Honda GB500

Café rocket

Old Bike Australasia - - CON­TENTS -

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 (mo­tor­cy­cle­[email protected]­pond.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 ([email protected]­hon­daltd.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.

ABOVE Mick Bulman’s XBR500 at the re­cent VJMC Rally. LEFT The GB500’s cousin, the XBR500.

Tra­di­tional look­ing in­stru­ments with clip-on han­dle­bars bolted above the top fork yoke. LEFT & BE­LOW The GB500 as re­ceived from New Zealand.

Stylishly sculp­tured fuel tank holds 16.5 litres. TOP LEFT Hous­ing for the starter mo­tor dom­i­nates the left side of the en­gine. TOP RIGHT Braided hoses carrying oil to and from the tank, lo­cated above the swing­ing arm. ABOVE LEFT Orig­i­nal rear shocks have been re­placed with Koni units. ABOVE CEN­TRE Drum brake is all that’s needed at the rear and looks the part. ABOVE RIGHT Sin­gle disc front brake is per­fectly matched to the weight and power.

Graham Caul­ing­ford’s GB500 at the 2018 Mac­cles­field Show in South Aus­tralia. It is fit­ted with a Jack Bat­son stain­less ex­haust sys­tem. Zero-miles 1990 GB500, one of the last built, sold at the 2019 Me­cum Las Ve­gas auc­tion.

Delke­vic slip-on muf­fler fit­ted to Rob Carmichael’s GB500.

Iden­ti­cal 1987 model Ja­panese im­port GB500s (eas­ily iden­ti­fied by the dif­fer­ent front mud­guards, rid­den by own­ers Brian Kohlhoff and Kel Han­nan at the 2019 Easter Bathurst Rally.

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