Bazza,
Your magneto does have a cam to open the points. Assuming you are using the BTH M2 series magneto, the cam is a ring, with two opposed areas on the inner diameter that have a smaller radius than the rest of the ring. This pushes the shoe of the contact breaker arm inwards, and opens the contacts.
If you can not probe the piston to tell when it is at TDC, take the timing cover off, remove the crank pinion nut, and look at the key for the pinion. It points directly at the crankpin for the rear cylinder. So when the key is horizontal, both pistons are at TDC (opposed twin, remember?) Arrange a suitable indicator, and mark the rim of your flywheel with a felt tip pen. Then you can lay out your 'advance' mark. Rather than use the 5/16" BTDC, which you can not see, the EW handbook also recommends timing at full advance to 50 degrees BTDC; further 10 degrees is the equivalent of 3/4" measured along the circumference of the flywheel (assuming you have the proper size flywheel, 8-5/8", I think, at least the math works out right for that diameter.)
It does not matter if you time the front or the back cylinder. Which ever cylinder has both valves shut, that is the one on the firing stroke (unless you timed your cams completely wrong!
) Make sure the magneto slip ring is directing the spark to that cylinder, and not the other one. Use very thin paper, and not regular writing paper. Or use a continuity meter across the points. With the points closed you will have a dead short. When the points open you will get a reading back through the primary windings, but with the right settings you should notice the difference between the points open (nil resistance) and points closed (higher resistance.)
-Doug
