Yamaha RZ350 and RD350LC - CLASSICS REMEMBERED

Technical Editor Kevin Cameron takes a look back at the classic motorcycles of yesterday

What most of us remember is an attractive, fast, decent-handling racer replica bike with modern water-cooling and (in the RZ, revealed in 1983 but available in the US from 1985), the same powerband-broadening variable-height exhaust port technology used from 1978 on Yamaha's 500 GP bikes.

Here’s how the claimed power numbers look over the years and models;

CLAIMED POWER NUMBERS
PRODUCTION BIKES
R5 350 (1970-72) 36-hp @ 7000, piston-port intake, air-cooled
RD350 (1973 – onward) 39-hp @ 7500, reed-valve intake, air-cooled
RD350LC (1980-86) 47-hp @ 8000, reed intake, water-cooled, not much action below 6000
RZ350 (1983) 52-hp @ 8750, reed intake, water-cooled
THE ROAD RACERS
TR3 (1972) 58-hp @ 9500, piston-port intake, air-cooled
TZ350C (1973-on) 60-hp @ 10,000, piston-port, water-cooled
TZ350G, H (1980-81) 72-hp @ 11,000, piston-port, water-cooled, 6 transfer ports

All of the above 350 twins had the same bore and stroke of 64 x 54-mm. It was reckoned that a top European 350 GP rider such as Jon Ekerold might get as much as 80-hp from a modified TZ350. To put this in historical perspective, the 350 GP class ended in 1984 and just ten years later, factory 250s were reaching 100-hp.

Two-stroke production bikes eventually ran into problems similar to those that previously afflicted race bikes. As power rises with development (better port design, steadily improving exhaust pipes), first-generation cast-iron cylinders (c. 1965) had to be replaced by higher heat-conductivity second-gen aluminum cylinders with iron liners, and they in turn gave way to bore surfaces of thin hard plating deposited directly on aluminum. And finally – the transition to a cooling medium with 830 times the density of air – water. All this is because a two-stroke’s most difficult problem is keeping its pistons cool enough to at least last through the warranty period! The hotter a stressed aluminum part operates, the faster invisible material defects turn into cracks. Motorcycle manufacturers not only look at warranty reports, they also test constantly to keep track of parts longevity. This has led to some well-publicized recalls – something everyone would like to avoid.

Yamaha RZ350 Kenny Roberts Special of 1985.Mark Romanoff at English Wikipedia, via Wikimedia Commons

Another aspect of the same problem is lubrication. The hotter the top piston ring groove operates, the harder it is to prevent gumming (polymerization) of the lube oil (if you ever tried outboard oil in an air-cooled motorcycle engine, you are familiar with this problem). Each succeeding generation of engines has required yet higher standards of resistance to piston ring sticking, from oils containing ever-higher loadings of ‘dispersants’ (the ‘engineer word’ for detergents), with increased resistance to oxidation.

Yamaha’s versatile modular two-stroke production of the 1970s was based upon a pressed-together 6-piece crankshaft (four flywheels with integral-forged mainshafts, plus two crankpins) having a 54-mm stroke. This served fairly well for R5, RD250/350, and the road racers TZ250/350 and TZ750. Yet even in the early days there were signs of trouble. The early 278-prefix crankpin had a 10-mm hole through its center, but as the models made more power at higher revs in the mid-1970s, the tighter fit of the solid 1A1 crankpin was needed to prevent slippage. R5s and RDs made great clubman road racers, but if modified engines were revved over 10,000 with the stock (heavy) ignition rotor, the left-hand flywheel would “walk” off its crankpin at a rate of several thousandths per race weekend. Riders using lightened or aftermarket ignition rotors did not have this problem.

This resulted from the “elephant ears flapping” mode of flywheel vibration. The heavier the ignition rotor, the lower the frequency of the flapping mode – just low enough to be excited by the 10,000+ rpm of a modified RD. See, if we’d just run all our bikes stock, there would never have been a problem!

Each inner flywheel had a main shaft – solid on the ignition-side wheel and tubular on the other. These pressed together with a 7-ton fit. But when the solid 1A1 crankpin came into use around 1975, its tighter fit led to cracking of the drive-side inner flywheel with the tubular main shaft. In extreme cases, that flywheel would completely split in half, becoming a 10,000-rpm pair of steel brake shoes. Fortunately, dealer price on a 250/350 crankcase set was right around $100 (this is why most of the people going racing in those days were mechanics or parts-men).

Another problem of the RD module was main bearing outer race rotation. If the outer races of main bearings are not in some way prevented from rotating in the crankcase, they will spin and self-polish to ever-increasing clearance. Yamaha in these models used a small steel ball, half-protruding from a shallow hole in the surface of the outer race, for this purpose (during assembly you had to remember to line up these balls with their seats in the case). Sometimes it didn’t work! Even in production cases (which had cast-in iron saddles for the bearings) the ball would occasionally be dragged around, cutting its way through the iron and leaving a jagged groove. Bearing friction was not the cause, but rather the orbiting imbalance of the crank, making the outer main bearing races act like Harmonic Drives (Google this – it’s interesting). More than one builder tried red Loc-Tite to fix this, but upon stripping an engine prepared this way and then raced, not a trace of the Loc-Tite could be found.

RELATED:

These were all 180-degree twins, so although they didn’t jump up and down, they did rock from side-to-side, creating unpleasant buzz for the rider and eventually breaking engine mounts – at least in racing. A whole lotta shakin’ goin’ on caused Yamaha to put the young Masao Furusawa on the problem of smoothing out the RD400 (it was given rubber engine mounts). Furusawa later went on to (1) trample Yamaha’s persistent problems with snowmobile suspension and (2) create solutions for Yamaha’s M1 MotoGP bike that would assist Valentino Rossi in winning four championships on it.

