When running a timing light on my gran sport, the marks line up where I want them, but every few seconds they flip to the backside of the wheel. Basically sparking at the 180 degree opposite of where I set it. Can't figure out why it may be doing this.. Did it with points, and just switched it to cdi today, still the same problem though. Btw, the bike is completely stock other than the cdi.
Any help would be appreciated