Cam ring lobes

[Pallijen15]

Hi, I have been looking at the cam ring as there is too much of a difference between points gaps. At each side of the cam ring high points there is a peak before and after where the points separate, as if there has been a drill grinder or similar used. Should the cam high points rise gradually or is there a peak before this? Thanks, Paul

[Doug]

Paul,

I had this problem with the Lucas magneto on my Mark 3 Sports. Manufacturing eccentricity between the magneto body, end housing, and cam ring bore meant the point gap was unequal between the two cylinders (cam lobes). However the point gap is not as critical as when the points open, and that should be as near as 180 degrees apart for even timing of the spark. On mine, I took a die grinder with a stone about 2/3rd the cam ring bore size and ground back the leading ramp of the lobe that was too far advanced. I did not bother to try and grind down the height to get the gap consistent.

How long the points stay open does effect the internal timing of the magneto and more gap (increased lift) will extend this and less will decrease it. But you won't notice the difference as much as getting the points to break exactly at 180 degrees. In my case, I split the difference of the recommended point gap between the two; one being a little over and the other a little under. It has worked fine for almost forty years.

-Doug

[Pallijen15]

Ok thanks Doug. So I grind the part of the cam where the spring part first meets to begin opening the points. I did use  a sandpaper spool off the end of a drill to try to take off a bit from the side which needs to have the points gap decreased. Been out today, seems to be running a little better. Was measuring each time, didn't take much off but will try where you say, in small amounts

[Doug]

Paul,

Yes, the side with the larger gaps will open earlier.

I found it a little easier and more accurate to do with the magneto off the machine and a degree wheel applied to the drive gear. Since the magneto runs at half engine speed, a two degree error at the magneto (cam ring) equals a four degree error at the crankshaft. So you want to be as accurate as humanly possible if timing just at the magneto. If done in-situ, a degree wheel to the end of the crankshaft or use whatever timing aid you prefer based on piston position to get both sides the same.

-Doug