A model is obsolete when its capacity for development is used up. At some point Yamaha decided that the rising piston temperature of the RD models, combined with the growing number of problems with the 6-piece crankshaft called for resolution in a new model.

First came the straight liquid-cooled RD350LC, imported into Canada from 1980, and then the much more advanced RZ350 which added two more technologies – the power-broadening YPVS variable-height exhaust port pioneered on Kenny Roberts’s 0W-48 transverse in-line four GP bike from 1978, and a pioneering attempt to reduce the two-stroke’s naturally high emissions of unburned hydrocarbons (UHC) by burning them up in hot-running exhaust catalysts.

A new crankshaft was provided which made it unnecessary for an inner flywheel to have two press-fits in it. Each inner flywheel was forged in one piece with its crankpin, then a center shaft with bearings and center seal joined the two inner wheels (this was supplied as a unit for crank rebuilding). On the previous model, the two inner flywheels were splined together – accurate & strong but expensive. On the new model, the accuracy was built into the assembly tooling in which the wheels were indexed as they were pressed onto the shaft. On Kenny’s 0W-48 and -53 in-line 500s, an even more radical measure was taken to eliminate press-fits; the two inner flywheels and their connecting shaft were forged in one piece as a “dumbbell”. Fractured-race needle bearings and a split center seal were then assembled onto the connecting shaft.

Yamaha RD350LC, a water-cooled, two-stroke, parallel-twin motorcycle.By Ronald Saunders, via Wikimedia Commons

RZ main bearings were pegged against rotation in the cases – a more positive system than the previous ball-in-a-hole method.

The original R5 350 had piston-port intake, a system that generates a lot of signal (meaning the momentary vacuum in the carb venturi that results from opening the intake when there is a partial vacuum in the crankcase). This strong signal allowed R5 to work well with 28-mm carbs, but when tiny reed intake valves were adopted on the following RD models, cab size was reduced to 26-mm because reeds are open longer, with slightly less signal being generated).

RZ was given a nice closer-ratio six-speed. Rs and RDs had begun with 5 speeds. When the RZ engine was adapted to power ‘Banshee’ 4-wheelers (made until 2006), it had the closest ratios of any production six-speed.

With exhaust ports of fixed timing, as used on pre-RZ models, porting was a compromise – the exhaust not so tall that it completely killed bottom-end, but giving away top-end power that an earlier-opening exhaust could have made. But with the ability of the YPVS Powervalve system to advance exhaust port opening as revs rose, it was possible to break this compromise to a degree. When you switched-on your RZ, its Powervalves cycled to full-open and back as a means of sweeping out any gum that might have formed. As you rode away, the exhaust port height was continually adjusted as rpm rose and fell. This resulted in a more rideable vehicle. Every manufacturer of two-stroke motorcycle engines had to do something similar, all with different trademarked names such as KIPS, ATAC, AETC, etc. This is analogous to the variable valve timing systems used today on the four-stroke engines of many production cars and some motorcycles.

Yamaha RZ350 Road Test from July 1984 issue of Cycle World.Cycle World

Cylinder spacing was a problem on these parallel twins, for when a 250 (54-mm bore) was turned into a 350 (64-mm bore), the space between the cylinders through which the cylinders’ transfer ports had to pass became narrower. Dutch two-stroke engineer Jan Thiel has pointed out that one of the basic necessities of a hi-po two-stroke is that the transfer loops swoop out from the crankcase and then loop back into enter the cylinder on a largeradius. But the close 104-mm spacing of Yamaha’s modular parallel twins left no room to give such transfers to a 350. For this reason, when Yamaha in 1981 switched their TZ250 production road racer from the RD production case to special $1000 sand-cast cases, they increased cylinder spacing to 112-mm. This was an improvement in terms of allowing larger, more gradually curving transfer ports, but the vibration of a 180-degree parallel twin increases in proportion to the distance between cylinders. At this point the aluminum frame revolution was in progress, but the awful vibration of Yamaha’s TZ twins was a problem for crack-prone aluminum.

Why not just rubber-mount the engine? If you can’t use the strength of the engine as part of the chassis, you have to add extra chassis material to provide that strength. RD and RZ chassis were tubular steel, based on road race geometry (27-degrees, 3.8” trail) but heavier in order to last more than 18 months. Another problem with rubber mounting is that just letting the engine shake creates problems for carburetion.

The enabling answer was to build the 1991 TZ250B as a 90-degree V-Twin. Because each cylinder’s crankcase had to be separate, the cylinders were far enough apart to generate a rocking couple, but a balance shaft settled that, and otherwise, the 90-degree cylinder angle canceled primary vibration. The new V-twins could be bolted solidly to their chassis, and their cylinders could be given the gracefully curving transfer loops prescribed by best practice.

But alas, carbureted two-strokes had run their course. With 30% of the fuel passing through the engine unburned, muffler-mounted exhaust cats ran very hot. Yamaha put modern Direct Fuel Injection (DFI) on some of their two-stroke outboards but the safer, cheaper path was to build four-stroke motorcycles whose emissions-control technology had already been pioneered and bankrolled by the auto industry. That’s where we are today.

Slot: div-gpt-ad-leaderboard_sticky
Slot: div-gpt-ad-leaderboard_middle1
Slot: div-gpt-ad-leaderboard_middle2
Slot: div-gpt-ad-leaderboard_middle3
Slot: div-gpt-ad-leaderboard_bottom