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Honda CB 750 Four K7

 

 

 

 

Make Model

Honda CB 750 Four K7

Year

1977

Engine

Air cooled, transverse four cylinder, four stroke, SOHC, 2 valve per cylinder

Capacity

736
Bore x Stroke 62 х 62 mm
Compression Ratio 9.2:1

Induction

4x 30mm Keihin carb

Ignition  /  Starting

Electronic  /  electric & kick

Max Power

67 hp @ 8000 rpm 

Max Torque

6.1 m-kg @ 7500 rpm

Transmission  /  Drive

5 Speed  /  chain

Front Suspension

Telescopic forks non adjustable, 141.5mm wheel travel

Rear Suspension

Dual shocks, Swinging arm. 101.5mm wheel travel

Front Brakes

Single 293mm disc

Rear Brakes

180mm Drum

Front Tyre

3.25 H19

Rear Tyre

4.50 H17
Seat Height 825 mm

Dry-Weight

236 kg

Fuel Capacity 

19 liters   /  5.0 gal

Standing ¼ Mile  

12.4 sec

Top Speed

125 mp/ h  /  200 km/h

.The CB750K'77 Four K was sold in 1977 and was available in one of two colors: Candy Alpha Red or Excel Black. The gas tank stripe was gold with a white and red pinstripe. The "750 FOUR K" side cover emblem was gold. There was a two-throttle cable system (pull open and pull closed). 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 CB750-2700009.

Road Test Cycle World 1979

Never mind for the moment that style gets all the attention. Consider instead the list of modern motorcycles cance140 from the catalogue and returned to production because of public demand: Honda's CB750K.

Inspect the sales chart. Which street Honda is the most popular and has been for several years? The CB750K.

Take that, fashion. When Honda geared up for its evolution, the second year of new and technically creative models, one thing that must have been at the top of the priority list was a 750 Four with four pipes and wire wheels. We bike nuts like speed and we like engineering, but we all know there are some traditions too good to forget.

Especially if, like the twin-cam, race-based 1979 CB750K, they work.

To call the new 750 race-based is perhaps misleading. The dohc Four is not precisely descended from the RCB endurance motors. Nor is the Four a little brother to the CBX Six.

What we have with the new 750, and the RCB and the CBX, CX500 et al is simply a pool; of talent design. The men now running Honda Research and Development have common experience; they've been king together on the racing team, the emissions control programs, the blue-sky think tank. They have formed a considerable body of knowledge and through trial and error have come to believe that some techniques are better than other techniques that engines and frames and brakes. etc., can be improved if designed along certain lines.

The new 750, then, is part of the CBX, RCB, XL and CX family.

Like the other Honda multis, the CB750 engine uses plain bearings, two-piece connecting rods, a one-piece crank and high pressure oil system. At 62 x 62mm bore and stroke, it's exactly square, which is just what you'd do if you wanted a balance between a narrow engine and a low one.

The cylinder head has two camshafts with Hy-Vo chain from crank to exhaust and a second Hy-Vo chain from exhaust to intake. Each cylinder has four valves, two exhaust and two intake, with the combustion chamber a pent-roof shape and the spark plug centrally located. At 9:1 the compression ratio is on the high side of moderate. So is camshaft timing and lift, and so—considering the power potential of the valve train and valve area—is the 9500 rpm redline.

Balance. To the old requirements of power and reliability, add emissions and noise. Given enough c.r. for power, the chamber can be shaped for optimum combustion and thus reduced emissions. If you can bring down emissions that way, you can fit large carburetors and let the engine breathe and get the power, and so it goes. Honda makes cars, too, and that means R&D has been working on emissions and combustion efficiency right along with their racing programs. It shows.

Thought has also been given to maintenance vs complexity, i.e., adjusting all those valves. The CB and CBX engines share some valve train components and they also share a Honda tool, to keep each pair of valves open while the cam is rotated around and adjustment shims are popped in and out. The shims won't pop out on their own, by the way, because all the shims are thicker than the valve clearance and (Honda says) the light valves will follow the cam's contours well past redline.

Equally neat is a semi-permanent gasket for the cam covers. It's shaped to fit the channels cast into the covers and the top of the tower. You don't need gasket sealer, the material won't tear and it can be used several times.

The oil pan—this is a wet sump engine like other Hondas and unlike the earlier 750s. The designers now have other ways to improve ground clearance and the separate tank was always a source of leaks—has a replaceable fitting for the oil filter cover. If the mounting bolt gets stripped, as happens, the owner can replace the fitting rather than the entire pan.

