|Reprinted from the July 1983 issue of Cycle World|
Once before, this choice was offered. It was a generation ago, when big bikes meant big V-Twins and newcomers appeared with lighter weight and more performance from their smaller, higher revving parallel Twin engines. The V-Twins used to be Harley-Davidsons and, for a while, Indians. The newcomers were Triumphs, and then BSAs and Nortons and Royal Enfields.
These bikes were part of a glorious time when motorcycles were motorcycles and distinctions about on or off road, production or racing, didn't seem so important. The racing battles between the Twins Vee and parallel were epic. The men who rode these bikes are now legends.
Here we are again, looking at a new, exciting 650cc parallel Twin. It doesn't come from England. It comes from Suzuki. It's the GR650, otherwise known as the Tempter.
Historical perspectives are tough to avoid when looking at the GR650. For one thing, it has parts styled to like like a Triumph. The two colors on the teardrop gas tank would look at home on a Bonneville. Sitting on the GR650, riding it, all feels a little British. The motor isn't like any British design, instead it uses a pair of chain driven overhead cams to operate the two valves in each cylinder. The engine is over square in configuration with a 77 by 70mm bore and stroke. It has a wet sump oil system, another non-British design. And the appearance of the motor is very Japanese, angular lightweight covers on the ends of the motor instead of carefully rounded and distinctive shapes.
Inside those engine cases are some more non-British ideas. Most unusual of these is the two-stage flywheel. Heavy flywheels are useful at low engine speeds. Store lots of inertia in the flywheel and when the clutch is released with the engine ticking over, the engine doesn't die. The bike just chugs ahead. A bike with heavy flywheels can idle better and run smoother at low engine speeds. But at high speeds that heavier flywheel can make the engine unresponsive. On the GR650 there's an auxiliary flywheel on the left hand end of the crankshaft, inboard of the generator. Inside the auxiliary flywheel weight is a centrifugal clutch. Below approximately 3000 rpm the clutch engages the added weight, making the crankshaft inertia higher. Above 3000 rpm centrifugal force on the engaging weights disconnect this added flywheel.
Following the practice used on the smaller GS450, the 650 uses a 180 degree crankshaft with a single gear-driven counterbalancer. One piston is up when another is down, and the resultant imbalance can be cancelled by a single balancer shaft. A crankshaft with both pistons rising and falling together requires the use of two balancer shafts to run as smoothly. The only shortcoming of the simpler 180 crankshaft is the irregular firing pattern that may not create as plea
sing an exhaust note.
Plain bearings are used on the crankshaft and rods. on both sides of the center main bearings are oil jets that spray the bottoms of the pistons with cooling oil. This is an effective method
of cooling an engine and Suzuki has found it works better than running cooling jets from the connecting rods.
At the top of the Suzuki's engine are mostly conventional parts, but with a couple of new or refined twists. The cams are hollow to save weight. They push inverted bucket followers with adjusting shims on the top of the buckets, just like Suzuki's other engines with two valves in every cylinder. Everybody has some trick to induce swirling of the intake mixture. Suzuki is using a sub intake port. It's a small tube connecting the carburetor throat to the intake port just above the valve head. This tube is canted at an angle, so that when the valve is open, a stream of air from the tube is aimed at the edge of the cylinder, swirling the mixture. Suzuki claims this air induction enables the GR650 to operate with much leaner air-fuel mixtures, improving economy. This improvement is most effective at low