I've found the best thing to use as a gauge is an Allen Key. Mark it at the top of your cyl head hole with the piston at TDC with a junior hacksaw. Then mark it again 7/16" above. The L shape prevents it falling down the bore. If you get it spot on first time......."You are a better man than me Gunka Pooch" Ron
In that case you have what we call slack wire advance. Actually it's a better idea. If the cable snaps you will automatically be at full advance and therefore carry on regardless. The points should just release a fag paper as the piston reaches 7/16" BTDC. If you are unsure, it's best to double check. Ron
When you are very close to this fag paper point but not exactly right, you can tinker a little with opening or closing the point gap a little to get there. But don't make the gap either to small or big.
Easy trick I use myself every time and often I get it right in one go.
I can't help but ask how on earth did you go on a 50 mile run without knowing which way advance & retard is on your handlebar lever... anyway
.. the simple way to rule out all confusion when determining which way is advance on an M20 is to look at the points Cam plate while moving your lever & when the Cam plate is moving anticlockwise your advancing the ignition.
