Switching to Mag. at an idle, a slight increase in RPM has always been noticeable, but it seems much more obvious of late. Could this be an indication of the level of charge in the battery ?
It could, and that might show that your generator isn't working. Of course, you'd notice a difference in the cranking speed as well.
I should have noted it's a '13, no starter or generator. I'm wondering when the 12v lawnmower battery I've been neglecting will fail - then MY cranking speed will have to go up !!
The answer to your question is in this paper!
Thanks Dan !
It would depend on the position of the timing lever. You will notice that when you move the lever on battery, the engine speeds up a little with each notch on the quadrant, but when you move it while on magneto, it won't increase with each notch but move it a few notches and then suddenly it will speed up. Another thing about the magneto is that it puts out higher voltage as the engine speeds up, so if your engine is going faster when you make the switch you will notice more change. Just don't make that switch under power, or you might blow the muffler or manifold gasket!
Norm, my observation was made at idle with spark retarded. Switching to Mag. she picks up a few revs.
The discussion of this subject is a different paper.
More on Model T Spark Timing