achieved gradually because the
puck must be highly compressed before the entire surface of its face comes into
full-pressure contact with the disc.
The CB750 frame is hefty, consisting of a full
double cradle and maintube, bolstered by two smaller auxiliary top rails. These
rails, joined to the maintube by triangulating struts, extend straight back from
the front downtubes to function as the top rails of the rear subframe assembly.
The swinging arm assembly is mounted inboard of the cradle. The swinging arms
are two-piece welded stampings, rather than the usual tubing; due to the wall
thickness and overall size of the arms, they should do the job properly. Hard
power-on cornering gave no evidence of weakness.
As mentioned before, the suspension gives a
comfortable, but decidedly firm ride. Evidently the rear spring/damper units
(nitrogen-sealed non-serviceable De Carbon shock absorbers) are at the peak of
stiffness for a single rider weighing 160 lb. It is best to leave the springs at
their soft setting for one-up sporting endeavor. On the two harder settings,
spring travel is reduced enough to inhibit damping action and cause slight rear
wheel chatter under hard acceleration, braking and cornering.
The front fork springs also are stiff and travel
is restricted enough to prevent the machine from giving a violent heave forward
under heavy braking, or upward and downward in fast corners. The only unpleasant
aspect of the front suspension is its reluctance to recoil immediately with
minor road deflections. Thus the bike may begin a light up-and-down oscillation
at certain speeds when the deflection is periodic in nature - i.e., over regular
tar joints, or the junctions of repeating strips of concrete. The cause seems to
be the rather high breakaway friction of the front fork bushings and seals, and
studies are being made in Japan to find bushings and seal materials with a lower
friction coefficient. For the time being, the rider will have to adjust by
increasing or lowering speed. There is a chance that the problem could correct
itself after a few thousand miles, as the seals and bushings wear in. At any
rate, this is a small detraction from what must be regarded as the finest
handling machine in its weight class.
Many points about a machine other than the cold
(but in this case enervating) statistics of its performance come to light during
the CYCLE WORLD performance tests. The most obvious, during the tire-scorching
100-mph quarter-mile acceleration runs, was the excellent placement of a
gorgeous pair of instruments - the 11,000-rpm tachometer and the 150-mph
speedometer. Their faces are broad and easy to read. More important, they are
positioned upright so that they may be read from a racing crouch with no
difficulty at all. They are illuminated for night riding and give off a
beautiful, reassuring glow. Another important feature is found on the throttle
grip - a two-way ignition kill switch. Flip the switch with an easy movement of
the thumb in either direction from center and the engine is turned off; in
moments of panic, which could come if the throttle jammed on such a big machine,
it is impossible to flip it the wrong way. In this respect, it is superior to a
kill button, which requires constant pressure to keep the fire out.
The seat is broad, padded, great for two-up
riding and looks good with its pleating. The handlebars are "Western" style, but
not overly wide, and are comfortable against the wind. The bike is tall, overly
so for the short rider, but the footpeg position is high enough to suit most
riders, short or tall.
It is hard to keep from raving at the way the
hardware on the chassis has been arranged to allow what must be the ultimate
angle of lean for any big-bore. Nothing grounds. The four separate
megaphone-style silencers, which are joined by balancing tubes, are turned
upward, well out of the way. The side and center stands have no projections to
ground. The crankcases are narrow and have hardly any overhang. One CW staffer
was exiting from an unwinding 50-mph left-hander, and found a portion of his
sole touching the ground, even though he had the tip of his boot touching the
It is amazing that the tires—standard Japanese
Dunlops with ribbed pattern front and block pattern rear - hold that well under
such treatment. Japanese tire quality has greatly improved over the past few
years. Choice of the rear tire makes us curious, though, as it is not made of
one of the special long-wear compounds chosen by the manufacturers of other
high-powered Multis from Japan and England. Prognosis for good rear tire mileage
can hardly be optimistic.
But maybe some positive thinking is in order.
Like the old saw about buying a yacht. If you can afford to buy the finest
production road machine available (which also happens to be a bargain in its
displacement category), you're not going to ask how much it costs to keep it in
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