Big Japanese Inline Fours start with the Honda CB750. It’s cheap, invincible, and it’ll get you there on modern roads.
Four decades ago, Honda changed motorcycling forever by making the first mass produced in-line four. The 125mph CB750 had features that rival’s brochures just could not match. They included an engine with obvious grand prix ancestry, electric starting, a five-speed gearbox and a disc front brake. The Dream Four, as Honda originally named it, was unveiled at the 1968 Tokyo Show and was on US roads the following summer. It was sold in the UK from January 1970.
Despite huge development costs, the blockbuster four was keenly priced – it cost less than a Triumph's Inline-Triple in the US. They sold by the shedload. Hugely influential, the CB750 design remained in production until 1978, with updates and derivatives. An estimated total of 553,000 were built. Well equipped – and weighty – the CB750 offers a remarkably modern level of sophistication. Prices are rising and some spares are getting scarce, but the dependable Honda can take you many miles at modern traffic speeds with hardly any spannering.
Honda CB750 - The Engine !
Company founder Soichiro Honda set a challenging brief for the youthful project team who were charged with creating the company’s first 750cc motorcycle engine. The bike had to deliver stronger performance than any of its rivals, but with less vibration and more reliability.
Imposing appearance was important. Yet the engine had to be compact enough to avoid creating an unmanageable monster, while being cost-effective to produce and easy to service. Honda chose an in-line four engine over Horizontally-Opposed and Vee Four cylinders. The layout’s advantages included smoothness and an affinity with Honda’s successful grand prix racers – riders were going to be much more interested in buying motorcycles that were like the race winners.
The finalised design electrified the 1968 Tokyo Show, and set an industry pattern that endures today.
Plain bearings are used throughout the bottom end, with big-end journals disposed at 180-degree intervals: the inner two are at top dead centre when the outers are at bottom dead centre. The cam chain and twin primary chains are driven by sprockets cut in the crank metal between the inner cylinders’ webs. Valve gear is simple, with a single camshaft in plain bearings that opens eight valves via rockers with clearance adjusters.
The biggest challenge was how to cut down the weight and width of a large capacity four. To minimise engine width, Honda made a radical decision to sacrifice revs – the bore is 2mm less than the 63mm stroke. To reduce engine height, they tilted the cylinders forward. And to find more ground clearance, they used a dry sump lubrication system – almost every other Honda has a wet sump system.
Oil is circulated through internal galleries by a twin rotor trochoidal pump in the crankcase. Some lubricant is supplied to the gearbox from where, on pre-1976 engines, a trickle was passed to the chain across the inner face of the output sprocket.
Driven by the paired primary chains via a shock absorber, the multi-plate clutch is in the right-side portion of an elaborate horizontally - split crankcase. There are five ratios in a conventional two-shaft gearbox, with output from a third shaft carrying the sprocket. The alternator is a sophisticated three-phase 180 watt Hitachi type with exciter coils rather than permanent magnets. It mounts at the leftward end of the crankshaft.
Four 28mm Keihin round-slide carburettors with integral float chambers are operated by a rocking beam throttle mechanism. The earliest models have an all-cable one-into-four exhaust. Honda knew that four separate exhaust pipes were not essential for performance but instead used them for striking visual effect on its 1968 bombshell and all later CB750s except the Super Sports and Automatics.
Teething troubles included a leaky head joint, soon addressed by an extra holding-down bolt. Honda dealt with early drive chain failures by enlarging the gearbox sprocket. You can get cam chain rattle if the tensioner’s badly adjusted or carburation is unbalanced. Generally however, Honda’s landmark engine is trouble-free.
Riding The Honda CB750 !
Size and weight are the things that strike you when you first approach the CB750. Yet the Honda is far from being unmanageable. Rolling it off the stand won’t give you a hernia and the choke lever is readily to hand. A mere jab of the thumb sets the four-cylinder powerhouse burbling and its power delivering is pleasantly docile at small throttle openings.
We rode a 1976 CB750, coded K6. On several markets, including the UK, it was the first update of the four since the K2 of 1972. The 900cc Kawasaki Z1 and Honda GL1000 Gold Wing had arrived by 1976, so the 750 could no longer claim to be the ultimate bike. It was simply an affordable 110mph workhorse ready to go the distance, while offering a high level of comfort and convenience.