Oil supply to the camshafts and valve gear is via an external line. Used to be the oil travelled through hollow studs with O rings. The rings got pinched and oil seeped out or didn't reach the valves. The external line is simpler. (The same thing appeared on the latest Kawasaki KZ400 and the Yamaha 750. A trend or a way to keep engines cleaner.)

The relative narrowness of the Four vs the Six has made the design simpler. The pointless inductive ignition runs off the left end of the crankshaft and the alternator is on the right end. Nice language note here. The designers said the alternator is on the crank, for engine balance. Come again? we asked, wondering what the alternator had to do with vibrations and counterweights.

Nothing. The old 750 Four had the alternator on the left and the center of gravity was to the left of the wheels, so if the rider took his hands off the bars, the bike would wander to the left. Swapping sides cured this. Oh. The engine is now in balance. Of course.

The alternator also serves as a flywheel, and is on the crank because having the load of the heavy weight on the jackshaft would feed greater loads through the HyVo primary drive. The jackshaft runs the mainshaft through gears and then to the clutch, an eight-plate job with one plate a multi-material damper. Five speeds, of course. The engineers say the new drive train is 86 percent efficient, while the previous 750 system delivered 83 percent of the power developed at the crankshaft.

The 30mm CV carbs have an accelerator pump, to keep the correct mixture on sudden throttle openings while allowing lean mixtures at cruise and idle, as required by emissions control standards. The CV carbs need lighter throttle return springs than do slide-type carbs and the pull from the new 750 is restfully light.

There are four exhaust pipes and four honest exhaust pipes. No balance tubes, no collection box. Individual pipes run cooler and tend to rust earlier, so Honda has provided thicker metal on the outside and a tar-like coating on the inside.

Subtle tuning note here. The 750K engine is rated at 72 bhp and the 750F engine at 75 bhp. The only difference between the engines is the exhaust. The F has a 4-into-2 system. Although tuning theory says the four-pipe method gives more power than an unbalanced, that is, pairing cylinders that are 180° apart, system, the F pipes give more peak power. And the K gets most of it back, as the factory's power curves show the K to have more power and torque than the F below 8 thou, when it's most useful.

The 750K's frame is new but not like the other new Honda frames. It's a double cradle, full loop. Honda R&D has a preference for open loop/stressed engine frames, as with the CBX, CX500, XLs and Hawks, because they believe that system can be lighter and as strong as a full loop if the engine is suited to the job. The Six, the V-Twin and the counter-balanced Twins and Singles are. But the transverse Four generates vibrations that don't work with an engine that's also a frame member. The design team said they in fact experimented with stressed-engine 750s, but by the time the things were as strong as the full loop bike, they were heavier and more complex

The front downtubes are widely spaced and triangulated with a cross tube, and the steering head/backbone area is triangulated in three dimensions. The backbone is arched so the cylinder head can be removed with the engine in place. The right downtube has a removable section so the engine can come out easily. The frame is also triangulated below the seat, that is., between upper shock mount and backbone. The 750F frame has extra gussets and several sections of heavier tube. Could be because the F is the sporting model, and it could be because the added exhaust pipes and minor bits make the K weigh more than the F does. The lighter frame keeps this difference down to 7 lb.

Wheelbase is 60.5 in. and steering head angle is 27.5° from vertical. Handling characteristics are determined in large part by the wheelbase/rake ratio. The new 750 is a move toward the European long/steep school of thought, while the first Honda 750s had less steep rake angles and shorter wheelbases.

Front forks have beveled stanchion tubes within the sliders, for better oil flow, and low-friction seals. Another change is added wheel travel, 6.3 in. compared to 5.1 in. for the 1978 CB750. Added travel allows softer springs and smoother compliance on rough roads. (Interesting that the new BMW R65 has less front wheel travel than older BMWs, for less dive under braking and subsequent changes in steering rake and wheelbase.) The rear shocks are Honda's FVQ two-stage units, with five s; stings for spring pre-load. No innovations, in other words.

The 750K's cornering angles are listed at 50° from vertical, static, and 41° under load. Good for Honda. Somebody has finally considered that cornering clearance with rider aboard and with suspension compressed by cornering force isn't the same as when the bike's just sitting there. Too bad the others don't do this, because for now we have no way to compare them. The K's visible distinctions, aside from being a twin-cam 750 of course, are the pipes, the wire wheels, some extra kick to the seat, and the brakes. The K has one front disc, behind the sliders, and a drum brake in back. Honda has been working overtime on the wet-weather disc problem, but one of our men says as a touring rider he's mighty glad to see a good or drum on the machine.

