The way I got my 91FX to run right inverted in a razzle is to use a OS F plug. The little tip on the plug helps keep it from being covered or flooded out. I had no success with idle bars or hot plugs. They still blew out. Also I make first start of the day with an electric starter and the engine dry. No primeing or chokeing. Let the engine pump the fuel in while starting. After that it always startes on 1 or 2 flips with a chicken stick. The only problem I have with the OS Fs is after about 20 flights the shine comes off the eliment and they start quiting and I have to put in anew one (expensive). I had the same problem you are having untill I switched to the F.

Denis