My OS .55 is a great engine, really powerful, but is really touchy on carb settings even side mounted. Seems to need lots of warm up. Should probably stay away from the Fuel fillers which go in the carb line. Inverted engines just seem to be tough to get right, but some people have great luck once the engines are tuned correctly. That said I have a 20 year old Pilot Twin with 2 inverted K&B 28s that have settled down fairly well.
Good Luck - Darryl