Villiers MARK 12D Operating Instructions And Spare Parts List Download Page 12

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Tuning

Before. any attempt is made to tune the Carburetter

it is

Carburetter.

essential that the engine is in a good mechanical condition.

his means that there should be no air leaks at any of

the joints, there should be a good spark at the plug points and also

that there is no restriction in. the fuel supply. lt is also Important,
ol cours�::, that the carburetter is clean mternally, and that the

air

filter 1s not obstructed.

!'here are ±our adjustments for tuning the carburetter, but each

of  these has  its  full  effect  at  a  particular  part  of  the  throttle  range,
and  should,  therefore,  only  be  used  for  tuning  that  particular  part
oi  the  range.  There  is  also  a  definite  sequence  for  the  tuning,  and
this  also  must  be  adhered  to  in  order that  the  results  achieved  with
one  adjustment  are  not  upset  by  the  next  adjustment.
The  sequence  of  tuning  with  the  necessary  adjustments  is  given
below:-

(1) Main Jet. Throttle Range--t to Full.

In order to obtain the correct main jet size, the engine must be
tested at full throttle in top gear.

If the engine lacks power,

detonates badly or runs better with the strangler slightly closed,
a larger main jet is required. Should the engine " four-stroke "

or improve momentarily after the petrol has been switched off,
a smaller jet is required. After de-clutching and stoppmg the

engine quickly the sparking plug should have a shiny black

appearance if the correct main-jet is fitted. As an additional guide

the engine should tend to " four-stroke " at full throttle in bottom

gear on level ground (or high engine speeds in neutral), but not
in any higher gears.

(2)

Pilot Jet. Throttle Range-. Oosed to

open.

The pilot jet must be set when the machine is stationary with the

engine running at the required idling speed. To richen mixture,

screw in the pilot adjuster screw, and to weaken, unscrew pilot
adjuster. The mixture strength must be set as weak as possible

consistent with a steady reliable idling speed and good engine

acceleration from this throttle position.

the mixture strength

is set too rich, trouble will be experienced with the fuel build-up

in the crankcase when the throttle is shut with the engine still
running fast. Should this latter fault be present after adjusting the

pilot, unscrew pilot a further half a turn. Any weakness on

acceleration can be cured by throttle cut away as given below:-

(3)

Throttle Cut-Away. Throttle Range-! to

Open.

The throttle slide is made with a cut-away on the carburetter inlet

side  which  influences  the  depression  on  the  main-jet  system.  The
throttles  are  marked  with  a  number  which  represents,  in  sixteenths
of  an  inch,  the  amount  of  cut-away.

throttle with more cut

away will give weaker mixtures (over the particular throttle range)

and vice-versa.

the acceleration is weak, fit throttle with smalle1

cut-away, e.g. change from 3 to 2!. Should the engine tend to

" four-stroke " when the throttle is shut, fit larger cut-away.