There are myriad small touches. The front master cylinder has a phillips-head-secured cap, because screw-type caps get removed out of curiosity and uncovered brake fluid absorbs water. The fuse box is mounted on the top triple clamp, below the instruments. Easy to find, plus moving the fuses from under the seat allows more room for the airbox and a larger airbox can be both quiet and unrestrictive. The air filter is replaceable. Most of the engine and transmission bolts, the ones which usually are phillips head, are hex head, the better to not strip at the worst possible time. The taillight has two bulbs. The key switch is on the instrument panel, so is the choke control and the trip odometer resets with a twist-and-push button.

The mirrors are amazing. Honda is using hollow stems and weighted backs, so the natural resonance of the bars is the same as those of the mirrors.

They stay clear at all engine and road speeds. One nit-picker said he could detect a tiny buzz on the right one, but the consensus was we've never seen clearer mirrors on any motorcycle.

Styling is Honda; a big tank curved to align with the side panels, sort of a one-piece body look. The K has a tailpiece, like the F models old and new, with a wiggle-bar (one of the wives calls it that. When she needs to wiggle, she braces herself on the back seat railing). The bars are high and the seat is stepped. The F versions have most of this; there is less distinction, in looks and specifications, between the new K and F models than previously.

So much for the specs: What does the CB750K do?

Damned near everything.

When the original Honda 750 appeared, it shook the world. Quoting CYCLE WORLD from 1969, "during the tire-scorching 100-mph quarter-mile acceleration runs" that first 750 Four (we called it the ultimate, by the way. We won't make that mistake again.) did the quarter mile in 13.38 sec. at 100.11 mph.

Since then we've had the government with its emissions controls and noise rules and safety equipment and they've ruined all the fun, eh?

Quarter mile times for the twin-cam 750?

An astounding 12.69 sec. at 105.5 mph.

Better than the pre-control bikes, better than the old 750F and better than all the other 750s in production.

How about that?! Government regulation means more research and more research means better motorcycles.

Brake performance wasn't as impressive. The figures are just about equal to the first 750, despite some added weight. But the 750K needs more stopping room than does the CBX. Some of this may be tires, as illustrated in this month's front tire comparison. Some of it may be sheer brake area, as in one disc vs two.

And some of it may be Honda's work on disc pad material. Discs are notorious for losing power in the rain. Honda has developed a pad material that isn't as affected by water. The test 750K had some of the out-of-round feel noted with the test CBX, and the 750 required more pressure on the lever, and the front brake squeaked and warbled under moderate lever pressure like the birds of spring.

Stopping distances and pressure and control are all respectable, mind. It's just that we'd expected superlatives here as well.

The engine is nearly flawless. Fires up in maybe two spins, hot or cold. The choke is calibrated so you can set a nice high idle while you're buckling your helmet. The CV carbs surge a bit on damp mornings for the first five miles or so 'and then smooth out so well you'd hardly know they were CV carbs. Control placement and function are fine, excepting possible the oil pressure warning light, which reflects the sun and gave us a start.

The turn signal switch isn't as good as its intentions. There's a lane-change position; a nudge that doesn't lock, and a lock for turns. The nudge is not quite light enough and the detent takes practice. Good idea, though, and one that will work right .in time.

Driveline snatch is notable by its absence. Clutch pull is, well, we can't quite agree with the Honda man who said the wind at 95 will push the lever in, but we know what he meant. Shifting takes firmness. Too slow a toe and you'll find more neutrals than you knew were there. A good shifter, but not a great one. Internal ratios drew no comments, perhaps' because the engine is so strong that one seldom needs to downshift and so quiet that revs/road speed don't get noticed. Proper big fuel tank, with four gallons on normal and 1.3 in reserve. Ridden normally, you can go 200 miles between fill-ups.

The passenger may complain. Strange. The CBX has a short and tipped passenger section and nobody can complain, because two-up isn't something the CBX owner will do often. The passenger space is for an occasional short ride or a convenience. When your buddy needs a lift home, it's good to be able to offer it legally.

But the same general shape appears on the 750K, a bike that will be bought for two-up trips and touring. Such a machine should have a longer and less shaped back seat.

The operator's section is better. Some. The step does give you something to brace against, if you are of medium height. The curve in the middle works okay. It's a wide seat, though, and leveraging the legs outboard brings them further from the ground than the measured seat height would lead you to expect. That gives the bike a heavier feel in slow traffic than its weight would indicate.

Nor did the bars win our hearts. High, and with a bend that cramped some wrists, The height is most of the objection, as although they are fine at most speeds, when you're hard on it and/or have a headwind, the upright posture forces the rider to hang on or lean forward with arms bent. Both are tiring.

