tech10

Two stroke, ugh, you say. Well now, BSA had the Bantam and some of the
best vintage trials and motocross bikes are two strokes. So why not
understand how they work? So here goes, to the best of my ability.

Two Stroke Operation:

As most of you know a four stroke engine takes four strokes of the
piston, two up and two down, or 360 degrees twice equaling 720 degrees
to complete the engine's cycles. A two stroke needs only one up and
one stroke down. Multiple things happen in a two stroke in each stroke
of the engine. The key is that the engine delivers power and acts as
an air/fuel pump. Two strokes use strategically placed windows (holes)
in the cylinder walls to move gases inside the motor, into the motor,
and out of the motor. Below the piston in the cylinder rear wall is
the intake port(s) that allows air/fuel to flow into the engine
crankcase area below the piston using the vacuum created by the rising
piston. Above the piston are two classes of ports: exhaust and
transfer. As the piston falls from Top dead center (TDC) the exhaust
port(s) are revealed, at a critical point, such that the power is
extracted from the burning fuel and then the consumed air fuel
(exhaust gases) exit the motor via the exhaust port, all while the
piston is falling. As the piston falls it compresses the air/fuel
under the piston inside the crankcase cavity forcing it to exit the
crankcase just above the top of the piston through transfer ports.
These transfer ports are critically placed to be opening after the
exhaust ports have opened, which allows enough time for the exhaust
gases to leave the cylinder under pressure. The transfers are ports
connected to the crankcase via long channels running from the mouth of
the crankcase (on each side) to a point about half way up the cylinder
wall. So the power cycle is done from TDC to the opening of the
exhaust port. The exhaust cycle is done from the opening of the
exhaust port until the transfers start moving the fresh air/fuel
mixture to area above the piston. The intake cycle is done as the
piston rises, and the compression cycle is done as the piston closes
the exhaust port until TDC. The transfers on more modern bikes are
aimed toward the back cylinder wall to allow the air/fuel gas streams
to collide and to act to sweep out any left over exhaust gases as the
exhaust port remains open all during the transfer phase. The aiming of
the transfers minimizes the flow of air/fuel directly out the exhaust.
Older designs pre 1950's used a deflector on the piston top, sort of
wedge shaped, to attempt to keep the air fuel from being short
circuited out the exhaust.

As you look down into a two cycle barrel the highest port is in the
front, the exhaust port. On each side are one or more transfer ports
(can be two sets of three in some motors). Their top edge is below the
top edge of the exhaust port while both the exhaust and the transfers
bottom edges are at the piston's bottom dead center (BDC) point. The
intake port(s) is below the piston and is opened and closed by the
piston skirt's rear side.

Lubrication:

With all that air/fuel moving around inside the crankshaft
cavity, where's the oil? It's mixed in with the fuel and carried along
with the air/fuel mixture to plate out on all the internal surfaces.
Two strokes use all ball/roller bearing crankshaft bearings which
require very little actual lubrication. Some older two strokes had
plain bushes on the small end of the rod since all it does is rock
back and forth. How much oil, you say. In the 1950's and early 60's
the recommended ratio of fuel to oil was 20 to 1 or one quart of oil
to five gallons of gasoline. But those oils were regular car oil or
maybe oil with an additive to keep it in solution while in the gas
tank. Today with synthetic oils we run 32 or 40 to 1 or 16 oz. (1
pint) of two stroke oil to 4 or 5 gallons of gasoline. The synthetic
nature of modern two stroke oils makes the difference. They also
minimize the smoking that normally is caused by burning the oil with
the fuel as well as the minimize the build up of carbon in the
combustion chamber, the exhaust port, and the exhaust pipe .<underline>

Modern Updates:

Modern two strokes can use some form of exhaust tuning through
auxiliary chambers or can use exhaust port top edge timing changed by
a valve that is controlled centrifically from the crankshaft (termed a
power valve). Most modern two strokes remove the piston as the
controller of the intake timing. Those ones use a reed valve that
allows air/fuel into the crankcase only when the vacuum in the
crankcase is high enough to operate thin flat plates acting as valves
across the intake path. Those reed valve petals can be steel, fiber
glass, Kevlar, Titanium, or who knows what(?). Some engines have a
boost port(s) that are auxiliary transfers that are aimed from the
rear of the cylinder wall directly at the exhaust port to purge out
any remaining exhaust gases. They are normally only a very small
window in the cylinder wall.

