Yamaha VMX12H Supplementary Service Manual page 368

V- B O O ST SYSTEM
Troubl eshoot i ng C hart ( 2)
W hen t he m ai n sw i t ch i s t urned on, t he
but t erf l y val ves operat e one cycl e.
uni t and t he V- b
I
C heck the ignitor and the ignition coil.
I
O K
R eplace the V -boost valve control unit.
I