Low Tension Magneto Timing

Valve timing and ignition timing in relation to low tension magneto timing

| March/April 1984

  • Economy
    This Sears "ECONOMY" also uses the pushrod as the base reference mark to correlate with the TDC, or "SPARK" line on the flywheel.
  • John Deere E
    The pushrod, chalked white, is the base reference point on this John Deere "E." Note the SPARK line, also chalked, on the flywheel, adjacent to the pushrod.
  • Fig. 1
    Fig. 1 TDC Base timing mark is noted by the position of flywheel spokes on this NOVO "S" engine.
  • Novo 1 HP model S
    This "NOVO" 1 HP model "S" uses an imaginary horizontal line for base reference to indicate TDC.

  • Economy
  • John Deere E
  • Fig. 1
  • Novo 1 HP model S

The current flow and resulting spark to cause ignition of combustible gases in a cylinder was discussed in a a previous article on low tension magnetos. This article will deal with valve timing and ignition timing, as both are closely related. In timing any engine, all gears in the gear train, from the crankshaft gear to the final gear that drives the push rod for the valve, or the gear that rotates the magneto, must be in time. The only exception is the governor gear which requires no timing. There are timing marks of some kind on all mated gears. They may be ground-off corners of two adjacent gears that mate with a ground-off tooth of another gear, as in most magneto internal gears. There may be dots or punch marks or line marks, either raised marks or grooves. All of these marks should be aligned at the same time. In order to ascertain that timing is correct, it is only necessary to turn the flywheel and check the action. So far as valve timing is concerned, many engines have the Ex. open and Ex. closing marks on the flywheel rim. The marks should correspond to the TDC mark on the engine casting.

If you have difficulty locating the engine base TDC mark, you can check the position of the flywheel marking when the piston is at its end or top stroke. There should be some identifying mark on the engine block. As in the case of John Deere, Economy, Hercules and similar engines, the valve push rod is used as a horizontal mark that indicates TDC. On the McCormick Deering "M" the mark will be a notch on the base at the bottom of the flywheel. On the McCormick Deering "LA" the engine base marking is a notch at the top of the flywheel. The Novo "S" engine manufacturer indicated TDC or IGN position in relation to the spokes of the flywheel. In this case, the webbed spoke of the flywheel, also the keyway groove, was in a downward position about 50 degrees beyond the horizontal position of the spoke (See Fig. 1).

In any given case, if the flywheel marking is not in evidence, I locate the TDC position of the piston, with the head removed is most desirable, but it can be determined with a piece of wire inserted through an igniter or spark plug opening. Locate some convenient reference point on the engine and file a notch on the flywheel. With the TDC mark indicated, keep in mind when you are setting ignition that this TDC mark appears twice during a single cycle of events. In one position of TDC, the exhaust valve is just closing and the intake valve is just opening. This is precisely accurate for all four cycle, 4, 6, and 8 cylinder engines. You may keep this in mind when checking valve timing of these engines. However, on the little one lungers this may vary because the intake valve is operated by suction from the movement of the piston.

With the oscillating movement type of low tension magneto, there was no internal magneto timing required. The only timing necessary was to adjust the trip lever for the magneto to release at TDC of the piston, at the end of the compression stroke.

With the more prevalent rotary type of magneto, it was necessary to set the magneto gear in such a position that the igniter was tripped when the current was at its peak flow. On the John Deere "E" magneto, this peak flow could be checked by locating the "SPARK" line on the flywheel. Rotate this mark until it is adjacent and parallel to the horizontal push rod. With the flywheel in this position, place your finger over the pin on either side of the magneto rotor shaft, slowly rotate the flywheel back and forth about an inch or so. The pin should be moving in and out very slightly. Other magnetos may have line markings on the end of the magneto shaft and the end housing to indicate peak flow.

With the magnetos timed correctly, the trip lever, attached to the push rod, should be adjusted to release the igniter trip lever at precisely this time. This type of magneto could be checked on the engine, by removing the lead wire at the igniter terminal and in effect, scratching it, that is making and breaking contact, on the engine block while the engine flywheel is being rotated as quickly as possible. The "scratching" should produce sparks.


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