Yamaha RD350

Performance for the masses

The RD350
Image: J. Thulead

Yamaha got the formula right, and the formula was simplicity. The TR and TZ race bikes had been dominating their respective classes for years and Yamaha wanted to profit from it. Yamaha produced the RD350 from 1973 to 1975. It evolved directly from the piston port (pre-reed valve intake tract), front drum-braked, five-speed Yamaha 350 cc "R5". The engine had a six-speed gearbox although in some markets, such as the UK, the first model was sold in five-speed form. The stock bike produced 39 bhp, 32 bhp at the back wheel at 7500 rpm. This, combined with the lightweight package, made it a quick bike. It was to gain the attention of many racers in the years to come. American tuner-racer Don Vesco in an 18-foot-long streamliner powered by twin TR2 350cc racing engines, set a speed of 251.924 mph on the Bonneville salt flats in 1970. The RD is a direct descendent of the TR2.

Externally the engine is identical to the TR2

The RD series road bikes were remarkably similar to the racers and a lot of parts were interchangeable. It was the performance package for everybody. A standard bike would keep up with most bigger bikes and was faster than many of them. With not much more than a file and some sandpaper (to open up the ports) it could be turned into a rocket and several motorcycling publications published articles on how to do it. It was easy to work on.
There were no other performance bikes at the time that came in such a small and lightweight package and what's more, it was at a price that almost everyone could afford. For those who wanted to race, this was the bike to do it on. By raising the exhaust ports, opening the intakes, widening the boost ports, putting a 15mm spacer between the reed valve block and the cylinders, swapping the stock 28mm carbs for 32s or 34s, milling the heads for more compression, a worked over RD could make more than 50 horsepower.

The only difference in the frame layout to the TRs and TZs was the steering head angle. The RD had a 27.5-degree rake and the TZ had 25. While the increased head angle was designed to give more stability on the road, it still steered superbly on the racetrack. As long as the ground clearance issues were addressed, only a full race bike could get round a corner quicker. The front disc brake and rear drum pulled the bike up very well and this could be improved considerably by replacing the heavy seat and mudguards. Fitting a good set of expansion chambers would lighten it considerably more and were one of the biggest single performance improvements you could add to the engine.

As a novice racer gained more experience, TZ parts were usually added when the budget would allow. Many racers followed the full evolutionary process and ended up with a bike that was virtually identical to a TZ. The road hardware bracketing could be easily removed from the frame with a bit of cutting and grinding. Big gains could be made here as the frame weighed almost twice as much as the racer. Aftermarket shocks were usually fitted.
It was also relatively easy to fit a TZ monoshock. The standard swing arm could be used. A tube needed to be bent and welded to the swing arm with bracing to locate the rear of the monoshock and a forward attachment bracket welded to the frame underneath the tank. The TZ tank and seat could be fitted with little modification. Fitting the fairing only required welding some brackets to the frame. Add the required engine parts, ditch the auto lube, add alloy rims, a good set of tyres, a set of clip ons and you had a full-blown racer.

The RD350LC took the "racer for the road" to a new level

The 350 evolved into the more refined and cleaner running RD400 in 1976 but this model didn't seem to have the mass appeal of the 350. It wasn't until the release of the liquid cooled RD350LC, also called the RZ350, in 1980, that two stroke performance freaks could get exited again. This is a whole other story and took racetrack type street performance to another level. People are now starting to realize the significant contribution the RD made to motorcycling's performance culture and are re thinking their "four stroke only" attitudes towards collectability. Well restored examples are now bringing many multiples of their original purchase price. Two stroke engines are lighter than four strokes as well as being cheaper and easier to produce and the RD along with other two stroke models from Kawasaki and Suzuki, played a major part in bringing affordable performance to riders worldwide.

Written by Paul Harmon, 15-1-18

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