i had this trouble yrs ago wif some numpty on a moped.
one night, i cut his ht lead wif me snips.
just where it appears out from the magneto!
all quiet after that.
:)
switch the rear slave cyls over, o/s to n/s, that will eliminate a poss faulty cylinder.
it has been known.
ive filed off the edges of new shoes many times and cured similar probs.
try another drum. they were never the most precise things ever made!
thats neat coming from a country where coors is the national beer, ffs.
it taste like the Mississippi, and it looks like it too.
and they serve it at about -45 degrees f for some strange ungodly reason.