Jwolfe,
Often the impulse stop plate and the magnet armature shaft are worn where the plate slides over the armature shaft, so the proper position of the impulse trip ring (the thing you loosen 4 screws and rotate on the back of the mag) can be hard to determine. I prefer the following procedure, which is based on getting the running timing correct first, then adjusting the impulse-related starting timing (hopefully your tractor spends 99% of its time running and 1% of its time starting).
You will need a common old timing light and a battery to power it. Once you have these, measure the circumference of your flywheel. That corresponds to a circle of 360 degrees, so with that information, and the desire to make timing marks at 30, 25 and 20 degrees BTDC, you can calculate the distances on the flywheel corresponding to these degrees (multiply the degrees timing times the circumference of the flywheel, and divide by 360 to get the distance to measure). From the LH Impulse mark on your flywheel, measure the above distances in a counter clockwise direction and mark them with a piece of chalk, being sure to make the marks different lengths to differentiate between them. Hook your timing light to the battery, and to the spark plug on the flywheel side of the tractor. Start the tractor (making sure not to mess up the chalk marks), and using the timing light flash see which chalk mark lines up with the timing mark on the tractor case (used normally to set the impulse click). Rotate the magneto back and forth until the desired timing mark lines up with the case - I normally use 25 degrees if burning gasoline. When that is done, mark the magneto flange and governor case so the magneto can be returned to that position.Stop the tractor, remove the timing light, and roll the engine over slowly, noting exactly where the impulse click now occurs. The desired time is when the piston is at TDC - ie, when the flywheel and case marks line up. If it is before the mark on the case, you want it to occur later, so remove the magneto and rotate the impulse stop ring down a bit. Then replace the magneto, being careful to locate it via the marks you made earlier, and try again. Keep doing this until you get the impulse to trip at TDC. Then you have both the running and starting timing correctly set. You might note the position of the impulse stop ring - it will likely not be at the position recommended, due to the wear noted earlier. Good Luck - others may have different methods, but this is how I do it.
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