Some of the edge has gone off the grand prix howl of the original four’s exhaust note. But the environmentally corrected K6 engine still has a pleasant tone and a useful head of steam. It may not spin as readily as a modern four, but making music by winding up to 7000 RPM through the gears makes for an exhilarating ride.
For a quieter life, you can saunter along in a high ratio, knowing you only need snick the leftside pedal down to stoke up enough revs for strong forward thrust when it’s needed. The gearchange is notchy at low speed and the clutch on this machine is far from silky, but on the whole the bike is easy to control, letting you to concentrate on enjoying the ride.
The seat is really comfy, although tingly high frequency vibes penetrate the filling across a fair span of the rpm range. The bars and footrests are well placed for brisk, rather than frenetic, road riding, with windblast inevitably becoming an issue at motorway velocity.
Except when making tight turns, when topheaviness is apparent, the four’s bulk can be largely forgotten. But, although the front disc and rear drum offer effective braking for leisurely riding, when the machine’s 218 Kg - plus weight is rolling at speed it’s a mistake to expect too much from the disc.
On minor country roads the ride is bouncy, the impression being that the K6 has over-firm fork action and poorly-damped rear units. Suspension was one of the most frequently revised areas during CB750 production and many owners fitted aftermarket rear shocks. When the stock rear units on this bike are warmed-up, wavering can be felt when cornering with verve, although not to an alarming degree at legal speeds. Handling quirks are the price you pay for 68 HP allied to Sixties’ Japanese chassis design.
Modern Avon tyres are fitted although specialist restorer John Wyatt, who built this gleaming beauty out of a 100,000-miler, says he prefers the lighter steering of a slimmer front tyre with a ribbed tread of the type seen on original fitments.
On this model, which has a Honda replacement tank, the single petrol tap is on the left. Earlier fours had it on the right: not good for switching to reserve while operating the twistgrip at the same time.
Also on the left, under the forward part of the tank, the ignition key is awkward. It comes as no surprise that a US aftermarket conversion for re-siting the switch on the handlebar was a hot seller. A lock is provided for the seat, which lifts to give access to the battery and a tray for the rarely-needed tools.
Many refinements were made during the CB750’s long production life. New regulations had to be met and Honda tried to minimise risks, holding talks with US safety campaigner Ralph Nader. Faults and weaknesses that cropped up in service were addressed. As a result, although blander and slower than the earlier models, the less lusted-after later Ks offer more sophistication and dependability.
Direct comparison with a 21st century four may make the Honda seem rough, clattery and sluggish, but the CB750 is a spectacularly clever package for a 46-year-old design. Positive attributes, such as strong performance, decent brakes, dependable electrics and reliability far outweigh the shortcomings. It won’t win you friends among the more blinkered devotees of its British contemporaries, but this eye-grabbing classic gives you the freedom to go as far as you want whenever you want.
Honda CB750 Specifications
Manufacturing : Honda
Model : CB750
Production Year : 1969 – 2003, 2007
Engine : 4-Stroke, Inline 4-Cylinder, SOHC 8-Valve, Air-Cooled (1969-1978) | 4-Stroke, Inline 4-Cylinder, DOHC 16-Valve, Air-Cooled (1978-2003)
Bore x Stroke : 61 x 63 mm
Capacity : 736 cc
Compression Ratio : 9,0 : 1
Induction : Keihin Carburetor 28mm (1969-1978) | Keihin Carburetor 30mm (1978-2003)
Transmission : 5-Speed Gearbox, Constant-Mesh
Starter : Electric & Kick Starter (1969-1978) | Electric Starter (1978-2003)
Maximum Power : 68 HP @ 8.500 RPM (1969-1978) | 77 HP @ 9.000 RPM (1978-2003)
Maximum Torque : 60 N.m @ 7.000 RPM
Top Speed : 125 Mph ( 200 Km/h )
Frame : Tubular-Steel Open Duplex Cradle
Dry Weight : 218 Kg
Fuel Tank Capacity : 19 Litre
Dimensions L x W x H : 2200 x 890 x 1100 mm
Wheelbase : 1460 mm
Seat Height : 790 mm
Front Suspension : Telescopic Fork
Rear Suspension : Dual - Shock absorber, Swingarm
Front Brake : Single Disc Brake, 296 mm Disc
Rear Brake : 180mm Drum Brake
Front Tyre : 3.25 – 19 Inch
Rear Tyre : 4.00 – 18 Inch