This is not a call for help, but rather a tech tip that may help you with your K2Fc magneto if you are having trouble with a poor/no run condition. The time this happened to me, it was on a fresh build, complete motor build, complete mag rebuild. And the bike ran like a champ for two weeks before this issue reared its ugly head.
Symptoms:
Backfiring out exhaust, stalling out, and eventually a "no-run" condition, only occasional firing of either cylinder. It was much like having very fouled plugs, and having the kill switch "on".
Upon further trouble shooting, it began to run, but only with the cap off (even without the kill wire connected).
The issue turned out to be a slightly loose (not rattling loose, just not completely cranked down) center stud. After tightening the stud, it ran like a champ again, instantly.
I hope this info helps another one of us fools who are addicted to old British iron.
Cheers, Dan.
This is not the mag in question, but it is a K2FC...
