usually no oil in starting fluid... piston cylinder scaring
do you not have a timing mark on the fly wheel that you can check the timing with instead of pulling it off to check?
if it back fires 95% of the time when trying to start it i would say timing is the issue
is it a rather violent back fire?
and, if you have gas in the engine is should still back fire the same, maybe more
try pulling the bike backwards if it starts and runs backward your timing is off
by a good bit