Fergi,but its chucking more fuel back out than is going in:
Welcome to the group. I think the word you are looking for is
reversion. This is when the air has already atomized the fuel
into vapor, than for some reason the newly mixed charge is
forced back out of the carburator. Now this will happen a little
bit with these bikes. This is because of the short intake track
and the return wave of air/fuel backing up in the intake track when
the intake valve goes back onto its seat. However, in a normal
engine this is very little amount and is sucked right back in during the
next intake event.
As Jim stated, if you have a lot of this going on something is
a miss with the sequence of the events for the intake charge.