Yamaha Yr2 And Yr2c
YAMAHA YR2 and YR2C
CYCLE WORLD ROAD TEST
ENTHUSIASTS once considered that two-stroke motorcycles were not motorcycles. These machines were regarded as fussy and underpowered devices for transporting people short distances. Large displacement fourstrokes were the “real” motorcycles, said the enthusiast.
The Germans changed all that. Fast, powerful racing twostrokes, built first by DKW and then by MZ, smashed the pop-pop image sky high. In particular, the MZ factory laid down the highly successful formula of disc valve induction, tuned exhaust systems which aid cylinder breathing by correctly spaced pressure waves, and pistons specially shaped to improve scavenging of the combustion chamber.
In recent years, the Japanese factories have continued the rapid development of the two-stroke engine. Yamaha’s five port system is the latest in this line of major improvements.
Among the Yamaha machines to be equipped with these new engine components are two 350-cc roadsters, the YR2, and a street scrambler version, the YR2C. Both are fine motorcycles, fast, well made, and with excellent handling qualities to match their speed.
There is no mystery or mechanical complication about the five port system. It consists simply of two extra transfer ports with matching windows in the piston, located one each side of the intake port. They considerably aid cylinder breathing and scavenging.
In a two-stroke engine operating with piston port induction, and with the customary three ports, the two side ports tend to direct the fresh mixture across the top of the piston. But this mixture does not effectively sweep into the top of the combustion chamber, and push out the burnt gases. Moreover, when the piston is at the bottom of its stroke, the fresh charge also has a tendency to pass straight through the open exhaust port. The larger the cylinder, the more exaggerated these problems become.
A disc valve engine, which Yamaha uses on some of its production machines, and on its grand prix racers, does not have these disadvantages. With the valve located low in the side of the crankcase, an extra port can be designed into the rear cylinder wall—a space occupied by the intake port in a piston port engine. This extra port can direct the fresh charge in a high loop into the top of the combustion chamber, cleaning out all expended gases.
The five port system accomplishes a similar effect. The two ports at the rear of the cylinder wall straddle the intake port, and aim the fresh mixture in such a way that all the waste gases are blown out. Additional advantages are that cylinder head and spark plug cooling are improved. Yamaha also claims the five port system gives the engine a wider power band.
Most of the other specifications on the YR2 series remain virtually unchanged from the successful YR1 machines. Compression ratio actually has been lowered, from 7.5:1 to 6.9:1, oil capacity has been increased form 6.6 to 6.8 pt., and overall gearing has been slightly altered. Engine capacity remains identical, at 348 cc, and bore and stroke still are 61 by 59.6 mm. Primary drive is helical gear to the multi-plate clutch and five-speed gearbox.
five-speed Locating these components is a frame which works as well as it looks. Every section of the tubing is of generous diameter, and the highly stressed areas are well gusseted, particularly around the swinging arm pivot. Twin down tubes pass under the engine and terminate at the top of the rear suspension units, leaving a very wide open cradle occupied by the engine, transmission, carburetors, oil tank, and battery. A single top tube is used, while another tube runs from it, and divides into two to meet the rear frame legs. Its two arms act as attachment points for the battery and oil tank. A side result of the very open layout of the frame is that plugs and carburetors, and the engine in general, are easily accessible.
Bodywork on the YR2 kinsmen is very effective. The fuel tank is long and large, and the capacity is 3.8 gal., much greater than the sprint tanks on many machines. If Yamaha had tried to use these very small tanks, the results could have been awkward for riders. Large capacity twin-cylinder twostrokes have a colossal thirst for fuel, and the YR2s are no exception. Fast riding gives them a range of no more than 150 miles between fuel stops, even with the large tanks.
Yamaha has continued to use its distinctive, slim-atthe-nose, wide-at-the-rear, styling with the fuel tank.
The left side panel under the seat covers the tool compartment, battery, and one of the two air cleaners. The corresponding panel on the right side hides the other air cleaner, and the oil tank and its filler cap. There is no need to remove this cover when checking for oil level. A plastic window in the tank corresponds with a hole in the panel, providing instant reading of the oil level. The YR2 requires a pint of oil about one in every three fuel stops.
On the road the Yamahas are smooth, fast, and very well mannered. High performance two-strokes are always “peaky” to some extent. On the YR2, this sensation becomes apparent at 4000 rpm. Below that speed, power is smooth, but unhurried. At 4000 rpm, it is transformed into a rushing burst of acceleration that quickly takes the tachometer needle to the red line.
This line begins at the 7500 rpm mark, so the rider has a 3500 rpm power band to play with, if he requires maximum power. That is wide, by any standards. But at the same time, engine speed builds up very quickly, so that there is little time between gear changes. For less hurried riding, the rpm range is from tickover to flat out. When the bike is stationary, the engine burbles happily at just under 1000 rpm, and will pull away from that speed with a little coaxing from the throttle and clutch. Freeway cruising at 65-70 mph is achieved at a leisurely 5000 rpm.
Gear pedal action is arranged on a left foot one down, four up basis, with neutral between first and second. Ratios are excellently chosen. Accelerating through the gears produces a drop of about 500 rpm every time a change is made. This keeps the engine constantly at maximum power.
Handling is superb, whether the bike is trickling through low speed traffic or hustling along the open road. The bike can be leaned into corners with absolute confidence that the wheels will follow the chosen line. The suspension is a little soft, but certainly smooths out bumps, and in a rough corner the machine keeps right on line.
A bike so suited for fast riding needs powerful brakes to make it safe. The Yamaha has them, a two leading shoe unit on the front wheel, and a single leading shoe brake at the rear. Test riders voted them among the best brakes ever encountered in a road motorcycle. Their stopping power is tremendous, yet the lever action is not sudden or jerky. On a wet road, however, a rider must use considerable care to avoid locking both wheels. It is hardly fair to criticize a manufacturer for designing ultra-powerful brakes; it is up to the rider to learn how to use them.
Riding comfort is good on the YR2, poor on the YR2C street scrambler. The bikes have luxurious dual seats, but there the similarity between the two ends. The handlebars on the YR2 offer a perfect riding position. On the YR2C, the raised motocross type bars push the rider high into the wind, and make freeway riding unpleasant. High level exhaust pipes give the machine a fat profile, and force the rider to adopt an ungainly leg position.
Handlebars and exhaust pipes are the main differences between the two machines. Also, the street scrambler has rubber gaiters on the front fork, and exposed springs at the rear, and tires that are slightly, but only slightly, more suited to dirt riding than the covers on the YR2. On each machine, the driver’s footrests are folding, but on the street scrambler the passenger’s footpegs also fold.
The differences between the two bikes may not be great, but they certainly have a great effect on rider enjoyment.
The combination of exhaust pipes between the rider’s legs, and the high bars, made the street scrambler quite uncomfortable. Footpeg location also is different, and the result is that the foot continually fouls one or other of the pegs when the kickstart lever is operated. Yet another fault with the high bars is that the control cables obscure the speedometer and part of the revmeter. The rider has to lean forward to see them properly.
Perhaps it is hardly necessary to add that the street scrambler is by no stretch of the imagination suited to trail riding. Its special features are merely for show.
On both machines, a 12-V battery caters for lighting. The headlight throws out a wide spread of light, and enables high speeds to be used at night.
The YR2 and 2C are the direct relations of the machines which performed so well at Daytona. The race machines gave away many ccs of displacement to their rivals, yet were capable of outrunning most of them. That’s proof that five port power has arrived.
YAMAHA
YR2 and YR2C
$810 $840
