Understanding The Cylinder And Piston; Understanding The Transmission System - MINSK Repair Manual

petrol mix has too little air in it. Conversely, if the ceramic is white then there's
too much air in the mix.
Understanding the Cylinder and Piston
Around the head of the piston are two metal rings just the right size to allow
snug movement of the piston up and down the cylinder without any air or pressure
being able to pass them. The metals used are extremely strong and are designed to
expand and contract at the same rate when exposed to all the heat caused by the
exploding petrol. What stops the rings from grinding against the sides of the
cylinder is the cooling and lubricating effect of the oil mixed in with the petrol.
This oil does not combust when the petrol explodes. Instead, it covers the sides of
the cylinder and makes the movement of the piston easier and smoother. If you
buy petrol with not enough oil in it then the piston will seize up after about 20
minutes of driving because of all the friction caused. Some of this oil then makes
its way down under the rings to lubricate the cam disk and engine bearings while
the rest gets blown out the exhaust pipe. Rather a lot of oil and smoke in fact
spews from the back of the Minsk. It not the cleanest bike in the world by any
account.
Four stroke engines used for example in Honda Dreams or in any car do not
need oil mixed with the petrol because there is a permanent supply of cooling and
lubricating oil under the piston. This oil never gets above the piston and then out
the exhaust pipe so these kinds of engines are much cleaner. Four stroke engines
also use a complicated system of valves at the top of the cylinder to make them
much more fuel efficient than two stoke engines.
It is these valves which give rise to the two names – four stroke and two
stroke. Without going into the details, the valve system only found on four strokes
requires the piston to down then up and then down then up (hence the name four
stokes) for every time the sparkplug fires and the petrol explodes. In a two stoke
engine like the Minsk however the piston only goes down and then up for every
time the sparkplug fires. For this reason two stoke engines are more powerful than
four stokes because the energy of every explosion goes into pushing only one
rather than two revolutions of the piston. Two stroke engines are therefor used by
dirt bike racers who need power while four stroke engines are used by the public
who prefer efficiency and cleanliness. Two strokes are also much more simple,
have much fewer parts on the inside and therefor suffer from less problems.
The Minsk might be a little bike and it might be a dirty bike, but no one has
ever said that it lacks the power to get you up and over any obstacle thrown up by
the back roads of Vietnam.
Understanding the Transmission System
The up and down movement of the piston is converted into circular movement
by the cam disk, which is a large double disk deep in the engine under the piston.
©Digby Greenhalgh 2000
Minsk Repair Manual
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