by Stephen “Hell Cat” Heller
I can’t count how many times I have heard, “I love my scooter, I just wish it would go 5 mph faster.” It doesn’t matter if the person is driving a restricted 50cc scooter or a 250cc scooter, they think that extra little power would make their bike just that much better.
I know this statement is unique just to scooters. People were thinking “faster” when they stuffed a V8 engine into a motorcycle and they are definitely thinking “faster” when they are designing and building half-million dollar MotoGP bikes.
With faster in mind this month, I would like to talk about scooter tuning. I have been dabbling in tuning scooters for pretty much as long as I have been riding; about 10 years. It is the same case for tuning as it is for everything else: the more you know, the more you know you don’t know.
An engine runs when air and fuel is ignited under pressure. To make a scooter go faster, more air and fuel needs to be ignited in greater quantities. One of the ways to do this is to increase the size of the cylinder and piston. On a Yamaha Zuma, a 50cc 2-stroke scooter, this would mean switching to a 70cc cylinder kit. A kit consists of a cylinder, cylinder head, piston, piston rings, a wrist pin and wristpin snap rings.
Adding any different part to a system that has been engineered to work together creates bottlenecks and trouble in different areas. By adding a larger cylinder, the carburetor needs to be adjusted or replaced with a larger one to keep up with the fuel demanded by the new, larger cylinder. And so begins the chasing of the tail, upgrading one part, only to create bottlenecks or potential problems elsewhere.
Lets say that you have added a new cylinder kit, carburetor and exhaust and everything is running smoothly for you and your scooter. It is now time to look at your CVT (Continuously Variable Transmission) it is what saves you from having to shift gears, and is different than an automatic transmission on a car.
The CVT is a set of 2 pulleys with a belt running in between the two. The front pulley is connected to the piston via the crankshaft. It is made up of two pieces that are conical in shape and pinch the belt in between the two cones. The outside cone is fixed while the inside can move in or out. When the crankshaft spins, roller weights inside the pulley spin outwards due to centripetal force and the belt is squeezed in between the two pulley halves. This causes the belt to move further out on the pulley.
The rear pulley is also comprised of 2 cones that pinch the belt together. The rear pulley acts as a reset for the front pulley with a compression spring and a clutch that engages the transmission as it spins.
At idle, the back spring is compressing the belt, pushing it out on the pulley. The belt is all the way in on the front pulley. Acceleration squeezes the belt and pushes it out on the front, and in on the back. When letting off of the throttle, the rear compression spring takes over and squeezes the belt back out. This process varies the speed of the engine.
The CVT is set up from the factory to run in the RPMs that produce the most power out of the engine. By changing the engine, you are also changing its power band. Changing to lighter or heavier weights will not increase the horsepower the engine is making, but it will help the engine work more efficiently.
Thinking of the CVT as a bicycle in a high gear, it takes a lot of effort to get it moving. But once it is spinning, you can go very fast. Opposed to starting in a low gear, it is easy to accelerate, but you cannot reach as high of a speed. By adjusting the weights, you can find a point where there is a balance of acceleration and top speed. Roller weights come in .5-gram increments. Heavier weights are usually needed to balance the increased power of the engine.
The back pulley is connected to the transmission and rear wheel. The back pulley is comprised of the clutch and a compression spring. The clutch engages when it is spinning fast enough and engages the final drive transmission and the rear wheel. The compression spring squeezes the belt and decelerates the engine. With a weak spring, the engine will stay in “high gear” and make it difficult to get up any incline or apply anymore throttle. The spring should be changed when any variator work is being done, but is usually neglected. Getting the right balance between the rate of the compression spring and the roller weights comes through trial and error.
I have only scratched the surface of scooter tuning, giving a bit more insight of the uniqueness of the automatic scooter and its transmission. If you want to tune your scooter, you’re not alone. There are companies that have made it easier to match all of the parts you need to make your scooter go faster. Power packs have been put together with a cylinder, exhaust, carburetor and also various roller weights and springs for the transmission. Not ready to make the big jump with everything? A larger cylinder kit would be a good way to start in the dizzying world of scooter tuning.