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The Differences
Developed from the air-cooled motors with horizontal cylinders, these engines have had a number of changes over the years. The major changes concern the transmission parts: the rear pulley, the clutch diameter and the rear pulley diameter changed around 1996/7, but apart from cosmetic differences and the use of a disc brake on the rear, the motors are basically the same.
Derestricting
Derestriction usually involves removing the restricter from the front variator, removing the washer from the exhaust and cutting the front pipe 90° elbow off and rewelding. When derestricted by these methods the top speed is around 50 to 55mph on the speedo.
Exhaust
The usual way of improving performance after derestricting is to fit a better exhaust. The LeoVinci Handmades and the Giannelli NRX exhausts give a good boost in power, but with a narrow powerband they can give the transmission problems if roller weights are not spot-on for the individual motor.
The exhausts with wider more flexible powerbands are the Giannelli Sport Cee and the LeoVinci SP3. These give good performance but at lower RPM than the NRX and Handmade pipes. They are better suited to the 70cc kits for road use where a gear kit is to be fitted. Because the RPM is lower with these exhausts potential engine wear is reduced.
Further Improvements
Like most Yamaha automatic motors, the transmission allows large variations in RPM, so using aftermarket springs, variators, and a torque driver pulley will benefit performance.
The use of a 70cc cylinder kit with a larger carb and a gear kit will give another boost in performance, the increase depending on what other parts are fitted - around 8 to 9bhp is achieved with the basic performance parts and around 10 to 11bhp with the aluminium MHR kit from Malossi and the W-port aluminium kit from Polini. These higher performance kits will benefit from the race spec transmissions that Malossi make under the wide-range transmission format, which must be fitted and used as complete sets.
The cast-iron basic cylinder kits such as those made by DR, Top Performance, Malossi and Polini can use 17.5mm, 21mm or 22mm carbs, but the more expensive larger carbs only give a small power increase over the 17.5mm when used with the cast-iron cylinders.
When aluminium cylinders are used the larger carbs become more beneficial. Any of the cylinder kits will run on any of the carbs, including the standard one, but like everything else it depends on performance required and budget.
Developed from the air-
Derestricting
Derestriction usually involves removing the restricter from the front variator, removing the washer from the exhaust and cutting the front pipe 90° elbow off and rewelding. When derestricted by these methods the top speed is around 50 to 55mph on the speedo.
Exhaust
The usual way of improving performance after derestricting is to fit a better exhaust. The LeoVinci Handmades and the Giannelli NRX exhausts give a good boost in power, but with a narrow powerband they can give the transmission problems if roller weights are not spot-
The exhausts with wider more flexible powerbands are the Giannelli Sport Cee and the LeoVinci SP3. These give good performance but at lower RPM than the NRX and Handmade pipes. They are better suited to the 70cc kits for road use where a gear kit is to be fitted. Because the RPM is lower with these exhausts potential engine wear is reduced.
Further Improvements
Like most Yamaha automatic motors, the transmission allows large variations in RPM, so using aftermarket springs, variators, and a torque driver pulley will benefit performance.
The use of a 70cc cylinder kit with a larger carb and a gear kit will give another boost in performance, the increase depending on what other parts are fitted -
The cast-
When aluminium cylinders are used the larger carbs become more beneficial. Any of the cylinder kits will run on any of the carbs, including the standard one, but like everything else it depends on performance required and budget.
