What Is A Yoshimura?
WHAT IS A YOSHIMURA?
THERE ARE a number of people actively engaged in the production of Honda speed and racing equipment, but none is better known than Hideo (Pops) Yoshimura.
Although especially popular in Japan, Yoshimura racing equipment has only recently been available in quantity in this country.
Pops began his association with motorcycles in 1948 when he became a dealer for British machines near the U.S. Air Force base at Itazuke, Japan. During his years there Pops built, tuned and rode BSA motorcycles in local drag race competition and was responsible for building and tuning engines for his customers, many of whom were stationed at the Air Force base.
In the early 1960s, Pops moved his facilities to Tokyo and began modifying Honda Hawk-series machines for road racing. His business grew and he soon found himself doing development work on automobiles as well as motorcycles.
Contrary to popular belief, Pops never worked directly for the Honda factory, but he has served as a consultant on numerous occasions. Recently he was commissioned to restore a set of Honda road racing machines to racing condition for the Honda factory museum. It seems that none of the original engineers or mechanics were available to do the job!
Pops spends most of his time doing development work in Japan while his son Fujio, Setsuhara Tanaka and Jun Yamada perform racing conversion work at the Yoshimura Racing facility at 2190 Shasta Way, Simi Valley, CA 93065. Except for the forged aluminum connecting rods for Honda CB500 and CB750 engines, which are manufactured in America, all Yoshimura racing equipment is produced in Japan. The dynamometer at Yoshimura’s Tokyo shop is kept busy testing new parts and modifications before they are offered for sale.
Yoshimura Racing also has a fully equipped shop in which racing conversion work is performed. A typical job for the shop is giving a CB750 engine the “Road Master” treatment, which includes a different cam and a displacement increase to 812cc. This is the largest displacement increase recommended if you are using the stock connecting rods. At the other end of the scale is the full “Daytona” treatment, suitable only for all-out competition. A Honda CB750 prepared by Pops and ridden by Gary Fisher led the early laps at Daytona in 1971.
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These conversions certainly aren’t cheap, but after a visit to the Yoshimura facility in Simi Valley you can see some of the reasons for the high cost. There is a special “white room” through which only filtered air is circulated that looks more like a hospital operating room than an engine assembly room. Nitrogen instead of air is used to blow engine parts clean before assembly because nitrogen prevents the parts from rusting or corroding.
k Special racing parts for almost all Tlonda engines including the CB/SL100-125 models are available, and each engine can be had in one of several stages of tune.
The popular Honda XL250 Single is receiving a lot of attention from Pops. The standard, mildly tuned engine is built to provide many miles of troublefree service in its original configuration, but the four-valve powerplant responds well to high performance tuning. A complete racing cylinder head featuring enlarged ports, lightened and polished valves, reshaped combustion chamber
and racing valve springs is just a starter. Pops feels that the XL250 has a great deal of potential due to its oversquare bore/stroke ratio and the four-valve cylinder head which permits good engine breathing at high rpm when suitably modified.
The full treatment for motocross and flat track racing includes the racing cylinder head, a Yoshimura racing cam, reworked piston, racing exhaust pipe and a fully machined and balanced crankshaft assembly.
All Yoshimura racing parts are originally conceived. The camshafts, for instance, are not regrinds, but are ma-
chined and ground from solid billets. Pistons are cast to Pops’ specifications, and each type of component is both dynamometer and track tested in Japan before it is allowed to be placed in the Yoshimura Racing inventory.
In addition to his own equipment, Pops is also the Keihin racing carburetor distributor for the United States. These precision carburetors help a modified engine perform at its best. They are expensive, but the excellence of their design for high volumetric flow and the ready availability of different slides, needles and jets needed to razor-tune an engine make their price a secondary consideration.
Pops is primarily a Honda expert, but other engines also come under his scrutiny. Such an engine is the new Kawasaki dohc Z-l which Pops feels has excellent potential. Development has already begun and high performance equipment and modifications will soon be available for the big tourer.-Jody Nicholas |5I
