GAS EBGUBE VAKVE & IGNITION TIMING


| January/February 1975



Engine to determine easiest method of shifting spark

The following article is by the courtesy of Edward L. Fleming, 1027 Meadowbrook Road, Iola, Kansas 66749 and he states: 'This is a copy of a gas engine timing instruction sheet distributed by Kansas State College (now Kansas State University) about 1935. These instructions were given to me by my uncle who was a vocational agriculture instructor. I thought it might be helpful to a lot of your readers.'

PURPOSE: To give a definite procedure for checking the valve and ignition timing of a single cylinder gas engine and for retiming if necessary.

EQUIPMENT: Rule and string, or tape, wrenches, pliers and tram.

EXPLANATION:Farm gas engines are generally of the four-stroke cycle principle and use valves to admit and exhaust the fresh and burned gases. Contrary to the belief of many, the valves do not open and close at the ends of the strokes, but vary from this as much as 45 degrees. A timing that gives maximum power and best operation is determined experimentally for each engine, and used. Naturally, therefore, the exact time at which the valves should open and close, varies with engines of different design. If the manufacturer's instructions are at hand they should be followed. The following suggested timing is average for small engines running between 400 and 600 rpm and will give good results where the exact timing is not known:

Exhaust valve open - 35 degrees to 45 degrees before ODC on power stroke. Exhaust valve close - 5 degrees after IDC on intake stroke. Inlet valve open - 10 degrees after IDC on intake stroke. Inlet valve close - 30 degrees after ODC on compression stroke. (NOTE: Inlet valves are usually automatic; above applies when they are mechanically operated.)

From the above, the range of opening can be calculated. It would be from 220 degrees to 230 degrees from the time the exhaust valve opens until it closes and for the inlet the range is 200 degrees. The range is adjusted by the tappet or screw on the rocker arm and it is absolutely necessary that this range be correct before the valves can be timed accurately.