Hemispherical Combustion Chamber
A fast combustion speed is necessary to increase combustion efficiency. Since the compact size and shape of the hemispherical combustion chamber adopted on MZ250/MZ300 reduce the distance that the combustion flame ignited by the spark plug must travel, less gas remains unburned and the combustion speed isincreased. This results in improved fuel efficiency.


To enable a hemispherical combustion chamber shape, the intake and exhaust valves are set at a 22° angle. To accommodate this angle, the intake port shape was changed from the previous design. The interior of the combustion chamber is conducive to creating a swirl that speeds up combustion, boosts combustion efficiency, and helps achieve better fuel efficiency.


Since the hemispherical combustion chamber increases combustion speed compared to the previous engine design, the ignition timing (advance) has also been changed from BTDC 23° to the new model's BTDC 20°.

Ignition timing (or ignition advance) means the timing of the firing of the spark plug. It is expressed in terms of the number of degrees of [crank] angle when the ignition fires before the piston reaches top dead center in its compression stroke. For example, BTDC 25° (BTDC = Before Top Dead Center, which is designated as 0°) would mean that the ignition fires at the point where the crank angle is 25° before top dead center. The reason for such an advance in the ignition timing is because it takes some time for the ignition flame to spread through the air-fuel mixture in the entire combustion chamber. If the ignition was fired when the piston reached top dead center, combustion would occur too late to deliver maximum power and efficiency.


(Only on Silent Semi-Dry Series)
The latest generation MZ250 and MZ300 models have a new air cleaner design with the intake vents located at the top of the box where the intake air is less influenced by engine heat and less likely to draw in dusty air. Also, a new filter material has been chosen to minimize dust intake and improve fuel efficiency and maintenance.


The previous MZ300 model was the same size as the MZ360, but now design changes throughout the engine have reduced the size of the compact new model sufficiently to make it the same size as the MZ250. First of all, the forward incline angle of the cylinder was changed from 28° to 22° from the horizontal for decreased overall height. At the same time, the shape of the piston and its skirt was changed and the length of the connecting rod shortened (110 mm > 102 mm) for an optimum design that reduced the dimension in the direction of the cylinder head. In addition, a review was made of the head assembly from the standpoint of space efficiency, resulting in a reduction in overall width by setting the breather chamber in the head at an angle.

Furthermore, the recoil assembly was made thinner through design changes like positioning a pair of cooling air ducts on two sides, and a thinner air cleaner design was also adopted to contribute to overall compactness. Despite its compact design, the new MZ300 maintains the same level of power output as the previous model. As a result, it now has the largest displacement of all competing models in its size category*.

* As of June 2012, according to Yamaha Motor surveys.


A chain is included to prevent loss of the fuel cap during refueling.

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