The Motorbike Archives

Among the four big Japanese firms three specialize in two-cycle engines and the other builds four-cycles. The Yamaha and Suzuki two-cycles are piston controlled inlet types (although both make rotary valve machines on a small scale), while Kawasaki specializes in engines utilizing rotary disc inlet systems.

To explore the advantage of the rotary disc we must first understand the workings of the piston controlled two-cycle engine. As the piston goes up and down in the cylinder there is a compression and vacuum on top of the piston and a similar action in the crankcase beneath it. To get the fuel into the crankcase there is a passage in the cylinder that is opened and closed by the skirt of the piston as it travels up and down.

The passage opens at so many degrees of crankshaft rotation before top dead center and closes at exactly the same number of degrees after top dead center. So, if the opening cycle is changed to happen sooner (this is done simply by lopping off the skirt of the piston on the inlet side), then it must close later. With this type of engine there is a considerable amount of flow-back coming out of the carburetor at slower engine speeds.

Now the rotary valve uses the same compression/vacuum principle in the lower cases but has a circular disc valve to open and close the inlet passage. The advantage is simple; they can increase the inlet cycle and shut it off anytime they please. 'No spit back.

In practice the engine has a larger powerband, more torque, and is easy to start. But you can't have your cake and eat it, too, so here are a couple of the negatives. The carburetor is placed inside the outer crankcase making the engine package a bit wide. And being situated inside the case makes it a bear to fiddle with.

Our test bike is the Kawasaki F8. It has a square engine having a bore and stroke of 68mm. The total displacement is 246.8cc. It's an air-cooled two-stroke single that is equipped with a primary kick-start system. This simply means that you can start the thing in gear by just pulling in the clutch and whaling on the lever.

The engine is fed through a 30mm Mikuni carburetor and utilizes a cone-shaped polyurethane foam air filter. The cases have a built-in air passage from the air filter to the carburetor cavity that would put the designer of the Pharaoh's tomb to shame. The carburetor is completely sealed in the cases with the cables and hoses passing through plastic caps and gaskets. However, there is a handy idle adjustment next to where the throttle cable enters.

The F8 is still using a pointed magneto. It has a very small 6-volt battery to operate a stoplight and horn and is fitted with lighting coils that get into the act when the switch is thrown. The 35/35w sealed beam headlight is quite good for night travel throwing an oval beam on the roadway.

In keeping with the Japanese motorcycle industry the F8 uses a separate oil supply system they call Injectolube. It has a 1.7 quart capacity and should be good for 500 miles under normal usage without a refill.

They rate the engine at 23.5 hp at 6,800 rpm. However, with the rotary valve inlet it gets a tremendous amount of power right down at the bottom. Coupled to a wide ratio gearbox the motorcycle is flexible enough for picking your way through difficult trails or cruising down the highway at 65 or 70. Low and even second gear are good for pulling stumps.

The motorcycle has a soft positive ride and is equally at home on the highway or off. It's a bit heavy at 295 wet, but the ideal sitting/ standing position rates it among the best. The adjustable Hatta forks worked fine out of the box and we didn't find any reason to mess with the spring rates. We did, however, try the different axle positions.

Changing the front axle to either of the other two positions results in a difference in steering that you can really feel. From one that is overly precise to the other that is just too clumsy and heavy. We feel the Hatta adjustable forks is a great method to better understand the mysteries of fork geometry, but they're at their best when left in the middle position.

The F8's rather wide seat has the proper padding and coupled with the soft-acting rear shocks produces a comfortable ride. The handlebars are positioned correctly to the footrests which are a bit further from the seat than is normal. The distance from the top of the seat to the footrests is a long 21 inches so there's not much to standing from sitting. In fact, people with a short inseam measurement may feel they are standing all the time even when sitting.

The Kawasaki is equipped as an enduro bike and is ready right out of the box. The front wheel drive speedo has an odometer trip and there are other tantalizing goodies to go along. The hand levers have neat rubber shields to keep the mud and crud from the works, and the waffled hand grips work super. The brake lever is a cobby thing with teeth that won't pack up with mud, and their only goof was to cover the footrests with rubber. Rubber soled boots don't stay on rubber footrests when they get wet.

At 11 inches the footrests are a bit low and the width of the engine places them a little wide, also. We got our feet knocked off the pegs once or twice while riding some rather narrow trails in the mountains. While standing, and we rarely did, the kick-start lever would interfere with the rider's calf. When seated it was no problem at all.

The F8 has the shortest shifter throw in the whole world. It's incredible, you move the lever about an inch to change from one gear to the next. With or without the smooth working clutch the thing would jump into the next gear. The only problem we had was finding neutral, but that's really not a problem when you consider it can be started in gear.

A box-like muffler comes out the left side and is tucked well out of the way. The pipe is shielded but we never even noticed it. The muffler has a U.S.D.A. approved spark arrester device built in and the exhaust note is pleasing. The outlet spews exhaust guck all over the L/H shock absorber turning the chrome plated spring all black. It's a shame, but this could be cleaned each time the bike's ridden.

The riding test we put this Kawasaki through was one of the most difficult we have ever done, and the F8 passed with flying colors.

With an out of the crate machine and no jetting changes we started at 3,000 feet and climbed a narrow switchback trail. The bike climbed well using 1st and 2nd gear. As the altitude increased we could feel it get rich on the needle and main jet but it still went. We crested at 8,000 feet and the F8 didn't give us a bit of bother.

On the trails it has a stable feel and steers precisely where you point it. Its low c.g. allows it to corner well, just don't try to pop wheelies. The trials tires give it good footing and the bike is controllable even in mud.

Under a pull you can throttle it up and down without fear of having the engine load up. We dropped it into our test stream and played in the water long enough for any ordinary motorcycle to fill with water and quit. The F8 didn't. And when we were all done trying to find a fault with the motorcycle we discovered it was getting better than 50 miles to a gallon, trail riding.

So if you're looking for a sensible dual-purpose machine that will do most anything, the Kawasaki F8 is certainly one that should not be passed up. This rotary valve single is a good one. ■