Purebreed Motorcycles CB750
By Ross Sharp - 03 Jul 15
Tuning engines is a double edged sword and away from the racetrack makes no sense at all. More or less any vehicle can be rendered a time sapping money-pit in the quest for improvement. You fight for every extra pony and will the dyno trace to steepen its accent, when simply buying the next engine size up would yield a similar result. But that's like wearing elasticated triple pleat trousers and velcro shoes, practical but oh so very dull. We are here for a good time, not a long time; so fill you life with flames, pops, bangs and the symphony of costly endeavours rattling away between your legs. Guillaume of Purebreed Motorcycles featured here a while back with a CB360 he built for his girlfriend Maude. So, he has a hot girlfriend who rides cool bikes, a great workshop and customers happy with his handiwork, a man fulfilled right? Nope, Guillaume decided that he wanted to build the fastest CB750, period. Before you get excited and expect to read a how-to on squeezing poke from Honda's venerable four-banger, the formula is a relatively guarded secret. Probably because Guillaume would rather not be reminded how fast this thing can burn a $100 note. The base lump is a SOHC unit from 1976 CB which rolled off the production with around 67hp on tap, Guillaume wanted to crack the ton, and not the MPH version. Experimentation was the underlying method of extracting more power. The cylinder head has undergone significant porting and polishing, and we're not talking about a rough stab in the dark with a Dremel here. Rather than re-profile the existing camshaft and new one was cast and machined to spec, running considerable overlap to scavenge the cylinders, increasing not only power but also the length of flames on overrun. Well, not that long as it happens, the flat-slide carbs have been set-up on the dyn0 to ensure efficient combustion and no melting of pistons. With tolerances this tight and a heady compression ratio one can imagine the margin for error is as narrow as the squish band. The skin of this engineering grenade has been lavished with similar attention, painted and machined barrels and sidecases look factory fresh. A warm golden sheen on the bespoke stainless headers a sign of the well-tuned motor and the stubby muffler has its work cut out to baffle the racing rasp when this thing comes on song. The oil tank is slimline and canted forward, not only centralising weight but freeing-up the rear triangle, a look easier to achieve with the wet sumped CB550. Of course, all that go is wasted without considerable upgrades to the chassis. Anyone who's at least pushed a stock one of these around will concur, they are flippin heavy. Lightweight engine internals go someway to reducing heft but binning cast and heavy seventies ironmongery is an easy win, if a potentially expensive one. First to go was the stock front end, replaced by upsidedowners from a GSXR 750 braced by Purebreed's own machined yokes and stem. And who would shy away from using a Motogadget speedo these days? The simplistic and multifunctional all-in-one Motoscope Classic is housed in within the top yoke. The wiring harness is all-new, through the M-Gadget system and is powered by the now ubiquitous lithium battery, hidden away of course. The nearly radial brakes are mounted to a hub manufactured by fellow CB modernisers, Cognito Moto. The widest possible Sun aluminium rim was then laced on. Out back the stock caliper, rebuilt of course, stands in as back up should exuberance overwhelm the pair of Tokico four-pots. Piggyback rear shocks are by Fox, Street Performance RC1s. All that speed is nothing without looks so a handmade café tail and seat unit takes care of the aesthetics, dark blue always looks smart and sophisticated which is something Guillaume's builds exude. And here he is, the 100 horsepower man. You may think that extracting that many ponies from an engine a half century old in its design would result in an unreliable, jerky beast of a bike but the running-in period proved to be smooth and trouble free. Now loosened up and the internal components familiarised with each other Guillaume has been able to stretch the CB's legs. A 600km rip through the mountains proved his hard work to be worthwhile, the motor spins freely and pulls really hard, sounding truly fantastic in the process. Too see some of the other bikes in Guillaume's stable head to the Pure Breed Fine Motorcycles website, where you can also buy merchandise. Keep an eye on his Facebook page too, as the salt flats of Bonneville are calling.