I have a 3W 60k that has allot of flights on it . Since it was new it will kick back hard IF it is really wet with fuel or the idle is set slightly high.
I have checked the timing , had the ign module checked and replaced , it still does it .. but only if i over choke it or have it set to high idle ...
I basically had to re-learn how to start this engine ... compared to my other gassers.
Since It appears that original post has had no running problems after started, but only a kick back problem occasionally while starting I would consider
that it might be getting too wet and kicking back .