Re: How Important is OMC Ignition Timing?

[Anonymous]

quote legendre:

Are you sure that the magnet is always in the flywheel? On the OMC magneto I know, it’s in the rotor, which is a separate part from the flywheel. While the flywheel must be removed to access the points, the rotor may remain in-place. In fact, it must, as one of the tuning parameters is rotor / coil shoe gap..

The tools Frank makes (superior) reproductions of are (to the best of MY knowledge) only for OMC "universal magneto" engines, with magnets in the flywheel. On those engines there is no shoe gap spec, just a machined reference boss to line up the coils with. Frank also makes a coil alignment jig for that task.

quote :

And once again, why not just put a mark on the rotor, to align with a coil shoe, if our only concern is that the magnet(s) properly time the ignition event to the point where the magnet(s) – on the rotor – are properly aligned with the coil pole(s)?

Do you not see how this would obviate the OMC timing tool?

OK, I do now! I don’t know why they didn’t find/make a "built-in" way to do it, since OMC engines were made to be more friendly to shade-tree mechanics than Elmer’s were….

quote :

None of this makes any sense.. since when is an engine timed, based solely on peak spark power? Have you ever encountered any other engine, or family of engines, that didn’t time the ignition relative to crankshaft position (in degrees, BTDC)?

The tool is for setting the points (the fixed timing between magnet, points and coils), not the adjustment of "advance"

The change in spark timing relative to piston position (advance) is done by rotation of the magneto "plate" which also controls the carb, so when the engine is "wide open", the spark is "earlier" (relative to the piston position) to give the flame front time to propogate (since the pistons speed up, but the speed of combustion doesn’t).

Adjustment of the "advance" is done by tweaking the cam on the bottom of the magneto plate, which is always backwards from what I expect– for a given number of degrees of advance before TDC the adjustment determines how much fuel the engine gets.

The mark on the cam on the bottom of the magneto plate is flat at the low end– at the low end, speed is changed only by advancing the spark (wasting less fuel?) while keeping the butterfly closed. The mark on the cam indicates the point (which no doubt could be expressed as degrees BTDC) when the throttle begins to add fuel as the timing is advanced further.