LINE ART: 1971 YAMAHA TD2B
Yamaha proved to the world that they could build a solid, over-the-counter racer which was good enough to win world titles. This was that machine…
Yamaha's first over-the-counter two-stroke race bikes, the TD1 series did not have the power or handling to be competitive at world championship level. When Yamaha withdrew their exotic 250cc four-cylinder factory machines at the end of 1968 they needed to maintain their presence in the world championships. To achieve this, a new production race bike was launched early in 1969, the TD2, more powerful than the TD1C and with a better chassis.
Sold through selected dealers, the TD2 promised to offer performance needed to be competitive on the world stage. Based on the street
YDS6, the vertically split TD2 engine had new, hard chrome plated aluminium cylinders with porting developed from years of GP experience.
The grooved TD1C rear transfer ‘booster' ports were now true ‘tunnel-like' ports augmenting the larger transfers. Plus widening and raising the inlet port and changes to the direction, the ports produced 6bhp more than the TD1C.
Gone were the ‘Amal-like' remote float carburettors, replaced by flexibly-mounted Mikunis with integral float chambers. Ignition was magneto, triggered by contact breaker points: these would later push the limits of reliability at 11,000rpm!
Expansion chambers were a spring fit at the head with spring mounts under the footrests. Some idiosyncratic ‘street features' found their way onto the race bike, a kick-start was fitted as was the Yamaha Autolube system. Autolube fed oil from a two quart tank in the seat via an engine driven pump to the inlet tracts augmenting the premix.
The chassis was all new, featuring a Norton Featherbed-like frame developed from Yamaha's RD56 GP bike. Sitting on top of the frame tubes was a large six gallon aluminium gas tank, its rear extending down between the tubes. Huge double-sided 4LS front and SLS rear brakes took care of complaints about the TD1C'S poor brakes.
Mounted inboard of the clutch lever was a neat thumb-operated, spring-loaded choke lever to help with starts. A white fairing similar in shape to the 1968 factory bikes had an aluminium belly pan and a broad red stripe down each side, matching the red stripes on top of the gas tank and seat hump.
Realizing the TD2 was at last the real deal, they sold out quickly. Generally buyers were happy with their purchase, but like all production race bikes, careful pre-race examination and assembly increased performance
“THE TD2 WAS - AT LAST - THE 'REAL DEAL' AND THEY SOLD OUT QUICKLY. WHEN PHIL READ WON THE 250CC WORLD CHAMPIONSHIP IT PROVED THAT OVER-THECOUNTER RACE BIKES COULD TAKE TITLES.”
and reliability. The edges of the ports in the hard chrome cylinders needed careful blending with a fine grinder to reduce the tendency for the chrome to flake and cause seizures.
Checking the street YDS6 crankshaft for trueness and making adjustments reduced vibration and increased its short life. The oil-tank in the seat was often modified to house spare spark plugs and plug wrench, a saviour on long races like the TT.
The introduction of the TD2 heralded the start of the privateer age, modifications by gifted mechanics gave their riders the performance needed to score 250cc world championship points. However, a four-stroke Benelli ridden by Kel Carruthers won the championship, although Kent Anderson on a TD2 finished 2nd, just five points adrift.
For 1970, Yamaha decided to field a factory team of modified TD2S with Kent Anderson and Rod Gould as riders. Modifications to expansion chambers and cylinders by experienced mechanics like Randy Hall, Kel Carruthers (also rider) and Ferry Brouwer increased performance to the point where TD2S were dominating the 250 class at National and World level.
Typically the header sections of the pipes were shortened by about 25mm, with the middle, parallel parts of the expansion chambers shortened about 17mm. Unlike four-strokes, (where changes to cam-lift and timing is expensive) two-stroke mixture timing is determined by port heights in the cylinders.
Armed with hand tools, mechanics could modify piston
skirt length and piston crown height to experiment with timing. If the experiments were successful those modifications could be transferred to the cylinder ports for stock pistons to be used for future races. Different sets of cylinders and pipes would be needed for high-speed tracks compared to stop and go tracks.
Anderson and Gould's 1970 factory TD2S were prototypes for the following year's customer bike, the TD2B. Gould won the 250cc world championship, Carruthers 2nd and Anderson 3rd, all on TD2S. In fact, nine of the top 10 finishers were Td2-mounted!
For 1971 most of the modifications pioneered by the factory were incorporated into the TD2B: big-end bearing diameters were increased to enhance reliability, 30mm was cut from the header sections of the pipes, piston ring thickness reduced to cure chattering in the bore at high rpms, intake and exhaust port width increased and transfer port timing slightly altered.
While the factory riders had six-speed transmissions, the customer TD2BS retained fivespeed gearboxes. Carruthers left Europe for the USA at the start of 1971; his place as top Yamaha privateer was taken by Phil Read riding Cheney-framed TD2BS heavily modified by Brouwer and Helmut Fath. Read won the 250cc world championship beating factory rider Gould by five points. Yamaha had achieved its goal of providing over-thecounter race bikes that could win world championships.