Honda's sohc SWANSONG

The CB750F Super Sport machines of 1976 and 1977 were the final shots in Honda's iconoclastic single-overhead-camshaft series of fours. John Nutting tested the last bikes of the line.
THERE’S nothing like a kick in the ribs when you’re having an afternoon doze to bring you to your senses.
That’s what it must have been like for Honda in 1976 when Suzuki launched its four cylinder GS750 four, the factory’s first four stroke multi. Honda’s own four cylinder CB750 had been top dog in the class since its launch eight years earlier.
But what with the need to establish its car range, get the Gold Wing into the US market and enliven its smaller models, the bike that had started the superbike age was beginning to look jaded.
The Suzuki was lean and fast and, more significantly, it handled more like a European machine. Its engine featured more durable architecture, featuring a roller-bearing mounted crankshaft, gear primary drive and double-overhead camshafts, just like Kawasaki’s 900cc Z1.
Honda’s engineers weren’t completely caught napping though, and having got wind of Hamamatsu’s plans, started work in 1975 on sharpening up the handling and performance of their 750, topping off the exercise with restyling to bring the bike in line with the looks of the CB400F and CB550F models.
Key to the handling improvements was a new frame with a revised steering geometry and stiffer suspension. The single front disc was augmented by a larger rear disc that replaced the old drum.
Engine performance was improved, though the changes were subtle, so much so that Honda didn’t quote a power increase over the original machine’s 67 bhp at 8500 rpm. The five-speed single-overhead camshaft four retained its undersquare (61 x 63mm) bore and stroke dimensions and valve sizes, lift and duration were unchanged.
The compression ratio was raised slightly from 9.0 to 9.2:1 but more importantly, the gearing was lowered by use of a higher countershaft ratio and a one-tooth smaller 17-tooth gearbox sprocket.
The bike was called the CB750F Super Sport and it looked the part with a sleeker tank, a streamlined seat and four-into-one exhaust system. To satisfy the traditionalists, the uprated CB750K7 sibling in the range used the same engine but had four pipes and normal styling.
Press bikes
AS a staffer at the weekly Motor Cycle newspaper, I got my hands on a yellow CB750F early in 1976, soon after its launch. During the previous year, Honda UK had woken up to the need for more press test bikes but that had been well after the appearance of the first 750.
The only CB750 I had tested had been a K2, two years earlier, and although it looked great, it wasn’t a brilliant performer, either in its roadholding or power delivery.
The Super Sport I took to the MIRA test strip on a mild day in February completely converted me. I’d been impressed by the more responsive steering and tauter handling, not to mention the better throttle response, but I hadn’t expected the transformation in speed.
The ‘mean’ flat out figure of 122.34 mph was almost eight mph up on the old model and was close enough to the Suzuki’s top speed as to make no difference. With the rider normally seated, the bike could pull an impressive 106 mph, which made 90-95 mph a comfortable cruising speed with plenty in reserve.
The lower gearing (higher engine revs at any road speed) had enabled the 750F to rev harder in top whereas the original bike’s top gear was more like an overdrive. The gearing also vastly improved acceleration, the standing quarter mile times being cut by about 0.3s with the terminal speed up to just over 100 mph - not bad for a motorcycle with a test weight of 490 pounds (223kg).
Honda’s first CB750 four had been introduced a year after the factory had pulled out of an illustrious period of Grand Prix racing in 1967. Almost a decade later Honda was making plans to return. It wouldn’t be in the GPs but in endurance racing where adapted versions of the 750cc four were used. The first machines to appear at a six-hour race at Zandvoort in the Netherlands had proper racing frames and engines with 16-valve double-overhead camshaft top ends that took the capacity up to 1000cc. Against the dealer-backed teams, the works operation - with armies of mechanics - cleaned up.
No doubt keen to capitalise on this, in 1977 Honda brought out the more highly-tuned CB750F2 Super Sport that for the first time featured fabricated Comstar wheels similar to those used on the racers but with heavier steel spokes. Like the racers, the engine was finished in menacing black.
Inside, the motor had been heavily breathed upon. Inlet valve sizes were upped from 32 to 34mm diameter, while the exhausts were increased from 28 to 31mm. The camshaft lobes had a revised profile and longer duration and the valve springs were 25 per cent stiffer. To accommodate the larger valves, with their slightly altered angles, the combustion chamber shape was modified and this reduced the compression back to 9:1.

