Never had an Avistar engine. But when I had my Rascal, and other planes with an inverted engine, you have to get the low end setup for it to run right. After you get the low end then you redo the high speed needle. I also used a glow plug with a idle bar.

When I got the inverted engines running I didn't have any more trouble out of them then any of my engines.

Hope you get it going.