Just as I thought. Especially in inverted mounting, it is important not to let the engine get too much fuel when standing still, or at idle.
Reduce idle mixture by clockwise turning of the idle mixture srew (closest to engine). check throttle response, it should still be crisp. Redo until crispness goes away.
(then do: Link to be added soon)

When standing still, do not rock that prop. Rocking the prop will pump fuel, and that fuel gets into the engine without getting burnt. It seeps right down to the plug eventually.

Starting procedure for first start of the day:
ignition on, no choke.
Flip a few times to check if any fuel went into the engine. If engine starts, all is OK.
If not,
close choke, ignition still on. Idle a few clicks open.
flip until the engine burps. (my engine 2 flips)
choke off
flip until the engine starts. (My engine 5 flips, yours may differ)

When I prepare the engine at home like mentioned above, engine starts first flip on the field like this: (I need about 3/4 hour to get the plane up at the field)
ignition off.
bump prop to and fro against compression with idle two clicks open a few times (do not flip).
ignition on.
first or second flip sets the engine going. Spectators jaws drop in awe.
(don't tell this to anyone)

edit:
warm start, or engine just cooled down.
ignition on, start flipping. First or second flip should set you engine off. If not, your idle setting is too lean.