Honda GB500
Café rocket
When the Honda GB500TT came into existence in 1985, there was not a road test or review that did not compare it to the exalted and extinct great British singles, especially the BSA Gold Star, Velocette Venom Clubman and Thruxton, Norton Featherbed International and so on. Certainly, the overall specification was of the same ilk; a sporty single cylinder four stroke with minimal weight, basic instrumentation, few concessions to comfort, and impeccable handling. But beyond the initial glance the Honda had a lot more going for it, notably its engine. But more of that later. The Japanese factories have made some curious marketing decisions over the years, but probably none more curious than for the GB500TT. One may have logically expected that its primary market would have been Britain – home to the legendary marques, home to the Ace Café and where the café racer culture had its roots, home to generations of enthusiasts who would annually flock to the Isle of Man to savour the sights and sounds of Manx Nortons, AJS 7Rs and Matchless G50s – the big overhead camshaft singles that were still winning races long after they should have stopped. But no, Honda chose not to release the GB500TT in Britain (nor in Australia for that matter, another nation with an established taste for sporting singles, although the GB was sold in New Zealand), for reasons known only to itself – the only plausible explanation may have been the cost, given the exchange rates of the mideighties. Some even say that Honda’s GB moniker stands for Great Britain, others for Gor Blimey... Honda apparently saw their primary market for the GB500TT as the USA, a view not shared by the market itself. One factor would have been the price – US$4198 when launched, which was $199 dearer than Harley-Davidson’s 883cc Sportser. The US model GB was primarily offered in green as well, traditionally an unfavoured colour in the States, although in reality, the GB’s green was almost black. In Japan, the GB was initially released in 400cc form only (with bore and stroke dimensions of 84mm x 72mm), with slightly different graphics, a different front mudguard and a ‘bikini’ style fairing. In other markets (Britain and Australia among them), the lithe GB concept was given a makeover to emerge as the XBR500, complete with the unloved Comstar pressed-up alloy wheels, a different fuel tank, shorter wheelbase, twin silencers instead of the two-into-one system on the GB, and a dual seat. The XBR appeared in mid 1985 and in Britain at least, was very well received in a market still miffed at missing out on the GB500TT. In USA, stocks of unsold GBs languished at dealers and in Honda’s warehouse, until reputedly a private European businessman did a deal to purchase the entire stock of unsold models, bundling up around 1,000 crated bikes and shipping them to Germany, where they were received with much greater appreciation. The GB continued in production until 1990, with only cosmetic changes, while the GB400 went on for a little longer in Japan. The GB’s powerplant owed much to its forebear, the XR500 trailie released in 1983. The standout spec of this engine was the chain-driven single overhead camshaft cylinder head, with four valves set in radial fashion, ie; not parallel to each other, just as Rudge had done in the ‘thirties. Honda called that system RFVC (Radial Four Valve Cylinder) and that train of thinking meant that the rockers operating the valves needed to incorporate extensions to reach the valve caps. A hemispherical combustion chamber had the spark plug centrally located. Whereas the XR500 used a pair of 28mm carburettors opening at different stages, the GB had a single 42mm CV Keihin. Unlike the British single cylinder road bikes (and the 500cc Manx Norton), which favoured usually square bore and stroke dimension – 86mm x 86mm or thereabouts, the GB was well oversquare at 92mm x
75mm, coincidentally the same as a Matchless G50. With a compression ratio of 8.9:1, that equated to 33 horsepower at 7,000 rpm, measured at the rear wheel, although the crankshaft figure is officially quoted as 42 hp. A five-speed gearbox, with straight-cut primary gears and a wet clutch was attached to the rear wheel via chain. Underneath the right side cover, just above the swinging arm, sits the oil tank, with the drain plug carefully placed so that the oil dribbles down the frame during oil changes, which need to be frequent. A pair of large diameter braided hoses transfer the precious liquid to the engine and back to the tank. Of course one word inevitably associated with big singles is ‘vibration’, and the GB’s answer to this is a bob-weight inside the front of the crankcase, connected by gear to the right side of the crankshaft. Fuelled up and ready to go, the GB weighed 177kg, similar to the British bikes, but this weight included something the Goldies and Velos did not have – an electric starter. And rather than Lucas magneto ignition, a modern CDI unit provided the spark. A kick starter was fitted, but rarely used, at least when the Hondas were new. To keep the handling sharp, a 1412mm wheelbase was achieved with a fork rake of 30 degrees. The frame itself was a single front down tube style, largely dictated by the need to use a pair of exhaust pipes (which joined via a collection box to emerge in a single megaphone style muffler), splitting into a cradle downstairs, with an alloy boxsection swinging arm. Suspension at both ends was quite conventional; 35mm telescopic fork at the front (with very British rubber gaiters), twin rear shocks, and 18 inch spoked wheels with anodised silver DID alloy rims. Visually, the biggest difference between young and old was the front disc brake, clamped by a twin-piston caliper, while a drum was considered satisfactory at the rear.
