I have an inverted 0S91. This like all my inverted engines needs to be started unprimed and the glow plug kept very clean during layup.

GLOW
Use a warm-hot glow.

TANK
Position so fuel is gravity fed to carb in airframe's starting position.

LOW NEEDLE
Low end mixture is on the leanish side.
Close Throttle to idle.
Fuel should drip slowly from the carb opening.

START
Do not prime.
Turn engine by hand two turns to check all is well.
Open throttle 20%.
Energise glow.
Turn engine rapidly with starter.
Engine sucks in fuel drops and fires in a couple of seconds.
If not check glow or for flooding.

AFTER USE
Probably the most important procedure.
Always burn off fuel in engine: empty tank, energise glow, apply starter for a couple of seconds.
Take care engine may run on.
Glow off.
Quick squirt of afterrun oil deep into carb, I use WD40, never ever had any problems.

STILL PROBLEMATIC
Check plug before starting, this can gather engine combustion byproducts and oil.
If needed, remove plug after each flying session, burn it clean ans store it for next time.

OBSERVATIONS
When cold the OS91 can be very tight.
You will need a powerfull starter.
I have never had a flame out following this procedure on an engine that is setup correctly.
Watch the richness of your needles keep them reasonably lean.
Good luck