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


| August/September 1988



Diagram of time gear mark

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.