One tends to forget (or forgive) the above once on the road, Even without air forks or adjustable damping, the K's ride is smooth, over little bumps or big ones. Good compliance at both ends. This is a sporting suspension, even so, and although the bumps can be felt, they have been cushioned before arrival.

Handling is as happy a medium as we can recall. There's a slight feeling of falling into turns at drive-way- entrance speeds, but from there up, the 790K goes where you point it.

Medium. Perhaps because the two models are fresh in our minds and are themselves such a contrast, at speed the 750K fits between the BMW R65 and the CBX.

On the same turn, same speed, the BMW sweeps through guided by the rider's wishes. The CBX walks about on bumps and must he forced down and around.

The 750K tends to run wide but gentle pressure on the bars will tighten the line exactly where you want it to be and around you go.

There is no shake, no wobble. Side by side with the CBX across slabs in the pavement and such, the 750K can be felt working over the ridges. But it feels like one solid piece as the suspension, well, suspends and the dampers damp. What we have here is a good strong 750 that feels like exactly what it is, as solid and agile a bike as there is in class.

That's on the street. As usual, since finding the performance limits of a large bike on the street is dangerous at best, we took the CB750K to Ontario Motor Speedway for an American Federation of Motorcyclists (AFM) club road race. Like most street motorcycles with compliant suspension, the CB750K wallowed and wobbled on the track, especially in sweeping, highspeed curves, when exiting turns on the gas, when entering bends and at top speed on the straightaway. The condition was very distracting during the first 15-minute practice session.

During the second practice, the rider concentrated on making very smooth transitions, as when changing from upright to leaning and from braking to turning. Avoiding abrupt changes in direction and rolling on the throttle at turn exits brought the wobble under control in the infield, and pushing against the handlebars and sitting perfectly still kept the bike stable on the straight.

The footpegs were the first things to drag during hard cornering. With the side and center stands removed, the next things to touch down were the brake lever on the right and the exhaust header pipes on the left. Any more lean lifted the rear wheel and started the bike sliding. Bridgestone Mag Mopus tires were well matched to available ground clearance, although they did heat up after several fast laps and begin to slip a little at maximum lean. The rear tire began sliding under power at turn exits, setting up a definite, but controllable, drift.

The front disc brake was adequate, but not outstanding; the rear drum brake very controllable and not as prone to locking up under hard pre-corner braking as the rear discs found on many 750s.

AFM Production rules allow internal engine modifications (such as cams, head porting, pistons), the removal of turn signals and taillights, use of accessory shocks, short handlebars, wider rims and racing tires. But despite the fact that it was flung around the track with all the street equipment (except mirrors and stands) attached, and despite the fact that the K model is more touring oriented than the sporting F model, the Honda performed very well. It accelerated harder than all but one of the dominate-model Suzuki GS750s entered, and matched all but one for top speed, indicating 125 mph in fifth, over-revving 500 rpm to 10.000 rpm.

In the race, a first-turn mishap kept the bike and rider from finishing well—the 2.19-mile course was run backWards, funnelling the combined Open-750-550 Production grid through what is normally turn 19, a tight, second-gear corner. Several riders got out of shape at the entrance and ran off the track. Faced with the choice of ramming the others or running off the track with them, our test rider rode onto the grass. By the time he had turned around and rejoined the fray in next-to-last place, the leaders were gone. The Honda finished sixth, but based on lap times, should have finished in the top three positions—probably second—if nothing had happened. Anybody who has been around racetracks knows that it is no disgrace for a stocker to give away two seconds per lap to a modified, accessory-shocked, racing-tired production racer. With shocks, rims and tires, the Honda CB750K could win; engine modifications would probably ensure victory. With the K version so close, the lighter, faster CB750F should be an instant winner.

In sum, the substance is better than the style. The flaws are in the area of bars and seat; personal, variable with rider size and weight and experience. Honda has kept the 750K flavor. The designers have done some things in line with what the other factories have been selling. If they've gone a bit far, so have the others. The cure for those who object is as close as the nearest shop with seats and bars for sale.

The important things, the engineering, are pure Honda. The engine is terrific. The frame and suspension are balanced and' engineered for each other and the engine. Just about every detail has been done correctly and the competence of the men who did it shines through.

So the four-piper Honda is 10 years, old. The new one does everything the first one did, but better.

That should entitle the 750K to 10 more years at least.

Source Cycle World 1979

 

 

 

NOTE: Any correction or more information on these motorcycles will kindly be appreciated, Some country's motorcycle specifications can be different to motorcyclespecs.co.za. Confirm with your motorcycle dealer before ordering any parts or spares. Any objections to articles or photos placed on motorcyclespecs.co.za will be removed upon request.  

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