MINSK Repair Manual page 11

through them and causes a spray of petrol to mix with the passing air. This
petrol/air combination then passes into the cylinder where it is compressed by the
upward movement of the piston into a tight space at the top of the cylinder just
below the sparkplug. Combine this with a spark and you get what you need to haul
your Minsk up a muddy track.
The two jets have different purposes and are of different length. The shorter
one called the idle jet has the job of making the air/petrol spray when the engine is
idling (when you have yet to turn the throttle) and is always open. The longer one
called the accelerator jet has a needle running down its centre which is closed if
you do not turn the throttle. When you accelerate the bike by turning the throttle
with your right hand, this needle rises inside the accelerator jet, thereby allowing
more and more petrol to pass though the jet and then on into the engine.
The different lengths of the jets means that if there is not enough petrol sitting
in the bottom of the carburettor, then only the longer accelerator jet will be
immersed in the petrol, leaving the idle jet dry. This will cause the bike to stall
when no throttle is applied (while not moving). Should the level be too high then
the action of both jets will be inhibited by to much petrol and the engine will
flood. The ideal level is just above the bottom of the idle jet. Not too high, not too
low.
Inside the carburettor is also a gate, sort of like a sliding trap door, which
moves up and down when you twist the accelerator throttle with your right hand.
The more you open this gate, the more the compression from the piston's
movement is able to suck in air from the air filter. The more air that passes over
the jets, the more the petrol is sprayed into the air/petrol mix and the greater the
power behind the explosion in the cylinder. When you open the throttle, the action
of the gate and the accelerator pin work to make the bike go faster.
The mixture of air and petrol in the spray needs to be in just the right
proportions to cause the most efficient and powerful explosion possible inside the
cylinder. It is possible to have too much or too little air in the mix and both will
cause sluggish performance. On the carburettor is a small screw which regulates
how much air is mixed in with the petrol. When you turn this screw inwards it
limits the amount of air in the mix and when you turn it outwards it allows more
air into the mix.
The correctness of the petrol/air mix dictates the colour at the bottom of the
sparkplug so a simple check will tell you if your carburettor is adjusted correctly.
A properly adjusted carburettor will cause the ceramic insulation in the centre of
the sparkplug thread to be dry and brown and the base of the thread to be dry and
black. If the ceramic insulation however is covered in dry black carbon then the
©Digby Greenhalgh 2000
Minsk Repair Manual
ceramic
insulation
in here
thread
8
BROWN – just right
WHITE – too much air
BLACK – too little air