Valve And Ignition Timing Of Four-Cycle Hit And Miss Engines

Bud Motry
August/September 1988
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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.


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