Cycle News - Archive Issues - 1970's

h o rr e nd o us porting jobs on the cylinders of the GP bikes. It wasn't clear that Mar ce l even ha d a porting tool amongst the con fusingly vast array o f machinery in his shop - an item that mos t American tuners would have gotten withdrawl symptoms without. Mar cel usually built his own engines which se t new standards for p ower an d reliabilit y in a Bul taco 250. These were the days , also , when carbure tion was sorted o ut . Several si zes of Mikuni, sever al sizes of Bin g, and at least two sizes of Spanish Am al ca rbs were bolted up to th e back o f th e cyli nders . Almost without exception, the 36m m Amal made th e bike run fastest an d cleanest. This is th e same carburetor right down to the little ru b ber mud guard which appear s on the production bikes . Mar cel sometimes used to favor a 3 6 m m Bing o n his bike for reasons which he only smiled abo ut. He was cautious about sneaking his GP bike back in the van or under a tarp whenever any of the Bulto family were visiting a circuit, since the Bing made it not to factory specs. ~ o ...... Besides, let 's face"it, J im had never ridden a ny th ing but standard bi ke s u p until last year an d knew o n ly that he liked them pretty well fo r power and handling while Mar cel had fin all y co me t o t he reali zati on th a t th e limitation last seas on lay in him an d not in his often su per i o r m a chinery . Occas io na lly, Rubi o would t ry something a little different in t he m otor and as oft en as no t, t he engine would go back t o sto ck or as J im often pro claim ed , "Slo wer than s tock !" T he re sult after a f ull sea so n on the GP circuit is th at the new GP engin e rem ains basically the same as th e standard ' 73 en gine and that the engine in t he Mk VII is th e sa me seve n-port as last yea r with a he avier fl ywheel an d, most im portantly , a n ew pipe. Bo th co mb ine to mak e th e pr oduct ion bike growl rat her than scream . The he ad seemed to get the most s eri o us re-working, in ternally and externally. T he squish area was smoot hed out and the actual vo lume lessened. Compression ratio got pushed up to a hefty 12: 1. Breaking of the frames at th e- front motor mount from downward ha mmering and vibration was even more cr ucial with the new c h ro me-m o ly steel . fra mes so the headstays were looked at very carefully. The result was a design change foc used on reinforcement of the head-to-frame brace to p revent cracking through engine vibration; T he design principle, but no t the exact item, carried over into production . bike man ufact ure. With most 250 m ac hines at the classical 246cc size, th e Pursang rem ain ed an undersquare 244cc. T he carefully asse m bled but basically stock engines from t he GP bikes with unsilenced wo r ks expansion chambers pro duced between 30 and 32 actual horsep o wer. One of the mass-produced engines with no sp ecial attention save to be car b uret ed co rrectl y, running with the- standard sile nced " tor q ue " pip e , will prod u ce a n a c tual 27 -28 ho rsepower. m ore than eno ugh to win and, if yo u're be ing ho nest with yourself. yo u kn o w it . An extra co up le of ponies can be achieved mainly by careful ma tch in g and polishing where it co un ts and jetting the Amal just right for your ra cing conditions. Un less you 're on the AMA pro circuit, it's not the extra two ho rses th at will make the diffe ren ce anyway. It is the smooth flow and transition ·of this Pursang's entire powerband from lo w to mid to high RPM ra nge run n ing that will make the difference , along wi th the individual twisting and throttle. The Bultaco ha s never been o u t of the motocro ss race in an y meaningful sense. It s till isn't . Th e Motopla t ele ctronic ign it io n , feeding spark to a Bosch platinum tip plug, auto mat ica lly ad vanced t o keep the timin g on. The co nt rol of the power , rather than the mid-range rush of some popular Japanese models, se em s to have been intentionally m ated to the handling resu lting fro m a frame and s uspens ion that are e xactly what Po meroy was riding on by the end o f the GP season. He had won the Sp anish 250 GP o n so me o ne-off Betors at the rear with the co nventional Betor fo rks • Arrayed in a f in e se lection of h is compa ny 's persona l gear, Bul re p Mike Hannon po ints t he Pursang away fr om a berm he's j ust used up. It looks like he k nows w hat he's doing . providing slightly less th an six in ches of incredibly smooth, progressive travel on the front . T he guy at the Betor factory could never rem em ber how to build those shocks again ; lost his design chart s or something, so that the final damper choice became the production Telescos whic h were lighter and maintained their damping ch ar act eristics for most riders througho