Golden YEARS

The memories come flooding back for Rod Ker as he is reunited with a GT250 two stroke just like the one he memorably crashed in the school drive as an 18 year-old.
GENERALLY speaking, Suzuki’s two stroke 250cc twins gradually gained speed and performance over the years, right through the range of Ts, GTs, RGs and RGVs.
Not that they were ever slow. As is well known, early Sixties models owed much to the development talents of Walter Kaaden, the man responsible for MZ’s racing success. When star rider Ernst Degner defected to Suzuki, all the best East German ideas went with him.
The T10 was the first fruit of the alliance but the T20 Super Six of 1966 put Suzuki on the motorcycling map as the manufacturer of a 250 capable of 100 mph. Theoretically. If reality was usually a bit slower, there was no doubt that the ‘Super Six’ lived up to its name by having six gears and being indisputably ‘super’, especially when compared with contemporary British quarter-litre offerings!
However, it could be argued that subsequent Suzuki 250s lost a bit of the magic. During the first half of the Seventies the T and GT series twins suffered from the effects of increasingly strict noise and emission regulations, with the result that they became slower and more refined.
The GT250K, which replaced the T250J (also commonly referred to as the Hustler, although that wasn’t an official name by then) in January ‘73, was the first sign of this.
Very similar to the previous model in essence, Suzuki’s two new secret weapons were a disc front brake and ‘Ram Air’. Unlike the big 750 triples, the twin’s previous two-leading-shoe front drum was perfectly adequate but discs were a status symbol in those days, so if Honda and Yamaha had them, so would Suzuki - even if you couldn’t stop if it rained!
Ram Air in this case referred to an extra lump of aluminium bolted across the top of the cylinder heads, rather than the sort of power-boosting forced-feed induction used on current sports bikes. The idea was to improve cooling but no-one ever really believed this claim.
What it definitely did do, however, was reduce the characteristic two stroke ringing from heads and cylinders. An unwelcome side-effect was to impede access for spark plug changes... not that you’d need to be doing that very often, of course.
Model logic
THE GT250K turned into the practically identical GT250L (but for a chrome headlamp) later in the year and the GT250M in October ‘74. This lasted for another year before Suzuki model ‘logic’ decreed that it became the GT250A, complete with a number of important modifications.
While the K, L and M had all been slower than earlier T20s (much to the disappointment of hordes of L-plated seventeeners), the new model stopped the rot. With a claimed 32 bhp at 7500 rpm instead of 30 bhp at 7000 rpm, the A was usually good for another 5-10 mph and gave more spritely acceleration.
The extra horses came mainly from revised cylinders featuring another pair of transfer ports and fed by a pair of 28mm Mikunis (up from 26mm). Bore and stroke stayed at the `square’ 54 x 54mm configuration used since the beginning but the crankshaft was now supported on four bearings and lubricated by a different system.
Tuners had, in fact, been adding another centre main to racing engines for years, so no-one was amazed.
What might have been a surprise was the disappearance of the Ram Air appendage. So let’s get this straight: Ram Air was supposed to improve cooling, yet when Suzuki tuned the engine, producing more heat, they ditched it! Hmmm. Those who thought Ram Air was just a marketing gimmick must have been amused.
To make use of the extra power the gear ratios were tightened up but that seems to be the extent of the mechanical changes - unless you count a one-piece cylinder head and more restrictive silencers.
Interestingly, after experiencing some ‘piston problems’ on early GT250As, Suzuki lowered the compression ratio and the specified ignition timing figure changed slightly. But the timing marks on the rotor weren’t altered to suit, so don’t try setting the ignition with a strobe unless you want ventilated pistons.
Almost identical B and C versions followed before the totally redesigned X7 hit the streets in mid 1978, just in time to tempt the last quarter-litre L-platers before the 125cc limit took effect.
That’s important, because the dearth of good Seventies 250s around now is a direct reflection of what happened to them in the first few years. Most seventeeners hardly bothered with maintenance and servicing, let alone running-in their GTs, KHs and RDs properly. I know, because I was one of them!