Performance Enhancers:

It was found in the 1950's that two stroke performance
could be doubled and more if some form of exhaust pipe tuning was
used. The basic two stroke pipe starts as a head pipe, sometimes
slightly tapered, into an opening cone, then a short straight section,
then a closing cone, with a longish small diameter straight tube
exiting to the outside world. Each part of the pipe makes reflected
pressure or vacuum waves inside the pipe to help scavenge or push back
in the exhaust gases, and the intake fuel/air charge. Every length and
diameter is important to help the engine make more power.

In more recent times electronic ignitions and automatic electronic
advance curves have helped increase performance, as well as spread the
power out over the entire RPM range. Some of the modern Supercross
bikes have switches on the handlebars so the rider can dial in the
correct advance curve for starts vs. the rest of the race. Early two
strokes used flywheel magnetos with fixed advance, usually set to give
maximum advance all the time. Some of the big two strokes were know to
break legs (seriously). The Maico being the biggest leg breaker. The
record is two legs in one day by the same Maico.

Servicing the two Stroke:

Since the metered air/fuel gets stored in the crankcase it
is imperative that no maverick air leaks into the engine. The sources
for those leaks might be the crankcase halves joint (including the
wall between the unit transmission and the crankshaft cavity), primary
side crankshaft seal, ignition side crankshaft seal, barrel to
crankcase joint, cylinder head to barrel joint, and intake to barrel
joint. To test for leakage we must do an Air Test. The principal is to
seal up the engine, pressurize it (or put under a vacuum), and measure
the time for the air to leak out (or in). The engine can never be made
perfectly air tight so a acceptable leak down rate is adequate.

To implement the test the intake must be blocked with a brass or
rubber freeze plug. The same must be done for the exhaust side with
the rubber freeze plug being the tool of choice after the loose carbon
is removed from the seating area for the test plug. I have modified a
spark plug that allows me to screw it into the head and attach a
rubber hose system with a gauge and a fill valve to a hose adapter
inserted and sealed into the plug's area where the electrode used to
be. Other tooling methods are possible as long as the tester can be
made air tight. The crankcase is then pressurized to no more than 7
psi. Anymore pressure and the seals will pop out and/or the rubber
plugs will pop out and shoot across the garage. The rate of leak down
should be no more than one pound (psi) a minute. Most engine will do
ten to twenty times better than that. When performing the test on a
leaky engine suspect the tooling first. Use soapy water and a brush to
find where it is leaking. A more through test is to then test a non
leaky motor with a vacuum pump, looking for gross leaks as sometimes a
seal will not leak under pressure but leak under a vacuum.

Two Stroke Troubles:

Two strokes, because the 4 cycles are compressed into one
revolution, have a heat dissipation problem. They also have twice the
opportunities to make power over a four stroke-meaning more heat. The
jetting has to be perfect, the ignition timing not too advanced, and
the engine has to be Air Test leak proof in order that the engine will
get rid of its' heat adequately. If it doesn't get rid of the heat,
the piston gets too hot, expands faster than the barrel and the piston
seizes in the cylinder bore. Two strokes that won't come down to an
idle after a hard run have an air leak and should not be ridden until
it is fixed. Too lean of a set of jetting gives sharp response to a
quick opening of the throttle and eventually a seizure. The saying is
that two strokes run best just before they seize. Jet up for the
desert where long wide open throttle sessions are a reality. Check the
plug for the correct color. Keep the ignition advance (timing) at the
manufacture's recommended setting.

Excessive smoking can be caused by either a leaky primary crankshaft
seal or a leak between the transmission cavity and the crankshaft
cavity. What is happening is the engine is consuming the primary or
transmission oil supply through the leak. Oil found in the ignition
cavity is coming, more than likely, through the crankshaft bearing and
seal. Don't ride it, fix it.

Well, that's about all I know. Well almost all. If you have a two
stroke project and need help, feel free to call for advice, free advice.