20201 Arthur Road, Big Rapids, Michigan 49307
Numerous inquiries as to the proper timing of four-cycle engines
indicates that the following article should be of keen interest to
some of the younger members of the fraternity of antique engine
enthusiasts.
A four stroke cycle engine is named a 4-cycle because it takes
four strokes of the connecting rod and piston to complete one cycle
of events to make the engine run.
We will name these four strokes: (1) compression, (2) power, (3)
exhaust, and (4) intake.
You should be able to trace these strokes on your engine. Crank
the engine over in the proper direction of rotation; that is, with
your hands on top of the flywheels, pull the flywheels toward you
as you stand at the rear of the engine. Continue turning until you
feel the piston coming up on the compression. This is COMPRESSION
STROKE-the crankshaft, connecting rod are pushing the piston to the
inner end of travel. The con/rod is in horizontal position, the
crankshaft con/rod arm is extended to its inner limit-the piston is
at T.D.C. THIS IS FIRING POSITION-IGNITION . At this point the trip
arm should just trip the igniter. (More on this later.)
With piston at T.D.C., continue to turn your engine over. The
crankshaft and connecting rod are pulling the piston out of the
cylinder. (In effect, after ignition, the combustion of gases is
pushing the piston out of the cylinder.) This is POWER stroke. As
you continue to turn the flywheels over you will note the
connecting rod arm of the crankshaft is at top of its travel.
Slowly, continue to turn the flywheels, about half way between this
top position of the crankshaft rod arm and the outer end of travel,
you will note the lobe of the camshaft engages the push rod. The
push rod has to move forward about half to three quarters of an
inch to close the gap between the push rod and the valve rocker
arm. The exhaust valve should begin to open when the piston is at
the outer end of travel-just beginning to move back into the
cylinder. The valve remains open for the duration of this stroke.
This is the EXHAUST stroke. There is an adjustment at the end of
the push rod or at one end of the rocker arm to make a correct
adjustment.
As you continue to turn the flywheel with the piston moving out
of the cylinder you should hear or see the intake valve spring move
to allow suction of the piston movement (actually it is difference
in atmospheric pressure) to take in a charge of air and fuel
mixture. This is INTAKE stroke. At the end of this stroke you have
completed one full cycle of events-ready to start on the
compression stroke again. Now, if you have followed this sequence
of events closely and everything is in order, your engine should
run.
Let’s go over the relationship of the camshaft and gear to
the crankshaft gear again. There are three meshed gears on your
engine-the governor gear is not timed-forget it. There are also TWO
LOBES on your camshaft gear. The shorter lobe operates the push rod
to trip the igniter at the proper time. The larger lobe operates
the same push rod to open the exhaust valve. In order to check the
accuracy of the timing marks and the camshaft to crankshaft gear,
you may do this:
With the piston at the end of the intake stroke, the connecting
rod at its outer limit, continue to turn the flywheels. At about 30
to 45 degrees (in reference to a revolution of the crankshaft)
beyond the con/rod outer limit, the push rod is at its most
rearward position. This position should permit the trip arm, which
is attached by a bracket to the push rod, to engage the igniter
lever. The trip arm bracket is adjustable on the push rod. There
should be a spring action to hold the trip arm against the igniter
lever. As you continue to turn the flywheel, on the compression
stroke, the igniter should trip when the piston is at top Dead
Center. A slight adjustment of the trip arm bracket may be
necessary. There are timing marks on the two gears, but they are
difficult to locate unless the gears are disassembled.
I might mention, with this type of ignition system or with any
Webster-Tri-polar, or Wico EK ignition system that uses the trip
mechanism, speed of rotation of the flywheels has little to do with
ease of starting. Spring action should take care of the quick
movement that is required.
The connections for your ignition with the battery and low
tension coil system is simple. One battery lead is connected to the
engine block for ground. The other battery lead is connected to one
terminal of the coil. The other terminal of the coil is connected
to the insulated terminal of the igniter. If you wish to use a
cut-off switch in your system, connect it to the lead from your
coil to the igniter.
The instructions outlined here apply to any type of ignition
system that uses an igniter, whether it is used with a magneto or a
battery and coil.