The SENSATIONAL Suzuki Square Four

Chris Pearson describes how the Suzuki factory progressed from a seizure-prone racer nicknamed ‘Whispering Death’ to complete domination of the 500cc grand prix class over a decade of development.
HISTORICALLY, the Suzuki RG500 (XR14) square-four grand prix engine’s beginnings were back in the mid Sixties when racing machinery was exotic to say the least.
The very successful twin cylinder 125cc Suzuki led to the square four 250, essentially two of the 125 engines coupled together. Known as ‘whispering death’ due to its ability to seize without warning, the 250 never finished higher than third throughout its career and the project
was shelved. Following rule changes, at the end at 1969, that restricted the amount of cylinders and gears in each class, the grids were robbed of such Japanese exotica. Consequently, apart from Italian MVs and Benellis, for five years GP grids consisted of converted road engines in race chassis.
What we did see before 1973 was a plethora of ingenious designs varying from road-based machines like the H1R Kawasaki and XR05 (TR500) Suzuki to the outboard motor powered Konig. In the hands of riders like Australian Jack Findlay or Britain’s Dave Simmonds, these were good enough to win - if the all-conquering MV of Giacomo Agostini failed to finish.
While other designs fell by the wayside the Suzuki twin continued throughout 1973 and ‘74, enabling designer Makoto Hase to assess the opposition and acquire much data in readiness for his new project. No surprise then, that Hase, who built the 125 and 250s of the Sixties, would revive his earlier concept of a square four, secretly enlisting Barry Sheene to lead the 500 team.
First seen in 1974 in the hands of Sheene and Jack Findlay, the new 500 square four - along with Yamaha’s across-the-frame four - sounded the death knell for the four strokes although MV battled bravely on and even won the title in ‘74 with Phil Read.
Alarming
THE first XR14 was not a pretty machine, with upright shock absorbers and a lumpy fairing to shroud the protruding carburettors. It became a little more attractive in ‘75 with laid-down shocks and a smoother fairing. More alarming though was a trait it shared with its Sixties 250 predecessor - a frightening ability to suddenly seize and dump the rider.
In an effort to reduce frontal area the original frame design had no down tubes and lower rails. This meant the exhausts and belly pan could sit much higher off the ground. As well as being mounted at the rear, the engine hung by a large aluminium bracket attached to the cylinder heads. This design worked well at the Suzuki test track but in fast and bumpy grand prix conditions it handled badly and soon separate down tubes were attached to strengthen the frame. Later in ‘74 a frame designed by Ron Williams of Maxton Engineering transformed the XR and the following RG series of machines.
Despite the early teething troubles the test riders agreed to a man that the XR14 was very fast, certainly faster than the 750 triple Suzuki were running in the Superbike classes.
Engine vibration caused external items to crack and fall off and many a race ended prematurely. In Sweden during that first season Sheene was well on his way to the XR14’s first GP win when the water pump failed. The temperature soared and the radiator bubbled over, spreading coolant over the rear tyre, ‘high-siding’ the hapless Sheene.
In another race the gearbox seized and spat him up the road and he was leading comfortably later in the season when vibration warned him of a gearbox failure.
After much deliberation he decided to take the soft option and pull in to the pits. True to Sheene’s on-board diagnosis, a shaft was on the verge of shearing, making it the equivalent of an unexploded bomb within the engine.
Before the next GP, only a fortnight away, a Suzuki engineer had returned to the Japanese factory and manufactured four new shafts out of solid steel billet. These were fitted in time for the race and the part never gave a problem again.