ut a 45-minute moto, even tho ugh they were thought to be slightly inferior in .performance to the Betors during the fir st 10 or 15 minutes of motos, The Betors n ever leaked, never kicked back after landings, never t opped . or bottomed, never were anything bu t ex cellent in pe rformance . Ra cing had reinforced, if not improved, the breed. The swingarm, now very light being a ch r o me-mo ly product, has a different, presumably more effective reinforcing brace ahead of th e wheeland heavier gusseting at: the rear bottom shoc k mounting positions. The overall Mk VII suspension is the best in sto ck form of an y o f t he 19 74 model moto cross machines. We ex cept the new-frame Pentons introduced after mid-year and Motocross des equipped with 1972-73 sea ts, he a lot around the Nations were to be th e sam e standard blew up . He does that factory. He was told there was no time to design new seats sui tease of dam per units and springs to . encountered this year. each GP to be tried o ut in training after the feedb ack of the previous week 's race. The result was the standard T he Pursang en courages you to ride the way you see Pomeroy an d , t o a lesser extent o t h er Bultaco pros like Zah rt , ride. Stand up , be o n th e throttle ne arly a ll the time and when yo u co rn er, get your weight way up o ver the which indeed may have been true. (T here are less than half a dozen co m petitio n design engineers a t Bultaco , which in the development o f truly great or distinctive racing machinery I count as a definite adva ntage. For success, look at Lyons and Jaguar , Ferrari and Ferrari, or Uh lenhaut and Mer cedes. There are too many negative examples to list though De troit co mes to mind a lot.) . So Ji m storms o ver to the seat factor y an d finds the ligh test seat base, the firmest fo am , and the nices t leather he ca n get his hands on in 15 minutes of yelling and lo oking and builds a seat this t ime with so me help from Arnie, who had co me o ver fo r the last three GPs. T he seat shop then built two that fro n t wheel, f o rcing it down , in, and o ut a fterno o n. of the tu rn . You st eer it o u t with t h e wid e, low bars rather than slide wast efull y and find yo urself using every gear but first to keep th e 25 0 buzzing. T he incr eas ed seat height m a kes it less fa tigu ing to go from the sitting t o stand ing m ode . You sh ouldn't be o n th e pad very m uc h any h o w with t his machi ne. Keep your weight on the pe gs. All d uri ng t he G P season , Jim and Marcel's Bultacos had co nven tional se ats whi ch , m an y rememb er, gave you the fee ling of sitt ing down inside th e mac h ine and were so soft that t he fender co nt inually smac ked your le a thers ; o f ten d istrac tingly an d pa in full y . By t he end of th e sea so n , Jim had had en ough of this. When he go t to Bar cel ona and saw th at his 400s for the Mot ocross des Natio ns bikes and exact replicas of th ose seats are on th e Mk VII Pursangs. Ever y b od y agr ees they're a operation. It go t to the point by the end of the seas on wh ere Oriol Bulto , fact ory competition manager, was carrying a suspension : Betor in front , which was the envy of nearly every other rider on the GP circuit , and th e Telescos at the rear which maintained their advan tage of reliability except for an o ccasional flawed damper unit. Jim arr ived for the Mo tocross des Nat ions in Sw it zerland wit h KaNis availa ble for his prototype 400 Bultaco . We played aro und with damping se tti ngs and spring rates and ende d up with a rear suspension that worked accep t ab ly and, best o f all, lasted for 9 0 minutes. The de cision on a prod uction sh ock rested on the fact that T eles co s are made in Sp ain and, needless t o say, the Telesco factory was eager t o co o p erate with Bult aco, also based in Spain. The Telescos that arrived on th e 1974-\1, model Pursangs were equ ipped with ny lon mud cov ers a n d 55 pounds-pe r-i nch springs and they wo rk very we ll indeed, with no bottoming or fading apparent in our shorter U.S. th e e xce s s of r e ar modificat ions on producti on s u s p ensi on bikes T hose t wo were on the bi g, bi g im provemen t. The new seats, at Ie a s t , > could have "Jim Pomeroy Replica" st amped on them, if it m atte red . The y ar e h igh , firm , sup porting, an d narrow. Where th e factory real ly ca m e thro ugh is t he wheels . T here a re very sma ll , very light d ural um in um hubs with cas t-in brake linings. T h e sho uideri ess Akront rims have t hicker walls now than th e very ea rly models (wh ich are" probably no lo nger aro un d or available) and are virtuall y ding·proof. To keep tire s from spi nning at the low pressures generally used, ca me a trick derived Cont inued o n page 42 35

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