In terms of creature comforts, the GB has traditionally-styled twin speedo and tacho instruments, very much in the style of the favoured café racer set up, and a racing style seat with a humped back not at all dissimilar to an AJS 7R, although in most markets this was fitted with a cowling that was colourmatched to the fuel tank and side covers. Clip-on handlebars sat above the top steering crown, with bar-end weights to dampen vibration.
A freshen up
This particular machine came to us from New Zealand, originally sold by Blue Wing Honda. The machine was substantially original and complete, save for a home-made single seat, and had been used for track days on several occasions. What it required was not so much a complete restoration as a freshen up, but this quickly became a full strip down and rebuild. One thing that becomes immediately evident in this process is there is very little wasted space on this model; everything is a tight fit. Accommodating the wiring inside the slim headlight shell can only be achieved by strictly following the official factory diagram in the combined parts book/workshop manual, which very fortunately came with the bike. The midriff section, largely hidden by the side covers, is also a very careful piece of packaging, which comes apart, and goes back together again, in a precisely monitored operation. To remove the airbox, and the carburettor, it is necessary, or at least highly advisable, to dismantle the rear end of the motorcycle, including the various sections of the rear mudguard, and remove the rear wheel. The airbox itself passes in and out of the rear frame section in one unit. The oil tank, a neat flat piece of alloy casting, sits above the swinging arm, so it can’t come out without removing the superstructure either. However once this process is complete and the centre section is laid bare, the carburettor can be removed, and in our case, discarded. Swatting up on the various forums dealing with the GB (and to a certain extent, the XBR), a near universal recommendation was to replace the CV Keihin carburettor
“I resisted the urge to pull on the pudding basin helmet and the white scarf, never having actually been part of the Café Racing scene in my youth, and set off for a 100km shake down one hot and humid Sydney summer’s day.”
with a Flat Slide Mikuni TM40 – ours was supplied by Mikunioz (www.mikunioz.com) in Townsville, Queensland . This involved only a small amount of fiddling, such as slightly shortening the twin throttle cables, and trimming the adaptor ring to fit the standard airbox. I also had to make a bracket to hold the choke cable which originally operated from a toggle on the left handlebar switch. This now sits under the left side cover, with the choke knob accessible below. Otherwise it was a straightforward exercise and as supplied, the jetting seems almost spot-on. As mentioned, the seat that came with the bike was non-standard (although a standard dual seat was supplied) so we set out to chase down an original single seater. Most reports gave us little chance of finding one, but there is an excellent Facebook page for the model (Honda GB500, GB400 TT, GB250 & XBR 500 Enthusiast), and a request there produced one in Melbourne, which was quickly snapped up. The seat cover, which is pretty special with its centre suede strip and gold stencilling had a couple of small tears, but these were expertly and invisibly repaired by Tony O’Connor at Eldorado Motorcycle Seats in Mount Barker, South Australia (motorcycleseats@bigpond.com.au).
