The Villiers manual states:
ENGINE SPECIFICATIONS
Mark 4F Unit
This unit consists of a 98 c.c. two-stroke engine and two-speed gearbox with two plate cork insert clutch, the clutch with primary driving chain and being contained in one common casing. The gear selector mechanism consists of a sliding sleeve with "face" dogs which engages either the top or bottom gears when operated through a bell crank lever connected to a " Bowden " type cable, the control lever being fitted to the handlebar of the motor cycle. The majority of the units in service are fitted with the Junior Type 6/0 carburetter, but current production units have the later Type S. 12 carburetter as standard equipment.
Mark 6F Unit
This unit differs from the Mk. 4F model only with respect to method of changing the gear. The change is made by foot lever instead of by a hand operated cable. The positions of the three frame fixing lugs are as for the Mk. 4F unit, but the clutch- case is deeper to accommodate the foot gear-change spindle and therefore the Mk. 6F unit will not fit all existing frames primarily built for the Mk. 4F model.
Mark 9F Unit This model is interchangeable with the Mk. 6F unit and although the capacity is the same the power output is greater, this being obtained by (I) cylinder head giving a higher compression ratio, (2) Cylinder having modified porting, and (3) Type S. 19 carburetter having a larger size of choke.
The manual may be downloaded from Icenicam.
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