Keep in mind that adjusting Ignition timing as we are discussing only applies to gasoline engines. Diesels are different.
Someone asked about setting the "Dwell". Dwell is the time that a spark coil has available to build up enough magnetic field to make a spark. It is critical in V8 high rev. race engines, but a low rpm tractor is going to have enough dwell time regardless. So it can be ignored.
Timing specs were always based on the fuel at the time. Basically the whole reason for timing is to allow the right amount of time to pass between the spark and the maximum expansion of the burning mixture pushing down on the piston.
At idle the spark can be nearly at top dead center (TDC). At full rpm, the centrifulgal advance mechanism makes the spark occur sooner to give the charge extra time to burn. If the spark is advanced too far you will get what is called "engine knock" due to the max expansion taking place before the piston reaches top dead center and being forced backwards/ So after setting the timing at idle as described, I will often rev it up and listen. If I hear the peculiar knock of too much advance, slightly ****** the timing until the knock goes away.
That last step is absolutely necessary in high performance gasoline engines, but rarely in tractors. I just do it because it is best practice.
Something to consider is that although getting timing too far RETARDED mostly just affects fuel efficiency at low RPM, having it too far ADVANCED at top RPM causes mechanical wear to pistons and bearings due to the knocking.
EDIT: Reducing the point gap slightly retards the timing.
rScotty