The rather unusual and very attractive ‘Aubergine’ paint on the fuel tank and side covers was in excellent condition and left alone – the Gold ‘wing’ tank stickers and GB500 side cover stickers are available as very good reproductions via TradeMe in New Zealand if required. Some models have the mudguards colour matched with the tank, but this one, and many others I checked out, had silver guards so they were redone in the standard colour. The wheels were entrusted to Doug Chivas at Chivo’s in Parramatta (02 9682 5950) who rebuilt the wheels with stainless steel spokes and Metzeler Lasertec tyres, which were developed in Australia by Peter Galvin and Jeffrey Sayle in 1983, so they are correct for the period and work extremely well. Kenma (www.kenma.com.au) supplied new SBS disc pads, rear brake shoes, and a Venhill front disc hydraulic hose.
The big bit, the engine unit, was entrusted to former Australian Period 4 500 Champion Bob Mariner, a Honda expert who competes very successfully in Historic Racing on his self-developed CB500/4. Upon stripping the engine, Bob discovered that although most parts were in good shape, the camshaft and two of the rockers were not, having worn through the hardening. Fortunately this rebuild happened just in time before serious damage was inflicted, and both components were able to be built up and reground and hardened. The other major casualty in the engine was the starter motor, and this is a very common cause for complaint. The one-way sprag clutch had collapsed, whereupon it had ceased to clutch anything apart from air, but this part was still available from Honda Australia. The other two bits of the starter assembly, the large driven gear and the outer plate, came from Econohonda in New Zealand (parts@econohondaltd.co.nz), along with piston rings, camchain, a gasket set, sprockets and a very hard-tofind item, the left side handlebar switch, which was broken. The fork gaiters, swinging arm bushes and various small rubbers came from CMS in Holland (www.cmsnl.com). The grease nipple for the swinging arm is in the underside so it tends to get overlooked.
And so with the frame recoated in rich black, everything was ready for reassembly, carefully following the process of disassembly in reverse. Getting the engine in single-handed is not the work of a moment, but eventually it all clicked into place. As previously noted, everything is a tight fit on the GB500. The GB500TT is now a complete and running unit again, and apart from the carburettor and Koni rear shocks, is 100% 1987 specification. I am not certain of the provenance of the muffler, which looks to be a replica of the Honda original, but it is in good condition, looks and sounds the part, so for now, it stays. There is a system that was made in USA by Jack Batson which is fully stainless and dispenses with the under-engine collector box, and a call to Jack found him just about ready to make another small batch, so we now have one of those which will be fitted in the near future. Delkevic (https://delkelvic.com.au) also make a slip on muffler that connects to the standard under-engine collector box and looks the part. GB500 owner Rob Charmichael from Melbourne has one on his bike and says it makes quite a difference in performance.
On the road
After turning over the engine many times via the rear wheel with the spark plug out to prime the oil pump and force lubricant through, a press of the starter button produced an instant result, with the big single quickly settling into a smooth, even tick over. I resisted the urge to pull on the pudding basin helmet and the white scarf, never having actually been part of the Café Racing scene in my youth, and set off for a 100km shake down one hot and humid Sydney summer’s day. What a delightful motorcycle! As the owner of a Velocette Sportsman, I’ve had plenty of time on big singles, some better than others (once you’re a Velo Fellow, you’re spoiled for life), and I say confidently that the GB500TT is right up there with the best ‚
of them. Like you’d expect, it is light and supremely agile, and the brakes are a darned sight better than the Velo! The gearbox is silky smooth in operation – for both up and down changes on big singles I always feather rather than shut the throttle and just slide the lever into the next gear, which makes the transition a lot less jerky. Mindful of running in the engine, I hardly took it above 4,000 rpm, where it was running at about 90 km/h on the gearing which is one tooth lower than standard on the rear. What can you say? The bike seems vice-free and an absolute pleasure to ride, and it sure does look cool. Since acquiring the GB500, I have become far more aware of the model’s presence in Australia, and for a model not officially imported, there are quite a few here, brought in from Japan (which has also supplied quite a few of the GB400 model), Europe (mainly Germany), USA and in this case, New Zealand. The owners are universally thrilled with their machines, and I now understand why.