T
he story of this bike’s 1983 race season is
a road race version of David and Goliath.
When the AMA reduced nominal engine
size from 1025cc to 750cc for the increasingly
popular Superbike class, it sent shock waves
through the racing community. All the knowl-
edge and painstaking work from previous years
was negated by the stroke of the rule maker’s
pen. The claimed rationale was to attract more
factory-backed teams; however, Suzuki and Ya-
maha watched from the sidelines, leaving only
Honda and Kawasaki participating with facto-
ry bikes. Honda was more than ready with the
VF750R (See the July 2018 issue of Roadracing
World, Historic Racebike Illustrations) that had
been developed at the end of 1982. But Kawasa-
ki’s U.S. race team did not get deliveries of the
new GPz750 until the start of the 1983 season.
Compared to Honda’s water-cooled, 4-valve V
VF750R racebike, the GPz was at best old-school,
almost vintage! The transverse Inline Four air-
cooled engine with two valves per cylinder sat in
a conventional twin-loop, steel-tube frame.
Predictably, Kawasaki factory riders Wes
Cooley and Wayne Rainey had a tough season
opener at Daytona. Reliability issues dogged the
pair in practice and were responsible for Cooley
retiring in the race. The best Rainey could do
was a distant fourth. As if Honda’s technical ad-
vantage was not enough, the grids were packed
with race-kitted VF750R’s fi elded by dealers.
Despite its “Uni-Trak” single rear shock
suspension and race-style front fairing, the
GPz750 was very much a UJM (Universal Japa-
nese Motorcycle) design, and looked very 1970-
ish. Despite its conventional appearance, the
frame needed little modifi cation for racing aside
from removing unwanted lugs and brackets
and adding fi ttings for pit stands and race-style
footpegs and controls. For race-duty, the alumi-
num swingarm was reinforced with triangulat-
ed bracing welded to the top. This made it ap-
pear like Yamaha’s Monoshock™ design but the
nearly vertically mounted GPz shock was acti-
vated by links between the frame and underside
of the swingarm. Changing link lengths altered
the ratio of swingarm to shock movement and
rate. Front forks looked stock but were 38mm
race units clamped in large triple clamps ma-
chined from billet aluminum. Dymag wheels
from England carried front disc rotors from the
European KR500 program and calipers from the
KZ1000 S1 racebike. The rear caliper on Rain-
ey’s bike was a small Brembo unit gripping a
massively drilled rotor.
While the chassis parts didn’t need too
much modifi cation for racing, the stock 80 bhp
engine certainly did! Kawasaki’s Rob Muzzy
assembled a team of talented race mechanics
and technicians to work on the engine. There
are basic problems with a two-valve, air-cooled
engine. Unlike a four-valver, housing two de-
cent-sized valves dictates a hemi-shaped com-
bustion chamber and matching piston crowns.
Air-cooling the large areas exposed to combus-
tion heat can never be as effi cient as water-cool-
ing. The other problem with this layout is a
restrictive skull-shaped combustion chamber
offering poor (slow) fl ame spread. Special drag
race style high-lift cams made by Megacycle™
increased torque and power, but initially caused
valve spring failure. Special spring material
sourced from the automotive industry cured the
problem early in the season. Vance & Hines car-
ried out extensive porting work on the heads for
Rainey’s bike; Cooley’s were modifi ed in-house.
To overcome the slow fl ame spread, heads were
also modifi ed for two spark plugs per cylinder
fi red by four double-outlet coils mounted on the
frame’s front down tubes. Ignition was by mag-
neto/CDI, and Keihin Smoothbore™ carbure-
tors replaced the stock units. Kawasaki Japan
supplied special transmission covers incorpo-
rating a dry clutch actuated by a left-mounted
cam pushing a rod through the hollow main-
shaft. Kawasaki technician Randy Hall had pi-
oneered tapered exhaust header pipes on Eddie
Lawson’s KZ1000 S1 the previous year; this fea-
ture was incorporated in the Kerker four-into-
one system for the GPz750.
So, with a lot of midnight oil burned after
the Daytona round, “David” had a well-devel-
oped slingshot with which to slay the techni-
cally superior “Goliath!” The next AMA round
was at Talledega, where Rainey fi nished sec-
ond, beaten by Mike Baldwin on his VF750F
Honda. Riverside gave the same result and the
next round at Mid-Ohio saw Rainey crash and
Cooley DNF. It was not until Pocono in June
that Rainey was able to beat the Honda hoard,
and he repeated that win at Laguna Seca butDNF one of the two races at Portland. By now,
though, David’s slingshot had the measure of
Goliath, with Rainey winning the next four rac-
es! A second-place in the last round at Daytona
netted Kawasaki and Rainey the AMA Super-
bike Championship by 41 points over Baldwin!
Racing doesn’t always reward the under-
dog; Kawasaki was out-spent, out-numbered
(there were at least eight factory or factory-sup-
ported VF750Fs) and out-teched, yet succeeded
through talented riding and resourcefulness. At
the end of 1983 Kawasaki issued a press release
congratulating Rainey on the Championship
and also announcing that the company—which
had introduced the new water-cooled GPz
(a.k.a. the Ninja 900R)—was quitting the 750cc
AMA Superbike series because it wasn’t selling
enough streetbikes! After winning the 250cc
race at Daytona, Rainey departed for Europe
and a tough year on a TZ250 in the 250cc World
Championship.
I would like to thank Randy Hall for his
help with text and illustration details. Look for
his new books on Kawasaki racing in the USA—
Lean Mean and Lime Green Vol. 1 deals with
the years 1968-1978 (ISBN 978-164440085-2.
Order from: https://leanmeanlimegreen.com/).
Vol. 2 (1979-1983) is due out later this year.
For more information on art-quality, 20-
inch x 16-inch prints (with a choice of white or
beige background) of this motorcycle and other
historic racebikes, see the "Historic Racebike
Illustration Prints" ad on page 56 of this issue. RW1983 Kawasaki GPz
historic racebike illustrations By Mick Ofi eld
Wayne Rainey (60) used an air-cooled, two-valve Kawasaki GPz750 to beat the water-cooled
4-valve VF750R Hondas and win the 1983 AMA Superbike Championship. Photo by Shigeo Kibiki.16—Roadracing World, August 2019